On 10/25/23 22:28, 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.
The 2020 event struck me because a Linux kernel would not even be
compatible with the device-tree of the same kernel release.
This year I once again had issues with booting an Allwinner board with a
device-tree from a different Debian kernel version. Both backward and
forward compatibility were broken. But at least I could boot with a
matching device-tree.
Distros will continue to load the device-tree that matches the kernel to
get the best possible board support and need to do this efficiently.
Right. Even if there is full backward/forward compatibility, you
probably want the latest device-tree to make sure you get the most
complete hardware support.
But this shouldn't be used as an argument to not care about
backward/forward compatibility.
Nobody in this thread suggested to not care about forward and backward
compatibility.
Best regards
Heinrich
