[U-Boot] efi_loader: Add distro boot script for removable media
I've spotted a couple of problems in 74522c898b35 "efi_loader: Add distro boot script for removable media". These help explain something I found strange in the commit description of the recently sent patch "jetson-tk1: Set fdtfile environment variable"; "The 4.5.0 kernel cannot cope with U-Boot's internal device tree".
"load_efi_dtb=" \ "load ${devtype} ${devnum}:${distro_bootpart} " \ "${fdt_addr_r} ${prefix}${fdtfile}; " \ "fdt addr ${fdt_addr_r}\0" \ \
The "fdt addr" command shouldn't be there. That affects the DT that U-Boot's internal commands operate on internally. That is entirely unrelated to the the DT that is passed to the Linux kernel. Instead, the EFI code should pass the DTB at ${kernel_addr_r} to the kernel, just like all other boot mechanisms do.
If by some chance U-Boot is configured by DTB and that DTB is fully suitable to pass to the Linux kernel, then the board-specific code can arrange for ${kernel_addr_r} to point at that same DTB, thus removing the need to load one. However, that's unlikely to happen too often at present; the most complete DTBs are housed in the Linux kernel source tree, and DTB ABI still isn't really a thing, so in practice one mostly wants to load a DTB that was built as part of the kernel being booted, and hence U-Boot's DTB isn't relevant.
On 13.04.16 19:54, Stephen Warren wrote:
I've spotted a couple of problems in 74522c898b35 "efi_loader: Add distro boot script for removable media". These help explain something I found strange in the commit description of the recently sent patch "jetson-tk1: Set fdtfile environment variable"; "The 4.5.0 kernel cannot cope with U-Boot's internal device tree".
"load_efi_dtb=" \ "load ${devtype} ${devnum}:${distro_bootpart} " \ "${fdt_addr_r} ${prefix}${fdtfile}; " \ "fdt addr ${fdt_addr_r}\0" \ \The "fdt addr" command shouldn't be there. That affects the DT that U-Boot's internal commands operate on internally. That is entirely unrelated to the the DT that is passed to the Linux kernel. Instead, the EFI code should pass the DTB at ${kernel_addr_r} to the kernel, just like all other boot mechanisms do.
I guess you mean $fdt_addr_r?
I wasn't sure which one to pick back when I implemented it. "fdt addr" seemed like a nice fit because it gets you an explicit fdt rather than a "this is an address, go and see whether there happens to be a dtb loaded there".
But I don't think it'll hurt to move it to fdt_addr_r instead.
If by some chance U-Boot is configured by DTB and that DTB is fully suitable to pass to the Linux kernel, then the board-specific code can arrange for ${kernel_addr_r} to point at that same DTB, thus removing the need to load one. However, that's unlikely to happen too often at present; the most complete DTBs are housed in the Linux kernel source tree, and DTB ABI still isn't really a thing, so in practice one mostly wants to load a DTB that was built as part of the kernel being booted, and hence U-Boot's DTB isn't relevant.
It depends on the camp you're coming from ;). If you come from the traditional server camp that used to work with server class ppc in the past, then device tree by firmware is a pretty natural concept. Also, all ARM servers available today have a stable dtb ABI, because they all provide dtb via uEFI.
In the embedded world this doesn't hold true quite as well unfortunately. People only realize that their device trees are incomplete / wrong when they write the respective drivers. So there we see much more churn.
Over time however as a platform ages, the "embedded" style systems should end up with stable dtbs as well, at which point having them provided by U-Boot straight away is a nice compromise.
Alex
On 04/13/2016 12:24 PM, Alexander Graf wrote:
On 13.04.16 19:54, Stephen Warren wrote:
I've spotted a couple of problems in 74522c898b35 "efi_loader: Add distro boot script for removable media". These help explain something I found strange in the commit description of the recently sent patch "jetson-tk1: Set fdtfile environment variable"; "The 4.5.0 kernel cannot cope with U-Boot's internal device tree".
"load_efi_dtb=" \ "load ${devtype} ${devnum}:${distro_bootpart} " \ "${fdt_addr_r} ${prefix}${fdtfile}; " \ "fdt addr ${fdt_addr_r}\0" \ \The "fdt addr" command shouldn't be there. That affects the DT that U-Boot's internal commands operate on internally. That is entirely unrelated to the the DT that is passed to the Linux kernel. Instead, the EFI code should pass the DTB at ${kernel_addr_r} to the kernel, just like all other boot mechanisms do.
I guess you mean $fdt_addr_r?
Sorry, yes.
I wasn't sure which one to pick back when I implemented it. "fdt addr" seemed like a nice fit because it gets you an explicit fdt rather than a "this is an address, go and see whether there happens to be a dtb loaded there".
But I don't think it'll hurt to move it to fdt_addr_r instead.
Sounds good.
If by some chance U-Boot is configured by DTB and that DTB is fully suitable to pass to the Linux kernel, then the board-specific code can arrange for ${kernel_addr_r} to point at that same DTB, thus removing the need to load one. However, that's unlikely to happen too often at present; the most complete DTBs are housed in the Linux kernel source tree, and DTB ABI still isn't really a thing, so in practice one mostly wants to load a DTB that was built as part of the kernel being booted, and hence U-Boot's DTB isn't relevant.
