[U-Boot] [PATCH] arm: enable unaligned access on ARMv7
Recent toolchains default to using the hardware feature for unaligned access on ARM v7, rather than doing the software fallback. According to ARM this is safe as all v7 implementations have to support this feature. (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/BABJFFA...)
To avoid CPU hangs when doing unaligned memory access, we have to turn off alignment checking in our CPU initialisation code. (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491c/CIHCGCF...)
Signed-off-by: Lucas Stach dev@lynxeye.de CC: Albert ARIBAUD albert.u.boot@aribaud.net --- arch/arm/cpu/armv7/start.S | 2 +- 1 Datei geändert, 1 Zeile hinzugefügt(+), 1 Zeile entfernt(-)
diff --git a/arch/arm/cpu/armv7/start.S b/arch/arm/cpu/armv7/start.S index 261835b..52f7f6e 100644 --- a/arch/arm/cpu/armv7/start.S +++ b/arch/arm/cpu/armv7/start.S @@ -316,7 +316,7 @@ ENTRY(cpu_init_cp15) mrc p15, 0, r0, c1, c0, 0 bic r0, r0, #0x00002000 @ clear bits 13 (--V-) bic r0, r0, #0x00000007 @ clear bits 2:0 (-CAM) - orr r0, r0, #0x00000002 @ set bit 1 (--A-) Align + bic r0, r0, #0x00000002 @ clear bit 1 (--A-) Align orr r0, r0, #0x00000800 @ set bit 11 (Z---) BTB #ifdef CONFIG_SYS_ICACHE_OFF bic r0, r0, #0x00001000 @ clear bit 12 (I) I-cache
On 06/05/2012 11:47 AM, Lucas Stach wrote:
Recent toolchains default to using the hardware feature for unaligned access on ARM v7, rather than doing the software fallback. According to ARM this is safe as all v7 implementations have to support this feature. (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/BABJFFA...)
To avoid CPU hangs when doing unaligned memory access, we have to turn off alignment checking in our CPU initialisation code. (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491c/CIHCGCF...)
Does this behavior change trickle down to Linux/... too, or would an OS completely re-initialize this state, and hence not be affected?
Hi Stephen,
Am Dienstag, den 05.06.2012, 12:42 -0600 schrieb Stephen Warren:
On 06/05/2012 11:47 AM, Lucas Stach wrote:
Recent toolchains default to using the hardware feature for unaligned access on ARM v7, rather than doing the software fallback. According to ARM this is safe as all v7 implementations have to support this feature. (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/BABJFFA...)
To avoid CPU hangs when doing unaligned memory access, we have to turn off alignment checking in our CPU initialisation code. (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491c/CIHCGCF...)
Does this behavior change trickle down to Linux/... too, or would an OS completely re-initialize this state, and hence not be affected?
Linux in particular does reinitialize this state and I expect any reasonable OS to do so.
-- Lucas
Hi Lucas,
On Tue, 05 Jun 2012 21:06:20 +0200, Lucas Stach dev@lynxeye.de wrote:
Hi Stephen,
Am Dienstag, den 05.06.2012, 12:42 -0600 schrieb Stephen Warren:
On 06/05/2012 11:47 AM, Lucas Stach wrote:
Recent toolchains default to using the hardware feature for unaligned access on ARM v7, rather than doing the software fallback. According to ARM this is safe as all v7 implementations have to support this feature. (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/BABJFFA...)
To avoid CPU hangs when doing unaligned memory access, we have to turn off alignment checking in our CPU initialisation code. (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491c/CIHCGCF...)
Does this behavior change trickle down to Linux/... too, or would an OS completely re-initialize this state, and hence not be affected?
Linux in particular does reinitialize this state and I expect any reasonable OS to do so.
Then what is the point of enabling it on U-Boot? Does it fix some issue whereby some mis-aligned piece of data cannot be properly aligned?
Amicalement,
Am Freitag, den 22.06.2012, 11:15 +0200 schrieb Albert ARIBAUD:
Hi Lucas,
On Tue, 05 Jun 2012 21:06:20 +0200, Lucas Stach dev@lynxeye.de wrote:
Hi Stephen,
Am Dienstag, den 05.06.2012, 12:42 -0600 schrieb Stephen Warren:
On 06/05/2012 11:47 AM, Lucas Stach wrote:
Recent toolchains default to using the hardware feature for unaligned access on ARM v7, rather than doing the software fallback. According to ARM this is safe as all v7 implementations have to support this feature. (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/BABJFFA...)
To avoid CPU hangs when doing unaligned memory access, we have to turn off alignment checking in our CPU initialisation code. (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491c/CIHCGCF...)
Does this behavior change trickle down to Linux/... too, or would an OS completely re-initialize this state, and hence not be affected?
Linux in particular does reinitialize this state and I expect any reasonable OS to do so.
Then what is the point of enabling it on U-Boot? Does it fix some issue whereby some mis-aligned piece of data cannot be properly aligned?
Yes, it fixes U-Boot USB on Tegra, when built with a recent toolchain. Fixing the alignment of some of the structures in the USB code should also be done, but this is a whole lot more invasive and requires some more thought, as the discussion about this on LKML shows. The issue doesn't show for older toolchains, as they by default emit code to work around unaligned accesses.
This patch fixes all unaligned issues, that may appear with recent toolchains. We avoid having to instruct the toolchain to work around unaligned accesses and gain better performance in cases where it is needed.
Thanks, Lucas
Hi Lucas,
Linux in particular does reinitialize this state and I expect any reasonable OS to do so.
Then what is the point of enabling it on U-Boot? Does it fix some issue whereby some mis-aligned piece of data cannot be properly aligned?
Yes, it fixes U-Boot USB on Tegra, when built with a recent toolchain. Fixing the alignment of some of the structures in the USB code should also be done, but this is a whole lot more invasive and requires some more thought, as the discussion about this on LKML shows. The issue doesn't show for older toolchains, as they by default emit code to work around unaligned accesses.
This patch fixes all unaligned issues, that may appear with recent toolchains. We avoid having to instruct the toolchain to work around unaligned accesses and gain better performance in cases where it is needed.
I am not too happy with enabling a lax behavior only to avoid an issue which apparently is diagnosed and could / should be fixed at its root. Can you point me to the relevant LKML thread so that I get the details and understand what prevents fixing this by 'simply' aligning the USB structures?
Thanks, Lucas
Amicalement,
Hi Albert,
Am Freitag, den 22.06.2012, 13:16 +0200 schrieb Albert ARIBAUD:
Hi Lucas,
Linux in particular does reinitialize this state and I expect any reasonable OS to do so.
Then what is the point of enabling it on U-Boot? Does it fix some issue whereby some mis-aligned piece of data cannot be properly aligned?
