On 5/1/23 16:31, Aaron Durbin wrote:

Hi,

On Mon, May 1, 2023 at 8:26 AM Heinrich Schuchardt <heinrich.schuchardt@canonical.com mailto:heinrich.schuchardt@canonical.com> wrote:

Linux distributions are interested in providing a single image which
enables a high number of boards to boot. This is simple if the boot
firmware (EDK II or U-Boot) is installed on flash.

For boards that expect to load a boot loader like U-Boot from an SD-card
it is necessary that the firmware locations for different boards
on the SD-card don't collide.

When loading from SD-card or eMMC the sector at which the binary
starts has to be identified. The following has been implemented:

- start from hard coded sector number
- load file from FAT file system
- load from given partition number
- load from partition with boot flag set (MBR partioning only)
- load from partition with specific type GUID

Loading by partition type GUID seems the most appropriate to avoid
collisions between the firmware for different boards.

Often firmware is separated into multiple parts due to firmware
restrictions, e.g. U-Boot SPL and main U-Boot (e.g. as .itb file).

Here the same considerations apply. Using a partition type GUID to
identify further firmware parts to be loaded is best suited to
avoid collisions.

I would suggest to add a recommendation to the EBBR specification
to use SoC specific partition type GUIDs to identify firmware to
be loaded from SD-card.

Who is loading the firmware that is identified by a GUID?  And how does that tie into Linux distros? I don't see anything wrong w/ the recommendation, but I didn't completely follow which piece of software is loading firmware and how the GUID reduces the problems. Also, is this "SoC specific partition type GUID" unique per SoC? Or is it expected to be a global GUID?

Booting firmware starts at boot ROM which may either directly load a piece of software from SD-card or may load a piece of software from flash which in turn will load the next step boot loader from the SD-card.

From the view of a distro it is best if each board uses a separate GUID. This allows to add multiple U-Boot versions (or other boot software) on the same SD-card.

Best regards

Heinrich