It depends on the camp you're coming from ;). If you come from the traditional server camp that used to work with server class ppc in the past, then device tree by firmware is a pretty natural concept. Also, all ARM servers available today have a stable dtb ABI, because they all provide dtb via uEFI.
In the embedded world this doesn't hold true quite as well unfortunately. People only realize that their device trees are incomplete / wrong when they write the respective drivers. So there we see much more churn.
Over time however as a platform ages, the "embedded" style systems should end up with stable dtbs as well, at which point having them provided by U-Boot straight away is a nice compromise.
Yes, unfortunately DTB ABI or completeness isn't something that many downstreams think about. I'm not convinced that will change much at the time boards (or drivers for specific HW modules) first appear upstream. You're right that boards that have been around a while will tend to stabilize.
For embedded targets, given that we already support loading DTBs separately, I'm not sure I would want to change away from that. The system works and people are familiar with it. Switching to a different scheme means everyone has to adapt. Still, it might work out well on a case-by-case basis.
On 13.04.16 20:31, Stephen Warren wrote:
On 04/13/2016 12:24 PM, Alexander Graf wrote:
If by some chance U-Boot is configured by DTB and that DTB is fully suitable to pass to the Linux kernel, then the board-specific code can arrange for ${kernel_addr_r} to point at that same DTB, thus removing the need to load one. However, that's unlikely to happen too often at present; the most complete DTBs are housed in the Linux kernel source tree, and DTB ABI still isn't really a thing, so in practice one mostly wants to load a DTB that was built as part of the kernel being booted, and hence U-Boot's DTB isn't relevant.
It depends on the camp you're coming from ;). If you come from the traditional server camp that used to work with server class ppc in the past, then device tree by firmware is a pretty natural concept. Also, all ARM servers available today have a stable dtb ABI, because they all provide dtb via uEFI.
In the embedded world this doesn't hold true quite as well unfortunately. People only realize that their device trees are incomplete / wrong when they write the respective drivers. So there we see much more churn.
Over time however as a platform ages, the "embedded" style systems should end up with stable dtbs as well, at which point having them provided by U-Boot straight away is a nice compromise.
Yes, unfortunately DTB ABI or completeness isn't something that many downstreams think about. I'm not convinced that will change much at the time boards (or drivers for specific HW modules) first appear upstream. You're right that boards that have been around a while will tend to stabilize.
For embedded targets, given that we already support loading DTBs separately, I'm not sure I would want to change away from that. The system works and people are familiar with it. Switching to a different scheme means everyone has to adapt. Still, it might work out well on a case-by-case basis.
I've seen it work pretty well on server class ARM systems, so I'm convinced the model can work. If people care :).
The interesting bit is that in the "real" embedded case, the model would work as well, because U-Boot would get updated in conjunction with the kernel, so they'd be in sync.
But what I'm aiming for is a waterfall model. Basically allow people to provide the device tree using
* internal U-Boot (either its own or a separate dtb) * distro override * grub override
Each come with their own pitfalls. Grub override is the worst of the bunch, because you're losing all of the device tree patching for memory size, simplefb, secondary entry points, etc. There's simply no efi hook for it and I'm not sure it's worth implementing our own protocol just for this use case.
But nobody forces you to use any of these, in any model. If U-Boot provides a dtb, you are free to use it or override it. If you provide a dtb at the known-good location, you can still override it using grub overrides.
In fact, if you don't want to you don't even have to use the distro bootcmd. You can create your own bootcmd that does
load mmc 0 $fdt_addr_r my_awesome.dtb ;\ load mmc 0 $kernel_addr_r some/random/path/grub2.efi ;\ bootefi $kernel_addr_r
and it would work just like you would expect it to work. The point about these defaults is that we have some path for users to not have to know these details :). I think one of U-Boot's great strengths is its modularity where you can do pretty much anything you want. But I guess I'm preaching to the choir ;).
Alex
Am 13.04.2016 um 20:47 schrieb Alexander Graf:
load mmc 0 $fdt_addr_r my_awesome.dtb ;\ load mmc 0 $kernel_addr_r some/random/path/grub2.efi ;\ bootefi $kernel_addr_r
If we're going in that direction, I would rather pass $fdt_addr_r as second optional argument to bootefi. That would be more consistent with bootm, bootz, booti.
While at it, is there a reason that bootz/booti create some output about memory locations of initrd/fdt but bootefi does not?
Regards, Andreas
On 13.04.16 20:54, Andreas Färber wrote:
Am 13.04.2016 um 20:47 schrieb Alexander Graf:
load mmc 0 $fdt_addr_r my_awesome.dtb ;\ load mmc 0 $kernel_addr_r some/random/path/grub2.efi ;\ bootefi $kernel_addr_r
If we're going in that direction, I would rather pass $fdt_addr_r as second optional argument to bootefi. That would be more consistent with bootm, bootz, booti.
Yes, probably.
While at it, is there a reason that bootz/booti create some output about memory locations of initrd/fdt but bootefi does not?
Is that important information? I don't quite get the feeling that it is.
Alex
participants (3)
-
Alexander Graf -
Andreas Färber -
Stephen Warren