> From: Simon Glass <s...@google.com> > Date: Thu, 19 Oct 2023 07:55:49 -0600 > > Hi Heinrich, > > On Wed, 18 Oct 2023 at 02:15, Heinrich Schuchardt > <heinrich.schucha...@canonical.com> wrote: > > > > On 10/18/23 05:33, Simon Glass wrote: > > > Hi Heinrich, > > > > > > On Tue, 17 Oct 2023 at 07:50, 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. > > >> > > >> 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> > > >> --- > > >> v2: > > >> Use a unique GUID to enable future U-Boot independent > > >> standardization. > > >> Do not try to add the variable on ACPI based systems. > > >> --- > > >> include/efi_loader.h | 5 +++++ > > >> lib/efi_loader/efi_setup.c | 30 ++++++++++++++++++++++++++++++ > > >> 2 files changed, 35 insertions(+) > > > > > > I was too slow to reply to v1. > > > > > > Does grub load the DT? I was assuming that U-Boot would pass it on? > > > What is the interface between U-Boot and grub? > > > > The device-tree built into U-Boot is often out of date and not usable to > > boot current Linux. A single device-tree can be loaded by U-Boot from > > file and passed on as EFI configuration table. This device-tree may not > > be compatible with all kernel versions exposed by GRUB. > > > > GRUB provides a devicetree command. It is disabled if you use secure > > boot. At least in Debian and Ubuntu GRUB invokes the > > EFI_DT_FIXUP_PROTOCOL exposed by U-Boot to run U-Boot's device-tree > > fix-ups after loading a device-tree. > > > > Vendor scripts for GRUB like Ubuntu's /etc/grub.d/10_linux add > > devicetree commands to the boot options in grub.cfg. > > Thanks. I wonder if you could document this somewhere? It seems like > there are a lot of options and it is quite complicated. > > Back to the question, I suppose you are expecting grub to load the DT > using this filename? But why doesn't U-Boot load it instead? It seems > very convoluted.
>From an OpenBSD perspective: * It allows us to load directly from an OpenBSD filesystem. (And yes I really think you don't want to add support for all the different flavours of the UFS/FFS filesystem to U-Boot). * It means we don't have to worry about breaking other OS installs on the same disk since we can avoid copying the device tree onto the (shared) EFI System Partition. * It means that we can provide a uniform user interface that is independent on the UEFI implementation; the user doesn't need to care whether they are dealing with U-Boot, EDK2 or some other proprietary UEFI implementation. Cheers, Mark