Yes, it fixes U-Boot USB on Tegra, when built with a recent toolchain. Fixing the alignment of some of the structures in the USB code should also be done, but this is a whole lot more invasive and requires some more thought, as the discussion about this on LKML shows. The issue doesn't show for older toolchains, as they by default emit code to work around unaligned accesses.
This patch fixes all unaligned issues, that may appear with recent toolchains. We avoid having to instruct the toolchain to work around unaligned accesses and gain better performance in cases where it is needed.
I am not too happy with enabling a lax behavior only to avoid an issue which apparently is diagnosed and could / should be fixed at its root. Can you point me to the relevant LKML thread so that I get the details and understand what prevents fixing this by 'simply' aligning the USB structures?
I'm with you that the issue for this particular fault that I ran into should be fixed at it's root and I will do so as soon as I have enough time to do so, i.e. within the next three weeks. You can find a thread about this here: https://lkml.org/lkml/2011/4/27/278 The problem here is that simply removing the packed attribute is not the right thing to do for structures that are used for accessing hardware registers. I have to read a bit more of the USB code to come up with the right thing to do for every structure there.
But apart from this, we certainly have situations where we have unaligned accesses that are justified and could not be removed. Activating the aligned access hardware check is crippling a feature of the ARMv7 architecture that's apparently useful enough that all recent toolchains default to using it and not using compiler side workarounds. Furthermore setting the check bit doesn't even deactivate unaligned access (there is also a bit for this, which is forced to 1 by all v7 implementations), but just traps the processor in case you care about such unaligned accesses. Linux for example only sets this check bit if you choose to install a trap handler for this.
I cannot see how enabling a hardware feature can be seen as allowing of lax behaviour. As some of the USB structs are used to access hardware registers, we can not align every struct there. Our options are either: 1. instruct the toolchain to insert workaround code or 2. allow v7 hardware to do the unaligned access on it's own My comment about fixing the USB code applies only to part of the structs used there as some of them generate _unnecessary_ unaligned accesses, the issue that all unaligned accesses fail with current U-Boot built with a recent toolchain has to be fixed either way and is exactly what this patch does.
Thanks, Lucas
+Tom
Hi Lucas,
On 06/22/2012 04:47 AM, Lucas Stach wrote:
Hi Albert,
Am Freitag, den 22.06.2012, 13:16 +0200 schrieb Albert ARIBAUD:
Hi Lucas,
Linux in particular does reinitialize this state and I expect any reasonable OS to do so.
Then what is the point of enabling it on U-Boot? Does it fix some issue whereby some mis-aligned piece of data cannot be properly aligned?
Yes, it fixes U-Boot USB on Tegra, when built with a recent toolchain. Fixing the alignment of some of the structures in the USB code should also be done, but this is a whole lot more invasive and requires some more thought, as the discussion about this on LKML shows. The issue doesn't show for older toolchains, as they by default emit code to work around unaligned accesses.
This patch fixes all unaligned issues, that may appear with recent toolchains. We avoid having to instruct the toolchain to work around unaligned accesses and gain better performance in cases where it is needed.
I am not too happy with enabling a lax behavior only to avoid an issue which apparently is diagnosed and could / should be fixed at its root. Can you point me to the relevant LKML thread so that I get the details and understand what prevents fixing this by 'simply' aligning the USB structures?
I'm with you that the issue for this particular fault that I ran into should be fixed at it's root and I will do so as soon as I have enough time to do so, i.e. within the next three weeks. You can find a thread about this here: https://lkml.org/lkml/2011/4/27/278 The problem here is that simply removing the packed attribute is not the right thing to do for structures that are used for accessing hardware registers. I have to read a bit more of the USB code to come up with the right thing to do for every structure there.
But apart from this, we certainly have situations where we have unaligned accesses that are justified and could not be removed. Activating the aligned access hardware check is crippling a feature of the ARMv7 architecture that's apparently useful enough that all recent toolchains default to using it and not using compiler side workarounds. Furthermore setting the check bit doesn't even deactivate unaligned access (there is also a bit for this, which is forced to 1 by all v7 implementations), but just traps the processor in case you care about such unaligned accesses. Linux for example only sets this check bit if you choose to install a trap handler for this.
I cannot see how enabling a hardware feature can be seen as allowing of lax behaviour. As some of the USB structs are used to access hardware registers, we can not align every struct there. Our options are either:
- instruct the toolchain to insert workaround code or
- allow v7 hardware to do the unaligned access on it's own
My comment about fixing the USB code applies only to part of the structs used there as some of them generate _unnecessary_ unaligned accesses, the issue that all unaligned accesses fail with current U-Boot built with a recent toolchain has to be fixed either way and is exactly what this patch does.
I think this issue was discussed before here(I haven't gone through all the details of your problem, but it looks similar). And I think Tom fixed this by wrapping the problematic accesses with get/set_unaligned().
Please look at this thread, especially starting from my post reporting the OMAP4 regression: http://thread.gmane.org/gmane.comp.boot-loaders.u-boot/113347/
best regards, Aneesh
On 06/22/2012 03:11 PM, Aneesh V wrote:
+Tom
Hi Lucas,
On 06/22/2012 04:47 AM, Lucas Stach wrote:
Hi Albert,
Am Freitag, den 22.06.2012, 13:16 +0200 schrieb Albert ARIBAUD:
Hi Lucas,
Linux in particular does reinitialize this state and I expect any reasonable OS to do so.
Then what is the point of enabling it on U-Boot? Does it fix some issue whereby some mis-aligned piece of data cannot be properly aligned?
Yes, it fixes U-Boot USB on Tegra, when built with a recent toolchain. Fixing the alignment of some of the structures in the USB code should also be done, but this is a whole lot more invasive and requires some more thought, as the discussion about this on LKML shows. The issue doesn't show for older toolchains, as they by default emit code to work around unaligned accesses.
This patch fixes all unaligned issues, that may appear with recent toolchains. We avoid having to instruct the toolchain to work around unaligned accesses and gain better performance in cases where it is needed.
I am not too happy with enabling a lax behavior only to avoid an issue which apparently is diagnosed and could / should be fixed at its root. Can you point me to the relevant LKML thread so that I get the details and understand what prevents fixing this by 'simply' aligning the USB structures?
I'm with you that the issue for this particular fault that I ran into should be fixed at it's root and I will do so as soon as I have enough time to do so, i.e. within the next three weeks. You can find a thread about this here: https://lkml.org/lkml/2011/4/27/278 The problem here is that simply removing the packed attribute is not the right thing to do for structures that are used for accessing hardware registers. I have to read a bit more of the USB code to come up with the right thing to do for every structure there.
