Hi Simon,
Le ven. 5 nov. 2021 à 17:12, Simon Glass sjg@chromium.org a écrit :
Hi François,
On Fri, 5 Nov 2021 at 02:27, François Ozog francois.ozog@linaro.org wrote:
On Fri, 5 Nov 2021 at 03:02, Simon Glass sjg@chromium.org wrote:
Hi François,
On Tue, 2 Nov 2021 at 10:03, François Ozog francois.ozog@linaro.org
wrote:
Hi Simon,
On Tue, 2 Nov 2021 at 15:59, Simon Glass sjg@chromium.org wrote:
Hi François,
On Mon, 1 Nov 2021 at 02:53, François Ozog francois.ozog@linaro.org
wrote:
Hi Simon,
this seems a great endeavor. I'd like to better understand the
scope of it.
Is it to be used as part of what could become a U-Boot entry ABI
scheme? By that I mean giving some fixed aspects
to U-Boot entry while letting boards to have flexibility (say for
instance that the first 5 architecture ABI
parameter registers are reserved for U-Boot), and the Passage is
about specifying either those reserved registers
or one of them?
The goal is to provide a standard entry scheme for all firmware binaries. Whether it achieves that (or can with some mods) is up for discussion.
If you say a) define a U-Boot entry ABI and providing a firmware-to-firmware
information passing facility which would be part of all firmware ABIs (as the projects decide to define their own ABI) it looks good.
but If you say
It is an ABI to be adopted by U-Boot but also other firmware. For example, if TF-A calls U-Boot it should use standard passage. If U-Boot calls TF-A or Optee it should use standard passage.
b) define a standard entry scheme (register map, processor state, MMU
state, SMMU state, GIC state...) that does not look realistic.
No I don't mean that. This data structure could be used in any state, so long as the two registers are set correctly.
I think you mean a) but just want to be sure.
Yes I think so.
Re the registers, do you think we need 5?
I don't :-)
Thinking entry ABI, here is what I observed on Arm:
Linux has two entry ABIs:
- plain: x0 = dtb; command line = dtb:/chosen/bootargs; initrd =
dtb:/chosen/linux,initrd-*
- EFI: x0=handle, x1=systemtable, x30=return address; dtb = EFI_UUID config table; initrd =
efi:<loadfile2(INITRD vendor media UUID); command line = efi: image_protocol::load_options
U-Boot (proper) has plenty of schemes:
- dtb is passed as either x0, x1, fixed memory area (Qemu which is
bad in itself), or other registers
- additional information passing: board specific register scheme,
SMC calls
- U-Boot for RPI boards implement a Linux shaped entry ABI to be
launched by Videocore firmware
Based on all the above, I would tend to think that RPI scheme is a
good idea but also
shall not prevent additional schemes for the boards.
I was not actually considering Linux since I believe/assume its entry scheme is fixed and not up for discussion.
I also did not think about the EFI case. As I understand it we cannot touch it as it is used by UEFI today. Maybe it is even in the standard?
It is in the spec and we are making it evolve, or its understanding
evolve (jurisprudence) for instance on initrd standard handling.
Well perhaps we could merge it with standard passage. But EFI is not going to want to use a bloblist, it will want to use a HOB.
Really I am hoping we can start afresh...?
What about a U-Boot Arm entry ABI like:
- plain: x0=dtb, x1=<Passage defined>, x2-x5 = <reserved>, other
registers are per platform, SMC calls allowed too
Hmm we don't actually need the dtb as it is available in the
bloblist.
If you don't have x0=dtb, then you will not be able to use U-Boot on
RPI4.
Unless you want to redo everything the RPI firmware is doing.
That's right, RPI cannot support standard passage. It is not open-source firmware so it isn't really relevant to this discussion. It will just do what it does and have limited functionality, with work-arounds to deal with the pain, as one might expect.
So you are seeing two "all-or-nothing" options: <specific>: U-Boot entry is board specific as it is today <purepassage>: A new form where the only parameter is a head of
bloblist, one of those blobs contain a DT
You propose to mandate a DT for all boards make sense in that
environment.
For RPI4, you just ignore everything the prior boot loader does because
it is not <passage> compliant.
It's not that. It's just that it is closed-source, so not relevant to the discussion here. They could open-source it and then we could consider it, but it has been closed-source for years now, so why would we think that would happen?
This reinforces my opposition to the mandatory DT proposal.
a third option is I think way more attractive: <optpassage>: shaped after the architecture Linux entry (ie. first
parameter is dtb) [+ passage head (i.e. second parameter is pointer to passage head)]
This way, you make U-Boot entry clean in RPI4, Apple M1, Qemu,
SystemReady contexts
and get a well deserved standardized information passing between prior
loaders and U-Boot.
The three options are possible though, you could select a U-Boot entry
CONFIG option for:
<specific> <optpassage> <purepassage>
But despite it would be technically feasible, I don't think it is goes
in the right direction.
OK. Do you think we need a separate devicetree pointer, rather than forcing it to be inside the created bloblist?
I'd like to understand what problem you are solving with this. I am trying to figure out a firmware-to-firmware mini-ABI (just a few register values) that can be used in open-source projects. The ABI is not intended to be used with Linux (I am unsure of the benefit it would give and whether it is feasible to change the current one).
You are talking about the Linux entry mechanism. What relevance does that have for firmware?
I understand that some projects already implement the Linux mechanism, but that is because they expect to jump straight to Linux, not have U-Boot in the path. So IMO standard passage offers no benefit to them.
To address them in turn:
- rpi4 - closed source, who cares?
- Apple M1 - we could probably expand it to pass a bloblist, but it
would be confusing unless we share registers, as you suggest
- Qemu - I already tried to update that and got pushback...do you
really think those guys are going to want to add a bloblist? So again, who cares?
