Re: [PATCH 0/2] efi: device tree fix-up

[Heinrich Schuchardt <heinrich.schuchardt@canonical.com>]

Hello Daniel,

I sent this series when you were in the middle of getting GRUB-2.06 out. 
Unfortunately I did not see any feedback yet. Could you, please, share 
your thoughts.

Best regards

Heinrich

On 04.02.21 14:15, Heinrich Schuchardt wrote:
> Operating systems need a description of the hardware. This description can
> either be supplied via ACPI tables or via device-trees. On the ARM
> architecture ACPI tables are not avaialable for most devices.
> 
> In an ideal world device-trees would be defined once by the hardware
> manufacturer and would not be subject to change.
> 
> In reality device-trees or subject to frequent changes. Linux has a long
> record of breaking forward and backwards compatibility between device-
> trees and the kernel. So for booting it is wise to use the specific device-
> tree that comes with the operation system. This requires reading the
> device-tree from file.
> 
> One approach is to leave loading the device-tree file to the firmware. This
> works fine if the firmware is loading the triple of kernel, initrd, and
> device-tree. This is what Debian does with the flash-kernel package. It
> provides a boot script for U-Boot that loads the most recent kernel,
> initrd, and device-tree.
> 
> Before passing the device-tree to the operating as an EFI configuration
> table the firmware goes through the following steps:
> 
> * Fix-ups are applied to the device-tree. Examples of the changes include
>    the memory size and the RISC-V boot hart. Without the changes the
>    operating system may not boot at all or crash later.
> 
> * Memory reservations are added to the memory map according to the
>    /reserved-memory node and the memory reservation block of the
>    device-tree. Without these reservations the operating system may crash.
> 
> When using GRUB it is not known beforehand which operating system the user
> will choose. To guarantee compatibility with the operating system GRUB has
> to take control of the device-tree loading.
> 
> GRUB has a devicetree command to load a device-tree which is then passed as
> EFI configuration table to the operating systems. But GRUB lacks the
> information needed to apply fix-ups to the device-tree. Further memory
> reservations are not executed.
> 
> What is needed is to pass the device-tree loaded by GRUB via the
> devicetree command to the firmware to execute fix-ups and memory
> reservations.
> 
> U-Boot v2020.04-rc1 provides an EFI protocol for this purpose which has
> been defined in the EFI_DT_FIXUP_PROTOCOL specification [1].
> 
> With the first patch in the series the devicetree command is enhanced. It
> checks if an instance of the EFI_DT_FIXUP_PROTOCOL is available. If yes,
> the Fixup() method of the protocol is called when the boot command is
> invoked. The fixed-up device-tree is passed to the operating system as
> EFI configuration table.
> 
> The second patch adjusts the 10_linux template for grub-mkconfig. If
> GRUB_LOAD_DEVICETREE=true in /etc/default/grub, it looks for file
> dtb-${version} matching the Linux version. If the file (or file dtb as
> fallback) is found, a devicetree command is added to grub.cfg.
> 
> [1] EFI_DT_FIXUP_PROTOCOL specification
>      https://github.com/U-Boot-EFI/EFI_DT_FIXUP_PROTOCOL
> 
> Heinrich Schuchardt (2):
>    efi: EFI Device Tree Fixup Protocol
>    10_linux: support loading device trees
> 
>   docs/grub.texi             |  6 ++++++
>   grub-core/loader/efi/fdt.c | 35 ++++++++++++++++++++++++++++++++++-
>   include/grub/efi/api.h     | 22 ++++++++++++++++++++++
>   util/grub-mkconfig.in      |  1 +
>   util/grub.d/10_linux.in    | 23 +++++++++++++++++++++++
>   5 files changed, 86 insertions(+), 1 deletion(-)
> 
> --
> 2.30.0
>