Boot from specified partition (or FS) UUID without knowing Device ID

[AKASHI Takahiro]

On Fri, Feb 18, 2022 at 06:41:18PM +0100, Heinrich Schuchardt wrote:
> On 2/18/22 18:07, Tom Rini wrote:
> > On Fri, Feb 18, 2022 at 02:01:41PM +0700, Tom Hale wrote:
> > 
> > > I want to have a fallback boot situation where boot is first tried from one
> > > partition, and then from another partition (on a different device) if the
> > > first partition isn't available (eg, if the first device fails).
> > > 
> > > Can I boot from a specific FS UUID or a partition UUID without knowing a
> > > particular device ID?  (I'm not sure if my devices will always be detected
> > > in a repeatable order).
> > > 
> > > Can fatload be used with only a partition UUID?  Or can I somehow get the
> > > dev number from the UUID?
> 
> The UEFI boot manager uses device paths which comprise the partition
> GUIDs. Currently we use the full device path.
> 
> Windows and efibootmgr just store the partition and the file path in
> boot options and non-U-Boot UEFI implementation will work fine with

What do you mean by "efibootmgr"?
It sounds to be the name of U-Boot's implementation of UEFI Boot Manager.

> this. E.g.
> 
> HD(2,GPT,12345678-1234-5678-ABCD-123456789ABC,0x109000,0x32000)/File(\EFI\debian\shimx64.efi)

This is called a short path and already supported in U-Boot UEFI implementation.
See efi_dp_find_obj().

> I think this approach should also be adopted for U-Boot when the file is
> on a GPT partition.

To be clear, setting a device path in a boot option is not boot manager's
responsibility, but efidebug's, at least for now.
So even now, if efidebug is modified to allow for a short path, it should
work with current "efibootmgr" (although I have never tried it before).

# FYI, the current implementation of creating a "full" device path from
# U-Boot's probe mechanism/DM has a fatal problem of uniqueness. I have
# mentioned the issue partially in my "removable media" support RFC but
# will have to address it fully in the future.

-Takahiro Akashi

> Would this solve your problem?
> 
> Best regards
> 
> Heinrich
> 
> > 
> > As UUIDs are unique per device but our device probe order is (nominally,
> > there's counter examples) static, this would be a more board specific
> > way of going than knowing which media is which device.
> > 
>