[PATCH 4/5] pxe_utils: Support a backup for localboot

[Simon Glass]

Hi Tom,

On Fri, 20 Dec 2024 at 10:23, Tom Rini <trini at konsulko.com> wrote:
>
> On Fri, Dec 20, 2024 at 10:18:17AM -0700, Simon Glass wrote:
> > Hi Tom,
> >
> > On Fri, 20 Dec 2024 at 07:56, Tom Rini <trini at konsulko.com> wrote:
> > >
> > > On Thu, Dec 19, 2024 at 09:01:19PM -0700, Simon Glass wrote:
> > >
> > > > The current localboot implementation assumes that a 'localcmd'
> > > > environment variable is provided, with the instructions to follow. This
> > > > may not be included, so provide a fallback in that case.
> > > >
> > > > Add a test image and test as well.
> > > >
> > > > Signed-off-by: Simon Glass <sjg at chromium.org>
> > >
> > > This is a pretty niche feature, I had to dig around a bit to see how
> > > it's specified elsewhere (not really) and how it's used. And I think
> > > that based on how it's used, making up a bootcmd when localcmd is
> > > undefined is the wrong approach. It's the hook for "run what I defined
> > > in the environment", so if not set erroring back out seems appropriate.
> >
> > Yes, but unfortunately it seems to be used and we should support it.
> > The problem with scripts is that we don't know the boot device, etc,
> > so it needs to be integrated into PXE. I did consider putting
> > something in bootstd, but we only find out that it is requesting a
> > localboot when actually running the extlinux bootmeth, so this is what
> > I came up with.
> >
> > It will be interesting to see if any other cases come up.
>
> It would be helpful at this point I think if you can point to how the
> code for handling this case (the LOCALBOOT keyword followed by an
> integer) in other projects so that we can be compliant with what's
> expected, even if it's poorly documented.

Yes, it seems very hard to find anything at all.

The implementation in syslinux uses the BIOS to jump to a boot sector:

https://repo.or.cz/syslinux.git/blob/HEAD:/core/bios/localboot.c

I'm really not sure how it is supposed to work with a filesystem.

[1] is a usage of it for rpi

But other than that, even Gemini doesn't seems to have much idea.

Regards,
Simon

[1] https://github.com/ppisa/rpi-utils/blob/master/u-boot-setup/boot/extlinux/extlinux.conf