But apart from this, we certainly have situations where we have unaligned accesses that are justified and could not be removed. Activating the aligned access hardware check is crippling a feature of the ARMv7 architecture that's apparently useful enough that all recent toolchains default to using it and not using compiler side workarounds. Furthermore setting the check bit doesn't even deactivate unaligned access (there is also a bit for this, which is forced to 1 by all v7 implementations), but just traps the processor in case you care about such unaligned accesses. Linux for example only sets this check bit if you choose to install a trap handler for this.
I cannot see how enabling a hardware feature can be seen as allowing of lax behaviour. As some of the USB structs are used to access hardware registers, we can not align every struct there. Our options are either:
- instruct the toolchain to insert workaround code or
- allow v7 hardware to do the unaligned access on it's own
My comment about fixing the USB code applies only to part of the structs used there as some of them generate _unnecessary_ unaligned accesses, the issue that all unaligned accesses fail with current U-Boot built with a recent toolchain has to be fixed either way and is exactly what this patch does.
I think this issue was discussed before here(I haven't gone through all the details of your problem, but it looks similar). And I think Tom fixed this by wrapping the problematic accesses with get/set_unaligned().
Please look at this thread, especially starting from my post reporting the OMAP4 regression: http://thread.gmane.org/gmane.comp.boot-loaders.u-boot/113347/
BTW, I agree that enabling un-aligned access is not a bad idea.
br, Aneeesh
Hi Aneesh,
On Fri, 22 Jun 2012 15:13:39 -0700, Aneesh V aneesh@ti.com wrote:
On 06/22/2012 03:11 PM, Aneesh V wrote:
+Tom
Hi Lucas,
On 06/22/2012 04:47 AM, Lucas Stach wrote:
Hi Albert,
Am Freitag, den 22.06.2012, 13:16 +0200 schrieb Albert ARIBAUD:
I am not too happy with enabling a lax behavior only to avoid an issue which apparently is diagnosed and could / should be fixed at its root. Can you point me to the relevant LKML thread so that I get the details and understand what prevents fixing this by 'simply' aligning the USB structures?
I'm with you that the issue for this particular fault that I ran into should be fixed at it's root and I will do so as soon as I have enough time to do so, i.e. within the next three weeks. You can find a thread about this here: https://lkml.org/lkml/2011/4/27/278
From what I understand, the issue was not to allow unaligned access at the hardware level, but to modify the attributes of the structure, first by removing the packed attribute, then by reinstating the packed attribute along with align(4).
But apart from this, we certainly have situations where we have unaligned accesses that are justified and could not be removed. [...] I cannot see how enabling a hardware feature can be seen as allowing of lax behaviour. As some of the USB structs are used to access hardware registers, we can not align every struct there.
If the access is in true RAM, then we can always realign the data; and I don't know of memory-mapped registers which would be unaligned wrt their width. If some USB controller is designed so, then the fix should only and explicitly affect that controller, because we don't know it it will always be used with an ARM implementation that can do unaligned accesses.
I think this issue was discussed before here(I haven't gone through all the details of your problem, but it looks similar). And I think Tom fixed this by wrapping the problematic accesses with get/set_unaligned().
Could be a solution if the structures themselves cannot be fixed.
Please look at this thread, especially starting from my post reporting the OMAP4 regression: http://thread.gmane.org/gmane.comp.boot-loaders.u-boot/113347/
Thanks for the reference. There seems to have been no confirmation that the structures involved needed packing in the first place, and my general opinion on packing structures is "DO NOT" -- if packing a structure has an effect, it can alway be to de-align some field, which barely makes sense as far as HW prgramming is concerned (I can only see some point in packing a struct when you deal with network layer 7 data in very special cases).
BTW, I agree that enabling un-aligned access is not a bad idea.
Just being "not a bad idea" is not enough for me to accept this. It will have to be the sole sound solution to a problem, and at this point, I do not think it is as far as USB structure mis-alignement issues are concerned.
br, Aneeesh
Amicalement,
Hi Albert,
On Sat, Jun 23, 2012 at 2:01 AM, Albert ARIBAUD albert.u.boot@aribaud.net wrote:
Hi Aneesh,
On Fri, 22 Jun 2012 15:13:39 -0700, Aneesh V aneesh@ti.com wrote:
On 06/22/2012 03:11 PM, Aneesh V wrote:
+Tom
Hi Lucas,
On 06/22/2012 04:47 AM, Lucas Stach wrote:
Hi Albert,
Am Freitag, den 22.06.2012, 13:16 +0200 schrieb Albert ARIBAUD:
I am not too happy with enabling a lax behavior only to avoid an issue which apparently is diagnosed and could / should be fixed at its root. Can you point me to the relevant LKML thread so that I get the details and understand what prevents fixing this by 'simply' aligning the USB structures?
I'm with you that the issue for this particular fault that I ran into should be fixed at it's root and I will do so as soon as I have enough time to do so, i.e. within the next three weeks. You can find a thread about this here: https://lkml.org/lkml/2011/4/27/278
From what I understand, the issue was not to allow unaligned access at the hardware level, but to modify the attributes of the structure, first by removing the packed attribute, then by reinstating the packed attribute along with align(4).
But apart from this, we certainly have situations where we have unaligned accesses that are justified and could not be removed. [...] I cannot see how enabling a hardware feature can be seen as allowing of lax behaviour. As some of the USB structs are used to access hardware registers, we can not align every struct there.
If the access is in true RAM, then we can always realign the data; and I don't know of memory-mapped registers which would be unaligned wrt their width. If some USB controller is designed so, then the fix should only and explicitly affect that controller, because we don't know it it will always be used with an ARM implementation that can do unaligned accesses.
I think this issue was discussed before here(I haven't gone through all the details of your problem, but it looks similar). And I think Tom fixed this by wrapping the problematic accesses with get/set_unaligned().
Could be a solution if the structures themselves cannot be fixed.
Please look at this thread, especially starting from my post reporting the OMAP4 regression: http://thread.gmane.org/gmane.comp.boot-loaders.u-boot/113347/
Thanks for the reference. There seems to have been no confirmation that the structures involved needed packing in the first place, and my general opinion on packing structures is "DO NOT" -- if packing a structure has an effect, it can alway be to de-align some field, which barely makes sense as far as HW prgramming is concerned (I can only see some point in packing a struct when you deal with network layer 7 data in very special cases).
BTW, I agree that enabling un-aligned access is not a bad idea.
Just being "not a bad idea" is not enough for me to accept this. It will have to be the sole sound solution to a problem, and at this point, I do not think it is as far as USB structure mis-alignement issues are concerned.
My point is that enabling un-aligned accesses in itsown merit is not a bad idea, not as a solution to this problem. I have seen it being enabled in HLOS environment. TI's Symbian port for instance had it enabled for OMAP3. I don't know why Linux too shoudln't take advantage of such hw features. Perhaps you don't want to take it at this point of time to force the real solution to the USB problem, which is reasonable.