- SystemReady - not sure what this means in practice, but it would be
good if SystemReady could use standard passage
So let's say we have an optional standard-passage thing and we use registers such that it is similar to Linux and EFI and just expands on them.
The first problem is that Linux and EFI seem to be completely incompatible. Can that be changed, perhaps on the EFI side? If not, we need two separate protocols.
I'll ignore EFI for now. So we might have:
r0 = 0 r1 = machine number (0?) r2 = dtb pointer r3 = bloblist pointer, 0 if missing r14 = return address
or
x0 = dtb x1 = bloblist pointer, 0 if missing x30 = return address
That’s essentially what I proposed! you do not force the DTB to be found in the bloblist, and shape the U-Boot entry after the Linux entry ABI. Good ! I was saving a few registers for future ABI evolution so that boards can be guaranteed to have their board specific registers properly protected. The 5 registers, leaving 3 undefined was just « why not ». We could also have a cookie in x1: high 48 bits magic low 16 ABI version, x2=bloblist pointer.
For EFI, we could add a blob to the bloblist containing the system table and handle, perhaps? Otherwise:
x2 - efi handle x3 - system table
Is that along the lines of what you are thinking?
No, efi entry is only x0=efi handle, x1=system table . I was trying to find a way to have passage when U-Boot is loaded as a UEFI app (your other patch set to make U-Boot a more integrated UEFi app). Let’s say that a U-Boot aware DXE driver/protocol actually populate such a table, it could be a communication channel between that driver and U-Boot.
But still, please respond above so I can understand what problem you are worried about.
Regards, Simon
But I added an offset to it as a convenience.
- EFI: x0=handle, x1=systemtable, x30=return address; (when
U-Boot is launched as an EFI app)
dtb = EFI_UUID config table, + Passage = Passage UUIDconfig table
I don't understand the last line. Where is the passage info / bloblist? Do you mean it goes in the HOB list with a UUID? I suppose that is the most EFI-compatible way.
The Passage config table could just contain the "head" of the
bloblist/Passage information.
If UEFI wants to deal with standard passage, that is...
What do you think about the idea of using an offset into the bloblist for the dtb?
It is possible but as I said, failing to mimic Linux entry ABI would
miss the opportunity to just boot without changes on RPI4.
See above. Broadcom could look at open-sourcing their bootloader if
they wish.
Also, can we make the standard passage ABI a build-time option, so it is deterministic?
Looks good. I would look into stating that for SystemReady we would
advise to use that option and make it standard for Trusted Substrate (Linaro recipes that we upstreaming to make SystemReady compliance easy and consistent across platforms).
OK. I mean that if the option is enabled, then standard passage must be provided / emitted or things won't work. If the option is disabled, then standard passage is not used. In other words, we are looking for magic values in registers, etc, just enabling/disabling it at build-time.
We could further leverage Passage to pass Operating Systems
parameters that could be removed from device tree (migration of /chosen to Passage). Memory inventory would still be in DT but allocations for CMA or GPUs would be in Passage. This idea is to reach a point where device tree is a "pristine" hardware description.
I'm worried about this becoming a substitute for devicetree. Really
my
intent is to provide a way to pass simple info, whereas what you talk about there seems like something that should be DT, just that it
might
need suitable bindings.
I see your point and I agree It should not be a substitute. here is an expanded version of what I had in mind when I wrote those
lines.
cma, initrd and other Linux kernel parameters can be conveyed either
through command line or DT.
When using the non UEFI Linux entry ABI, you need to use the DT to
pass those parameters.
When using the UEFI Linux entry ABI, you *can* (not must) use the
command line to pass all information, leaving the DT passed to the OS without any /chosen.
When introducing Passage, I was wondering if we could pass command
line to Linux and, same as UEFI, leave the DT free from /chosen.
I am not sure it is a good goal though. I may be too pushing for a DT
free from parameters.
We could. Are there benefits to that?
I doubt we would pass the standard passage to Linux as a bloblist. I imagine something like this. The bloblist sits in memory with some things in it, including a devicetree, perhaps an SMBIOS table and a TPM log. But when U-Boot calls Linux it puts the address/size of those individual things in the devicetree. They don't move and are still contiguous in memory, but the bloblist around them is forgotten. Linux doesn't know that the three separate things it is picking up are actually part of a bloblist structure, since it doesn't care about that. Even a console log could work the same way. That way we don't end up trying to teach Linux about bloblist when it already has a perfectly good means to accept these items.
For ACPI I see things a similar way. The ACPI tables can point to things that *happen* to be in a bloblist, but without any knowledge of that needed in Linux, grub, etc.
As you know I have more expansive views about what should be in DT.
I think both of us are huge supporters of DT format and self
describing capabilities.
I am inclined to put rules into what fits into what lands in the DT
that is passed to the OS.
I am a fan of having DT used more in ad-hoc files.
Me too.
Cheers
PS: as Ilias mentions, this patch set contains bug fixes, non
immediately related additional functions (DM stats). It would be great to carve those out to fast path them and keep this one with the very core of your idea.
The DM stats is used in 'passage: Report the devicetree source'. I know it is sideways but I think it is better to make the output line more useful than just reporting the devicetree source.
I believe the DM stats has merits in its own. You could upstream this
independently and then Passage would be yet another "customer" of the feature.
I could, but it would just be a debug feature so people might not think it worth the code space. With the devicetree source it is more compelling.
The first patch is indeed unrelated. I will pick it up so we can drop it for the next rev.
[..]
Regards, Simon
-- François-Frédéric Ozog | Director Business Development T: +33.67221.6485 francois.ozog@linaro.org | Skype: ffozog