Re: [PATCH v2] Add 'bootsource' /chosen property

On Mon, 5 May 2025 at 09:34, Quentin Schulz <[email protected]> wrote:
>
> From: Quentin Schulz <[email protected]>
>
> Bootloaders typically can be loaded from different storage media, such
> as eMMC, SD card, SPI flash, EEPROM, but also from non-persistent media
> such as USB (via proprietary protocols loading directly into SRAM, or
> fastboot, DFU, etc..), JTAG, ...
>
> This information is usually reported by the SoC-ROM via some proprietary
> mechanism (some specific address in registers/DRAM for example).
>
> It would be useful to know which medium was used to load the first stage
> of the bootloader. SoC-ROM shall be ignored and not reported in this
> property.
>
> This can allow client programs to detect which medium to write to when
> updating the boot program, or detect if fallback mechanisms to
> unexpected medium were used to reach the client program's execution.
>
> In cases where a boot program is split into multiple stages (like
> U-Boot), it only represents the device that was used to load the very
> first stage (in case of U-Boot, VPL/TPL/SPL whichever is executed first)
> and not any of the later stages (in case of U-Boot, TPL/SPL/proper) or
> any client program. They may match, but they may not and this property
> is meant to only represent the device used for loading the very first
> stage.
>
> I have a board running U-Boot which currently has 9 boot scenarios
> (eMMC/SD/SPI-NOR for the first stage, eMMC/SD/SPI-NOR for the next
> stages; not counting USB loading yet, which would make it a few more). I
> cannot force the BootROM of this board to select a specific device aside
> from erasing the other media.
> The only way to identify which device was used for the first stage is to
> parse U-Boot first stage console output or add some custom logic for my
> board. To validate that a new version of the bootloader works, including
> the fallback mechanisms, I need to make sure the BootROM loads the first
> stage from the expected device otherwise I may have false positives.
> This would be useful for automated testing.
>
> I could also very well see this being used to identify where the first
> stage of the boot program is stored (which may differ from where the
> later stages are! that's the case for U-Boot proper for example!) to be
> able to update it from a client program.
>
> Note that Barebox has been using this property for a while already, with
> this very content[1].
>
> This is chosen as a string so that it matches other properties in
> /chosen (e.g. stdout-path) as well as allows for extending it, in case
> one needs to provide additional information (e.g. HW boot partition for
> eMMC, a specific disk on an AHCI controller, a specific USB device on
> a USB bus, etc.).
>
> [1] https://lore.kernel.org/u-boot/[email protected]/
>
> Signed-off-by: Quentin Schulz <[email protected]>
> ---
> Note that this property is already set by Barebox and I'm planning on
> adding it to U-Boot as well, specifically for Rockchip SoCs.
>
> I have some doubts about the wording, especially in the case of
> hypervisors or chained boot programs. I'm not entirely sure what would
> make the most sense to put in the property for those scenario.
> ---
> Changes in v2:
> - added usecases, non-usecases and increased verbosity of the definition
>   of the property name as requested by Simon,
> - Link to v1: https://lore.kernel.org/r/[email protected]
> ---
>  source/chapter3-devicenodes.rst | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>

Reviewed-by: Simon Glass <[email protected]>

Does this need to go into the schema as well?

- Simon

> diff --git a/source/chapter3-devicenodes.rst b/source/chapter3-devicenodes.rst
> index 8080321d6e60d6b1e86c81af86c6850246a0223b..98cab42c105d9a390f74a6f80fb2bba61d5bb919 100644
> --- a/source/chapter3-devicenodes.rst
> +++ b/source/chapter3-devicenodes.rst
> @@ -456,6 +456,23 @@ time. It shall be a child of the root node.
>                                                         the client program. The value could
>                                                         potentially be a null string if no boot
>                                                         arguments are required.
> +   ``bootsource``          O     ``<string>``          A string that specifies the full path to the
> +                                                       node representing the device the BootROM used
> +                                                       to load the initial boot program.
> +                                                       If the initial boot program is split into
> +                                                       multiple stages, this represents the storage
> +                                                       medium or device (e.g. used by fastboot) from
> +                                                       which the very first stage was loaded by the
> +                                                       BootROM.
> +                                                       It may differ from the device from which later
> +                                                       stages of the boot program or client program
> +                                                       are loaded from, as this property isn't meant
> +                                                       to represent those devices.
> +                                                       A later stage of the boot program, or the
> +                                                       client program, may use this information to
> +                                                       favor the device in this property over others
> +                                                       for loading later stages, or know the storage
> +                                                       medium to flash an update to.
>     ``stdout-path``         O     ``<string>``          A string that specifies the full path to the
>                                                         node representing the device to be used for
>                                                         boot console output. If the character ":" is
>
> ---
> base-commit: 5688e1c0b961d2ca5a32e3b624a9f4a9b433184f
> change-id: 20250205-bootsource-84df255e9e6c
>
> Best regards,
> --
> Quentin Schulz <[email protected]>
>