Hi Aneesh,
BTW, I agree that enabling un-aligned access is not a bad idea.
Just being "not a bad idea" is not enough for me to accept this. It will have to be the sole sound solution to a problem, and at this point, I do not think it is as far as USB structure mis-alignement issues are concerned.
My point is that enabling un-aligned accesses in itsown merit is not a bad idea, not as a solution to this problem. I have seen it being enabled in HLOS environment. TI's Symbian port for instance had it enabled for OMAP3. I don't know why Linux too shoudln't take advantage of such hw features. Perhaps you don't want to take it at this point of time to force the real solution to the USB problem, which is reasonable.
What is the (non-contrived) problem to which allowing mis-aligned accesses would be a solution?
Amicalement,
Hi Albert,
On 06/25/2012 01:34 PM, Albert ARIBAUD wrote:
Hi Aneesh,
BTW, I agree that enabling un-aligned access is not a bad idea.
Just being "not a bad idea" is not enough for me to accept this. It will have to be the sole sound solution to a problem, and at this point, I do not think it is as far as USB structure mis-alignement issues are concerned.
My point is that enabling un-aligned accesses in itsown merit is not a bad idea, not as a solution to this problem. I have seen it being enabled in HLOS environment. TI's Symbian port for instance had it enabled for OMAP3. I don't know why Linux too shoudln't take advantage of such hw features. Perhaps you don't want to take it at this point of time to force the real solution to the USB problem, which is reasonable.
What is the (non-contrived) problem to which allowing mis-aligned accesses would be a solution?
memcpy() when there is a mismatch in the alignment of source and destination buffers. Let's say the source buffer is 4 byte aligned but the destination buffer is only 2 byte aligned. I believe relaxed alignment requirements will help in writing an accelerated memcpy routine for this case.
Again, my point is that as a platform software provider I would like to enable such features to make maximum things work on my platform, where as the developer of a generic sw module should probably avoid depending on such features to ensure maximum portability.
br, Aneesh
Dear Aneesh V,
In message 4FE8DCE7.7090700@ti.com you wrote:
What is the (non-contrived) problem to which allowing mis-aligned accesses would be a solution?
memcpy() when there is a mismatch in the alignment of source and destination buffers. Let's say the source buffer is 4 byte aligned but the destination buffer is only 2 byte aligned. I believe relaxed alignment requirements will help in writing an accelerated memcpy routine for this case.
If memcpy() has problems with handling such a situation, then clearly memcpy() needs fixing.
Best regards,
Wolfgang Denk
Albert ARIBAUD albert.u.boot@aribaud.net writes:
I cannot see how enabling a hardware feature can be seen as allowing of lax behaviour. As some of the USB structs are used to access hardware registers, we can not align every struct there.
If the access is in true RAM, then we can always realign the data; and I don't know of memory-mapped registers which would be unaligned wrt their width. If some USB controller is designed so, then the fix should only and explicitly affect that controller, because we don't know it it will always be used with an ARM implementation that can do unaligned accesses.
The ARM architecture does not permit unaligned accesses to strongly ordered or device memory, so MMIO register accesses are always aligned.
Hi Albert,
Am Samstag, den 23.06.2012, 11:01 +0200 schrieb Albert ARIBAUD: [snip]
But apart from this, we certainly have situations where we have unaligned accesses that are justified and could not be removed. [...] I cannot see how enabling a hardware feature can be seen as allowing of lax behaviour. As some of the USB structs are used to access hardware registers, we can not align every struct there.
If the access is in true RAM, then we can always realign the data; and I don't know of memory-mapped registers which would be unaligned wrt their width. If some USB controller is designed so, then the fix should only and explicitly affect that controller, because we don't know it it will always be used with an ARM implementation that can do unaligned accesses.
My point is: on ARM platforms that can't do unaligned access in hardware the toolchain will automaticly emit helper code to work around this. On an ARMv7 platform hardware assisted unaligned access is mandatory, so toolchains will not emit helper code and rather let the hardware do it's job. Now the situation is as follows: hardware platforms without the ability to do unaligned access in hardware will just work even though the code is suboptimal, but users of an ARMv7 platform will encounter unexplained system hangs, which is bad imho. This patch just makes behaviour consistent across ARMv5 and ARMv7 platforms.
If we really want to disallow the use of unaligned memory operations in U-Boot we should make all platforms fail at compile time, not make one platform fail randomly at runtime. Do you think this is the way to go? I'm fine either way, I'm just not okay with the current situation where one platform fails while another just works.
Thanks, Lucas
On 06/22/2012 04:15 AM, Albert ARIBAUD wrote:
Hi Lucas,
On Tue, 05 Jun 2012 21:06:20 +0200, Lucas Stach dev@lynxeye.de wrote:
Hi Stephen,
Am Dienstag, den 05.06.2012, 12:42 -0600 schrieb Stephen Warren:
On 06/05/2012 11:47 AM, Lucas Stach wrote:
Recent toolchains default to using the hardware feature for unaligned access on ARM v7, rather than doing the software fallback. According to ARM this is safe as all v7 implementations have to support this feature. (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/BABJFFA...)
To avoid CPU hangs when doing unaligned memory access, we have to turn off alignment checking in our CPU initialisation code. (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491c/CIHCGCF...)
Does this behavior change trickle down to Linux/... too, or would an OS completely re-initialize this state, and hence not be affected?
Linux in particular does reinitialize this state and I expect any reasonable OS to do so.
Then what is the point of enabling it on U-Boot? Does it fix some issue whereby some mis-aligned piece of data cannot be properly aligned?
This is a new optimization feature in gcc 4.7 (and backported to some 4.6 versions like the ubuntu 12.04 arm cross compiler (4.6.3)):
http://lists.linaro.org/pipermail/linaro-dev/2012-June/012360.html
http://seabright.co.nz/2012/06/11/kernel-not-booting-with-linaro-gcc/
If you don't want to enable unaligned accesses, then "-mno-unaligned-access" needs to be added.
Regards, Rob
Hi Rob and all,
Am Dienstag, den 05.06.2012, 12:42 -0600 schrieb Stephen Warren:
On 06/05/2012 11:47 AM, Lucas Stach wrote:
Recent toolchains default to using the hardware feature for unaligned access on ARM v7, rather than doing the software fallback. According to ARM this is safe as all v7 implementations have to support this feature. (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/BABJFFA...)
To avoid CPU hangs when doing unaligned memory access, we have to turn off alignment checking in our CPU initialisation code. (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491c/CIHCGCF...)
Does this behavior change trickle down to Linux/... too, or would an OS completely re-initialize this state, and hence not be affected?
Linux in particular does reinitialize this state and I expect any reasonable OS to do so.
