Hi all,
Reviving this as it is still very much an issue, and especially relevant
for Qualcomm platforms.
On 11/3/23 20:44, Simon Glass wrote:
Hi Heinrich,
On Wed, 25 Oct 2023 at 15:22, Heinrich Schuchardt
<heinrich.schucha...@canonical.com> wrote:
On 10/25/23 23:13, Tom Rini wrote:
On Wed, Oct 25, 2023 at 10:28:05PM +0200, Mark Kettenis wrote:
Date: Wed, 25 Oct 2023 21:57:44 +0200
From: Heinrich Schuchardt <heinrich.schucha...@canonical.com>
On 10/25/23 20:23, Simon Glass wrote:
Hi Heinrich,
On Tue, 24 Oct 2023 at 18:02, Simon Glass <s...@google.com> wrote:
Hi Heinrich,
On Mon, 23 Oct 2023 at 23:20, Heinrich Schuchardt
<heinrich.schucha...@canonical.com> wrote:
Forward and backward compatibility of Linux kernel device-trees is
sometimes missing. One solution approach is to load a kernel specific
device-tree. This can either be done via a U-Boot scripts (like the one
generated by Debian package flash-kernel or by a boot loader like GRUB.
The boot loader approach currently requires to know the device-tree name
before first boot which makes it unusable for generic images.
Expose the device-tree file name as EFI variable FdtFile.
This will allow bootloaders to load a kernel specific device-tree.
kernel-specific
The variable will not be exposed on ACPI based systems or if the
environment variable fdtfile is not defined.
Signed-off-by: Heinrich Schuchardt <heinrich.schucha...@canonical.com>
---
v4:
Generalize the description of the content of $fdtfile.
v3:
Add documentation
v2:
Use a unique GUID to enable future U-Boot independent
standardization.
Do not try to add the variable on ACPI based systems.
---
doc/develop/uefi/uefi.rst | 14 ++++++++++++++
include/efi_loader.h | 5 +++++
lib/efi_loader/efi_setup.c | 30 ++++++++++++++++++++++++++++++
3 files changed, 49 insertions(+)
diff --git a/doc/develop/uefi/uefi.rst b/doc/develop/uefi/uefi.rst
index fb16ac743a..702c490831 100644
--- a/doc/develop/uefi/uefi.rst
+++ b/doc/develop/uefi/uefi.rst
@@ -916,6 +916,20 @@ So our final format of the FilePathList[] is::
Loaded image - end node (0xff) - VenMedia - initrd_1 - [end node (0x01)
- initrd_n ...] - end node (0xff)
+EFI variable FdtFile
+~~~~~~~~~~~~~~~~~~~~
+
+Ideally U-Boot would always expose a device-tree that can be used for booting
+any operating systems. Unfortunately operating systems like Linux sometimes
+break forward and backward compatibility. In this case there is a need to load
+an operating system version specific device-tree.
This seems to be a strong statement. Given the effort that goes into
the DT, changes are supposed to be backwards-compatible. Is this
generally true, or is it just that we want an up-to-date DT for the
kernel to enable new features?
Did you see this comment?
It would have been nice to put the person which made that comment on copy.
The truth lies in the world "supposed":
The idea of a device-tree that never needs to change is quite old and
never became true on ARM devices.
We all know Linux tends to break both forward and backward compatibility
of device-trees. Here is a nice example:
d0c6707ca423 ("arm64: dts: allwinner: H5: NanoPi Neo Plus2: phy-mode
rgmii-id")
Driver changes broke forward and backwards compatibility of a lot of
Allwinner boards.
Well, that happened in 2020. Things have gotten better over time.
(kinda off-topic context on DT version compat)
From what I've seen, there is not yet very much infrastructure or
common practise in place in the kernel to handle parsing DTB in a
backwards compatible way. For example there has been efforts to simplify
the dwc3 devicetree for Qualcomm platforms with a series dating back
about as far as this U-Boot patch!
https://lore.kernel.org/linux-arm-msm/20250318-dwc3-refactor-v5-1-90ea6e5b3...@oss.qualcomm.com/
Earlier versions attempted to convert the older DTS to the newer format
internally, but after much discussion it was decided that this wasn't
really feasible, instead the new approach is to duplicate the entire
dwc3 driver to maintain DT compatibility.
It's obviously good to see compatibility taken seriously, since it seems
clear that if we ever want to treat DT as firmware, the kernel will need
to do this (and eventually vendors could ship laptops with DT out of the
box which works with upstream -- crazier things have happened).
But I think in the mean time we still want to be able to drive distro
adoption, and the way I see it teaching GRUB and systemd-boot about a
new FdtFile EFI variable is going to make that way simpler...
Well, yes and no. Given the brief summary here, I bet this was just
like when phy-mode and am335x platforms had DT compatibility broken and
the answer was that it was OK because the DT was incorrectly describing
hardware. So this is the reminder that there are cases of breaking DT
compatibility that are allowed. Even if the DT has been out (and wrong)
for several years.
That's not the main point of this thread and I don't want to derail
things further along this point, I just want to note that the details
here reminded me of when things are allowed to be incompatible with
previous trees.
Unfortunately I was off the list for a week or so, but I see this one
so will reply here.
You are conflating things:
The EFI variable is a hint to the GRUB OS to find the correct
device-tree file without scanning hundreds of device-tree. You can not
build a compatibility check for it into U-Boot.
Actually we can...and I think that is what we should do.
No, how and where an EFI bootloader app chooses to load the FDT from is
entirely implementation dependent. It may be extremely costly to compare
compatible strings for every single dtb, and whether or not to do so is
the decision of the distro anyways, all we can do in U-Boot is try to
promote best practise (which clearly isn't compatible prop comparison
for hundreds of DTBs...)
I'll note that systemd-uki does actually do this compatible prop check,
but this would not be feasible for regular systemd-boot.
In distro-boot we are using the same value from environment variable
$fdtfile. Here you could check the compatible string but this might
break booting .
We need to figure that point out. The compatible string is
programmatically is defined to be correct, so it is the filename that
may/may not be useful. What am I missing?
Current distro images don't have a way to map from compatible strings to
a filename, but they ship DTBs as normal files on the ESP.. They often
have multiple kernel versions installed, and therefore multiple DTB
directories which are versioned.
U-Boot doesn't know what boot entry is picked (and should not NEED to know).
Providing FdtFile as an EFI variable will make it possible for grub and
systemd-boot to support the same kind of "fdtdir" property that extlinux
does, this would simply enable versioned DTB loading for distros and
make booting wayyyyy easier.
With regards to the design, I think a good follow-up patch would set a
default value for $fdtfile (and consequently FdtFile) from the
DEVICE_TREE build variable as a constant (maybe only if OF_UPSTREAM is
enabled).
This would be correct in almost all cases (and wayyy better than the
hacky fdtfile generating code we have in mach-snapdragon right now).
For distro-boot, do you mean the scripts? We are trying to deprecate
those. Of course we have brought in all the same work-arounds, etc.,
but with bootstd we can start to clean things up, I hope.
So let's think about how we can have U-Boot choose the right DT to boot with.
Regards,
Simon
Heinrich, since it's been a while, if you aren't super interested in
re-sending this patch I'd be happy to address the wording feedback and
re-spin it, along with a patch setting the default $fdtfile (else I can
send that as a follow-up).
I'll also open an issue for getting support for this added to systemd-boot.
--
Caleb (they/them)