Then what is the point of enabling it on U-Boot? Does it fix some issue whereby some mis-aligned piece of data cannot be properly aligned?
This is a new optimization feature in gcc 4.7 (and backported to some 4.6 versions like the ubuntu 12.04 arm cross compiler (4.6.3)):
http://lists.linaro.org/pipermail/linaro-dev/2012-June/012360.html
http://seabright.co.nz/2012/06/11/kernel-not-booting-with-linaro-gcc/
If you don't want to enable unaligned accesses, then "-mno-unaligned-access" needs to be added.
I verified it. Option "-mno-unaligned-access" works good.
include/mtd/cfi_flash.h
/* CFI standard query structure */ struct cfi_qry { u8 qry[3]; u16 p_id; <-- unaligned! ... } __attribute__((packed));
$ ${CROSS_COMPILE}gcc --version arm-none-eabi-gcc (GNU Tools for ARM Embedded Processors) 4.6.2 20110921 (release) [ARM/embedded-4_6-branch revision 182083] Copyright (C) 2011 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
===================================================================== Compiled without --mno-unaligned-access $ ${CROSS_COMPILE}objdump -d -S u-boot
info->vendor = le16_to_cpu(qry.p_id); cc88: e3003a1c movw r3, #2588 ; 0xa1c cc8c: e1dd11bb ldrh r1, [sp, #27] <-- this is unaligned access cc90: ... cc94: e18410b3 strh r1, [r4, r3]
===================================================================== Compiled with --mno-unaligned-access $ ${CROSS_COMPILE}objdump -d -S u-boot
info->vendor = le16_to_cpu(qry.p_id); cce8: e5dd101c ldrb r1, [sp, #28] <-- ccec: e5dd301b ldrb r3, [sp, #27] <-- separated 2 byte accesses ccf0: ... ccf4: e1831401 orr r1, r3, r1, lsl #8 ccf8: e3003a1c movw r3, #2588 ; 0xa1c ccfc: e18410b3 strh r1, [r4, r3]
Recent compiler generates unaligned memory access in armv7 default. But current U-Boot does not allow unaligned memory access, so it causes data abort exception. This patch add compile option "-mno-unaligned-access" if it is available.
Signed-off-by: Tetsuyuki Kobayashi koba@kmckk.co.jp --- arch/arm/cpu/armv7/config.mk | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/arch/arm/cpu/armv7/config.mk b/arch/arm/cpu/armv7/config.mk index 5407cb6..560c084 100644 --- a/arch/arm/cpu/armv7/config.mk +++ b/arch/arm/cpu/armv7/config.mk @@ -26,6 +26,8 @@ PLATFORM_RELFLAGS += -fno-common -ffixed-r8 -msoft-float # supported by more tool-chains PF_CPPFLAGS_ARMV7 := $(call cc-option, -march=armv7-a, -march=armv5) PLATFORM_CPPFLAGS += $(PF_CPPFLAGS_ARMV7) +PF_CPPFLAGS_NO_UNALIGNED := $(call cc-option, -mno-unaligned-access,) +PLATFORM_CPPFLAGS += $(PF_CPPFLAGS_NO_UNALIGNED)
# ========================================================================= # -- 1.7.9.5
Tetsuyuki Kobayashi koba@kmckk.co.jp writes:
Recent compiler generates unaligned memory access in armv7 default. But current U-Boot does not allow unaligned memory access, so it causes data abort exception. This patch add compile option "-mno-unaligned-access" if it is available.
Why not allow unaligned accesses instead?
Am Montag, den 02.07.2012, 10:53 +0100 schrieb Måns Rullgård:
Tetsuyuki Kobayashi koba@kmckk.co.jp writes:
Recent compiler generates unaligned memory access in armv7 default. But current U-Boot does not allow unaligned memory access, so it causes data abort exception. This patch add compile option "-mno-unaligned-access" if it is available.
Why not allow unaligned accesses instead?
IMHO, our recent discussion showed that both ways are wrong. "-mno-unaligned-access" works around misaligned data on the software level, while allowing unaligned access does on the hardware level.
What we really want is no unaligned access in U-Boot at all. Just because "-mno-unaligned-access" is the default on ARMv5, we should not consider it a gold standard.
Thanks, Lucas
Lucas Stach dev@lynxeye.de writes:
Am Montag, den 02.07.2012, 10:53 +0100 schrieb Måns Rullgård:
Tetsuyuki Kobayashi koba@kmckk.co.jp writes:
Recent compiler generates unaligned memory access in armv7 default. But current U-Boot does not allow unaligned memory access, so it causes data abort exception. This patch add compile option "-mno-unaligned-access" if it is available.
Why not allow unaligned accesses instead?
IMHO, our recent discussion showed that both ways are wrong. "-mno-unaligned-access" works around misaligned data on the software level, while allowing unaligned access does on the hardware level.
What we really want is no unaligned access in U-Boot at all. Just because "-mno-unaligned-access" is the default on ARMv5, we should not consider it a gold standard.
It's slightly more complicated than that. Data can be misaligned for a variety of reasons:
1. Errors in software. 2. Specified by a file format or communication protocol. 3. Deliberately misaligned by the compiler.
Misaligned data of type 1 should of course be fixed properly, not worked around in any way.
Type 2 happens all the time, and has to be dealt with one way or another. If the hardware supports unaligned accesses, this is usually faster than reading a byte at a time.
When targeting ARMv6 and later, recent gcc versions have started issuing deliberate unaligned accesses where previously byte by byte accesses would have been done. This happens with "packed" structs and sometimes to write multiple smaller values at once, typically when zero-initialising things. These unaligned accesses are *good*. They make code smaller and faster.
The real problem here is that u-boot is setting the strict alignment checking flag, invalidating the assumption of the compiler that the system allows unaligned accesses. For ARMv5 and earlier, setting this flag is usually advisable since it makes finding accidental unaligned accesses much easier.
This was debated in the context of the kernel a while ago, ultimately leading to strict alignment being disabled for ARMv6 and up [1].
[1] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=...
Hello, Måns Thank you for summarizing.
I am not against you. I'm OK either "Allow unaligned access in U-Boot setting" or "Specify compiler not to generate unaligned memory access" or other. I just want to solve hung-up by unaligned access. I follow custodian's decision.
(2012/07/03 1:14), Måns Rullgård wrote:
Lucas Stachdev@lynxeye.de writes:
Am Montag, den 02.07.2012, 10:53 +0100 schrieb Måns Rullgård:
Tetsuyuki Kobayashikoba@kmckk.co.jp writes:
Recent compiler generates unaligned memory access in armv7 default. But current U-Boot does not allow unaligned memory access, so it causes data abort exception. This patch add compile option "-mno-unaligned-access" if it is available.
Why not allow unaligned accesses instead?
IMHO, our recent discussion showed that both ways are wrong. "-mno-unaligned-access" works around misaligned data on the software level, while allowing unaligned access does on the hardware level.
What we really want is no unaligned access in U-Boot at all. Just because "-mno-unaligned-access" is the default on ARMv5, we should not consider it a gold standard.
It's slightly more complicated than that. Data can be misaligned for a variety of reasons:
- Errors in software.
- Specified by a file format or communication protocol.
- Deliberately misaligned by the compiler.
Misaligned data of type 1 should of course be fixed properly, not worked around in any way.
Type 2 happens all the time, and has to be dealt with one way or another. If the hardware supports unaligned accesses, this is usually faster than reading a byte at a time.
When targeting ARMv6 and later, recent gcc versions have started issuing deliberate unaligned accesses where previously byte by byte accesses would have been done. This happens with "packed" structs and sometimes to write multiple smaller values at once, typically when zero-initialising things. These unaligned accesses are *good*. They make code smaller and faster.
The real problem here is that u-boot is setting the strict alignment checking flag, invalidating the assumption of the compiler that the system allows unaligned accesses. For ARMv5 and earlier, setting this flag is usually advisable since it makes finding accidental unaligned accesses much easier.
This was debated in the context of the kernel a while ago, ultimately leading to strict alignment being disabled for ARMv6 and up [1].
[1] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=...
Hi Måns,
On Mon, 02 Jul 2012 17:14:40 +0100, Måns Rullgård mans@mansr.com wrote:
IMHO, our recent discussion showed that both ways are wrong. "-mno-unaligned-access" works around misaligned data on the software level, while allowing unaligned access does on the hardware level.
What we really want is no unaligned access in U-Boot at all. Just because "-mno-unaligned-access" is the default on ARMv5, we should not consider it a gold standard.
It's slightly more complicated than that. Data can be misaligned for a variety of reasons:
- Errors in software.
- Specified by a file format or communication protocol.
- Deliberately misaligned by the compiler.
Misaligned data of type 1 should of course be fixed properly, not worked around in any way.
Agreed.
Type 2 happens all the time, and has to be dealt with one way or another. If the hardware supports unaligned accesses, this is usually faster than reading a byte at a time.
Sorry but I don't accept a systemic change as a good solution to a specific issue. The best fix is "don't accept using a protocol or file format which was not designed to avoid native misaligned accesses", and the second best fix is "if you really must deal with a protocol or file format which causes native alignment issues, then the fix should only affect the protocol or file format's issue, not with your system which is not the root cause".
But to be honest, I haven't seen such badly a designed protocol or file format; their designers tend to make sure that (assuming the start of a protocol or file 'content' (frame, record, etc) is aligned, then all fields in it are aligned as well. Can someone point me to an example of a protocol or file format which does present such a misalignment risk ?
When targeting ARMv6 and later, recent gcc versions have started issuing deliberate unaligned accesses where previously byte by byte accesses would have been done.
Wrongly formulated: "mis-aligning deliberately" seems to imply that GCC did away with the possibility of properly aligning, which they of course did not. What they did is switch its default behavior regarding native misaligned accesses from forbidding to allowing them. The change here is of policy, a change which we may or may not want to follow.
This happens with "packed" structs and sometimes to write multiple smaller values at once, typically when zero-initialising things. These unaligned accesses are *good*. They make code smaller and faster.
Again, "good" is a policy, or subjective, statement, not an absolute. Just as "good" is correctly aligning data in the first place (to begin with, in protocols and file formats) and keeping the compiler's native misaligned access policy set to "do not allow".
The real problem here is that u-boot is setting the strict alignment checking flag, invalidating the assumption of the compiler that the system allows unaligned accesses. For ARMv5 and earlier, setting this flag is usually advisable since it makes finding accidental unaligned accesses much easier.
Just as it is for ARMv6 and up. Again, just because the compiler folks changed their default policy does not mean we should change ours, which is not based on the same goals.
This was debated in the context of the kernel a while ago, ultimately leading to strict alignment being disabled for ARMv6 and up [1].
[1] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=...
I'd rather have a link to the rationale than to the commit, but anyway, the kernel folks' decision is theirs and does not necessarily apply to us. I have mailed Catalin Marinas off-list to get details on the rationale and context of the kernel patch; I will report conclusions here.
Meanwhile, our policy regarding misalignment accesses is to only allow them when externally required (by something other than a bad design). Someone please show me such an external requirement for U-Boot, and I will reconsider -- and then, other arch custodians may have a problem with that too.
Regarding the origin of this patch, i.e. a mis-alignment of USB fields, and unless U-Boot USB folks say otherwise, this issue should be fixed by aligning said fields properly.
Amicalement,
on Thu, 5 Jul 2012 09:57:19 +0200, Albert ARIBAUD albert.u.boot@aribaud.net wrote :
Hi Måns,
On Mon, 02 Jul 2012 17:14:40 +0100, Måns Rullgård mans@mansr.com wrote:
IMHO, our recent discussion showed that both ways are wrong. "-mno-unaligned-access" works around misaligned data on the software level, while allowing unaligned access does on the hardware level.
What we really want is no unaligned access in U-Boot at all. Just because "-mno-unaligned-access" is the default on ARMv5, we should not consider it a gold standard.
It's slightly more complicated than that. Data can be misaligned for a variety of reasons:
- Errors in software.
- Specified by a file format or communication protocol.
- Deliberately misaligned by the compiler.
Misaligned data of type 1 should of course be fixed properly, not worked around in any way.
Agreed.
Type 2 happens all the time, and has to be dealt with one way or another. If the hardware supports unaligned accesses, this is usually faster than reading a byte at a time.
Sorry but I don't accept a systemic change as a good solution to a specific issue. The best fix is "don't accept using a protocol or file format which was not designed to avoid native misaligned accesses", and the second best fix is "if you really must deal with a protocol or file format which causes native alignment issues, then the fix should only affect the protocol or file format's issue, not with your system which is not the root cause".
But to be honest, I haven't seen such badly a designed protocol or file format; their designers tend to make sure that (assuming the start of a protocol or file 'content' (frame, record, etc) is aligned, then all fields in it are aligned as well. Can someone point me to an example of a protocol or file format which does present such a misalignment risk ?
When targeting ARMv6 and later, recent gcc versions have started issuing deliberate unaligned accesses where previously byte by byte accesses would have been done.
Wrongly formulated: "mis-aligning deliberately" seems to imply that GCC did away with the possibility of properly aligning, which they of course did not. What they did is switch its default behavior regarding native misaligned accesses from forbidding to allowing them. The change here is of policy, a change which we may or may not want to follow.
This happens with "packed" structs and sometimes to write multiple smaller values at once, typically when zero-initialising things. These unaligned accesses are *good*. They make code smaller and faster.
Again, "good" is a policy, or subjective, statement, not an absolute. Just as "good" is correctly aligning data in the first place (to begin with, in protocols and file formats) and keeping the compiler's native misaligned access policy set to "do not allow".
The real problem here is that u-boot is setting the strict alignment checking flag, invalidating the assumption of the compiler that the system allows unaligned accesses. For ARMv5 and earlier, setting this flag is usually advisable since it makes finding accidental unaligned accesses much easier.
Just as it is for ARMv6 and up. Again, just because the compiler folks changed their default policy does not mean we should change ours, which is not based on the same goals.
This was debated in the context of the kernel a while ago, ultimately leading to strict alignment being disabled for ARMv6 and up [1].
[1] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=...
I'd rather have a link to the rationale than to the commit, but anyway, the kernel folks' decision is theirs and does not necessarily apply to us. I have mailed Catalin Marinas off-list to get details on the rationale and context of the kernel patch; I will report conclusions here.
Meanwhile, our policy regarding misalignment accesses is to only allow them when externally required (by something other than a bad design). Someone please show me such an external requirement for U-Boot, and I will reconsider -- and then, other arch custodians may have a problem with that too.
Regarding the origin of this patch, i.e. a mis-alignment of USB fields, and unless U-Boot USB folks say otherwise, this issue should be fixed by aligning said fields properly.
We are nearing the release, and we obviously won't have misalignments fixed and tested in time for it.
So I suspect that if I want the ARM U-Boot release to work I'll have to allow this patch in my master branch to be pulled in for 12.07, so that the compiler keeps behaving as it did before gcc changed the default.
... but only for the upcoming release, i.e. I will revert the patch in my 'next' branch, which will apply right after 12.07 is out. Therefore, before next release, misalignments will have to be fixed at the root.
Amicalement,
On Thursday 05 July 2012 03:57:19 Albert ARIBAUD wrote:
But to be honest, I haven't seen such badly a designed protocol or file format; their designers tend to make sure that (assuming the start of a protocol or file 'content' (frame, record, etc) is aligned, then all fields in it are aligned as well. Can someone point me to an example of a protocol or file format which does present such a misalignment risk ?
simply search the kernel for get_unaligned then. there are plenty of examples in there. granted, many apply to stacks that don't show up in u-boot (yet?) such as bluetooth, wireless, and irda, but i'm pretty sure TCP/IPv4 has a few edge cases too. -mike
On Monday 02 July 2012 12:14:40 Måns Rullgård wrote:
It's slightly more complicated than that. Data can be misaligned for a variety of reasons:
- Errors in software.
- Specified by a file format or communication protocol.
- Deliberately misaligned by the compiler.
Misaligned data of type 1 should of course be fixed properly, not worked around in any way.
it's also a reliability aspect. people don't write bug free software, not bug free protocols, nor bug free compilers. when misalignment does happen in the field, it's a hell of a lot better if the software continued to execute correctly rather than randomly triggered an exception. -mike
Hi Mike,
On Thu, 19 Jul 2012 00:29:23 -0400, Mike Frysinger vapier@gentoo.org wrote:
On Monday 02 July 2012 12:14:40 Måns Rullgård wrote:
It's slightly more complicated than that. Data can be misaligned for a variety of reasons:
- Errors in software.
- Specified by a file format or communication protocol.
- Deliberately misaligned by the compiler.
Misaligned data of type 1 should of course be fixed properly, not worked around in any way.
it's also a reliability aspect. people don't write bug free software, not bug free protocols, nor bug free compilers. when misalignment does happen in the field, it's a hell of a lot better if the software continued to execute correctly rather than randomly triggered an exception. -mike
Nitpick: this is robustness, not reliability.
That being said, yes, this robustness is desirable when you do not control all of the SW running on the product; Linux, for instance, will have to execute processes which were built with any old (or new) compiler settings, thus the Linux folks have to make sure the kernel won't fail running those.
But the only uncontrolled SW U-Boot runs is its payload -- typically the kernel image -- which are usually very cautious in what they assume they can do, thus are unlikely to perform unaligned accesses.
(pasting your other comment)
simply search the kernel for get_unaligned then. there are plenty of examples in there. granted, many apply to stacks that don't show up in u-boot (yet?) such as bluetooth, wireless, and irda, but i'm pretty sure TCP/IPv4 has a few edge cases too.
I'll have a look, if only to lament that protocol are not what they used to be in the old days. :)
Anyway: as I said: performing *controlled* unaligned accesses for external reasons other than bugs is fine with me. Having our own get_unaligned() in such places would be fine with me.
Amicalement,
On Thursday 19 July 2012 02:28:05 Albert ARIBAUD wrote:
On Thu, 19 Jul 2012 00:29:23 -0400, Mike Frysinger wrote:
On Monday 02 July 2012 12:14:40 Måns Rullgård wrote:
It's slightly more complicated than that. Data can be misaligned for a variety of reasons:
- Errors in software.
- Specified by a file format or communication protocol.
- Deliberately misaligned by the compiler.
Misaligned data of type 1 should of course be fixed properly, not worked around in any way.
it's also a reliability aspect. people don't write bug free software, not bug free protocols, nor bug free compilers. when misalignment does happen in the field, it's a hell of a lot better if the software continued to execute correctly rather than randomly triggered an exception.
Nitpick: this is robustness, not reliability.
useless pedantry: by increasing robustness, the system is more reliable
That being said, yes, this robustness is desirable when you do not control all of the SW running on the product; Linux, for instance, will have to execute processes which were built with any old (or new) compiler settings, thus the Linux folks have to make sure the kernel won't fail running those.
But the only uncontrolled SW U-Boot runs is its payload -- typically the kernel image -- which are usually very cautious in what they assume they can do, thus are unlikely to perform unaligned accesses.
it isn't just that. there is no way you can guarantee both the linux kernel and u-boot code bases themselves are perfect. in fact, it's even worse when these are the ones that get tripped up because it means your system resets/hardlocks/kills a kitten. when doing driver development under the linux kernel, we would come across parts of core stacks that lacked alignment checking and would panic the system. sometimes it would always panic, other times it depended on factors that made life worse: the compiler version (newer ones always like to pack/optimize better), the actual data stream, or the execution paths.
simply search the kernel for get_unaligned then. there are plenty of examples in there. granted, many apply to stacks that don't show up in u-boot (yet?) such as bluetooth, wireless, and irda, but i'm pretty sure TCP/IPv4 has a few edge cases too.
I'll have a look, if only to lament that protocol are not what they used to be in the old days. :)
Anyway: as I said: performing *controlled* unaligned accesses for external reasons other than bugs is fine with me. Having our own get_unaligned() in such places would be fine with me.
i have no problem adding put/get_unaligned() to all the right places. that makes perfect sense. but, as an orthogonal issue wrt ARMv7, i don't see any problem enabling hardware functionality: it increases robustness (:P), shrinks the code base (all the get/put unaligned macros expand into a single memory access as they no longer have to do alignment fixups in software), and speeds up the runtime (a single unaligned memory access is always faster than address masking/multiple loads/bit shifting/etc... -- obviously this ignores multimedia type code that does alignment adjustment at the start, then lets of memory accesses, then another adjustment at the end, but that's not what we're talking about here).
if you want to tell people that if they found an unaligned access in code they must fix that, then great. make them fix it. then once that bug has been fixed, let's merge the purely optimization patch that allows the hardware to do unaligned accesses. -mike
Hi Mike,
On Thu, 19 Jul 2012 10:27:07 -0400, Mike Frysinger vapier@gentoo.org wrote:
On Thursday 19 July 2012 02:28:05 Albert ARIBAUD wrote:
On Thu, 19 Jul 2012 00:29:23 -0400, Mike Frysinger wrote:
On Monday 02 July 2012 12:14:40 Måns Rullgård wrote:
It's slightly more complicated than that. Data can be misaligned for a variety of reasons:
- Errors in software.
- Specified by a file format or communication protocol.
- Deliberately misaligned by the compiler.
Misaligned data of type 1 should of course be fixed properly, not worked around in any way.
it's also a reliability aspect. people don't write bug free software, not bug free protocols, nor bug free compilers. when misalignment does happen in the field, it's a hell of a lot better if the software continued to execute correctly rather than randomly triggered an exception.
Nitpick: this is robustness, not reliability.
useless pedantry: by increasing robustness, the system is more reliable
Your description confirms that robustness and reliability are not equivalent, thereby goes against your statement about pedantry... :)
That being said, yes, this robustness is desirable when you do not control all of the SW running on the product; Linux, for instance, will have to execute processes which were built with any old (or new) compiler settings, thus the Linux folks have to make sure the kernel won't fail running those.
But the only uncontrolled SW U-Boot runs is its payload -- typically the kernel image -- which are usually very cautious in what they assume they can do, thus are unlikely to perform unaligned accesses.
it isn't just that. there is no way you can guarantee both the linux kernel and u-boot code bases themselves are perfect. in fact, it's even worse when these are the ones that get tripped up because it means your system resets/hardlocks/kills a kitten. when doing driver development under the linux kernel, we would come across parts of core stacks that lacked alignment checking and would panic the system. sometimes it would always panic, other times it depended on factors that made life worse: the compiler version (newer ones always like to pack/optimize better), the actual data stream, or the execution paths.
Correct; here I was considering the requirements / operating conditions for both projects, I was not considering development issues -- bugs during development happen (morethan they do in the field, hopefully).
Do you mean you'd like to catch misalignments as early as possible, and would like to have both -munaligned-access and A=1 during dev?
simply search the kernel for get_unaligned then. there are plenty of examples in there. granted, many apply to stacks that don't show up in u-boot (yet?) such as bluetooth, wireless, and irda, but i'm pretty sure TCP/IPv4 has a few edge cases too.
I'll have a look, if only to lament that protocol are not what they used to be in the old days. :)
Anyway: as I said: performing *controlled* unaligned accesses for external reasons other than bugs is fine with me. Having our own get_unaligned() in such places would be fine with me.
i have no problem adding put/get_unaligned() to all the right places. that makes perfect sense. but, as an orthogonal issue wrt ARMv7, i don't see any problem enabling hardware functionality: it increases robustness (:P), shrinks the code base (all the get/put unaligned macros expand into a single memory access as they no longer have to do alignment fixups in software), and speeds up the runtime (a single unaligned memory access is always faster than address masking/multiple loads/bit shifting/etc... -- obviously this ignores multimedia type code that does alignment adjustment at the start, then lets of memory accesses, then another adjustment at the end, but that's not what we're talking about here).
I wouldn't care about the ARMv7 implementing explicit unaligned accesses with native instructions if it didn't mean it won't catch unwanted unaligned accesses any more, and thus such unalignments will be found in another context and will take some more time to trace back.
if you want to tell people that if they found an unaligned access in code they must fix that, then great. make them fix it. then once that bug has been fixed, let's merge the purely optimization patch that allows the hardware to do unaligned accesses.
My problem is that as long as people start configuring their HW and compiler to not care about unaligned accesses, they *won't* find such accesses when accidental, because nothing will tell them.
Here's my suggestion: when building U-Boot as usual, strict aligment policy is enforced, i.e. -mno-unaligned-access and A=1, and for platforms that could benefit from native unaligned accesses, a run-time warning is emitted on the console. However, with a specific command 'PRODUCTION' line option added, the constraint is relaxed, i.e. -munaligned-access and A=0, no run-time message, but a build warning is emitted stating that this build is afoul of the U-Boot strict policy.
This way, you get the robustness you want as you can easily build an ARMv7- efficient binary, and I get the one I want as the build used for development is slightly less MCPS-efficient but will catch potential issues.
Comments welcome.
-mike
Amicalement,
On 2012-07-02 03:42, Tetsuyuki Kobayashi wrote:
Recent compiler generates unaligned memory access in armv7 default. But current U-Boot does not allow unaligned memory access, so it causes data abort exception. This patch add compile option "-mno-unaligned-access" if it is available.
Signed-off-by: Tetsuyuki Kobayashi koba@kmckk.co.jp
arch/arm/cpu/armv7/config.mk | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/arch/arm/cpu/armv7/config.mk b/arch/arm/cpu/armv7/config.mk index 5407cb6..560c084 100644 --- a/arch/arm/cpu/armv7/config.mk +++ b/arch/arm/cpu/armv7/config.mk @@ -26,6 +26,8 @@ PLATFORM_RELFLAGS += -fno-common -ffixed-r8 -msoft-float # supported by more tool-chains PF_CPPFLAGS_ARMV7 := $(call cc-option, -march=armv7-a, -march=armv5) PLATFORM_CPPFLAGS += $(PF_CPPFLAGS_ARMV7) +PF_CPPFLAGS_NO_UNALIGNED := $(call cc-option, -mno-unaligned-access,) +PLATFORM_CPPFLAGS += $(PF_CPPFLAGS_NO_UNALIGNED)
# ========================================================================= # -- 1.7.9.5
Tested-by: Gary Thomas gary@mlbassoc.com
participants (11)
-
Albert ARIBAUD -
Aneesh V -
Gary Thomas -
Lucas Stach -
Mike Frysinger -
Måns Rullgård -
Rob Herring -
Stephen Warren -
Tetsuyuki Kobayashi -
V, Aneesh
-
Wolfgang Denk