[PATCH v4 5/8] sunxi: Move to bootstd

[Mark Kettenis]

> Date: Mon, 4 Nov 2024 13:34:29 +0000
> From: Andre Przywara <andre.przywara at arm.com>
> 
> On Sun, 03 Nov 2024 18:54:38 +0100
> Mark Kettenis <mark.kettenis at xs4all.nl> wrote:
> 
> Hi Mark,
> 
> > > Date: Sun, 3 Nov 2024 10:36:26 +0000
> > > From: Andre Przywara <andre.przywara at arm.com>  
> > 
> > Hi Andre,
> > 
> > > On Sat, 2 Nov 2024 11:35:31 -0600
> > > Simon Glass <sjg at chromium.org> wrote:
> > > 
> > > Hi Simon,
> > >   
> > > > On Thu, 17 Oct 2024 at 17:25, Simon Glass <sjg at chromium.org> wrote:  
> > > > >
> > > > > Drop support for distroboot and move to using bootstd instead.
> > > > >
> > > > > Signed-off-by: Simon Glass <sjg at chromium.org>
> > > > > ---
> > > > >
> > > > > (no changes since v2)
> > > > >
> > > > > Changes in v2:
> > > > > - Convert the other DISTRO_DEFAULTS in the Kconfig too
> > > > >
> > > > >  arch/arm/Kconfig | 10 +++++-----
> > > > >  1 file changed, 5 insertions(+), 5 deletions(-)    
> > > > 
> > > > Could this series be applied, please?  
> > > 
> > > So I played with that a bit:
> > > - The FEL script boot method works: it has the highest priority, which
> > >   is what we want, because the script would be deliberately injected by
> > >   the user to be executed. So thanks for the changes!
> > > - However the other global boot method (efimgr) takes precedence over
> > >   other, *higher* priority methods (extlinux.conf/boot.scr). This
> > >   leads to behavioural regressions, I think, like:
> > >   - Say there is a bootaa64.efi on some ESP on the eMMC. If you now
> > >     boot from SD card, with boot.scr or extlinux.conf on there, U-Boot
> > >     will still find the EFI app first and execute that. I don't think
> > >     that's what we want?
> > >   - Similar, if you have a boot script on the eMMC, and boot from
> > >     there, but with a USB stick carrying a bootaa64.efi plugged in.
> > >     Then EFI would win as well. This might be intended, but maybe not,
> > >     and I don't see how one would influence that?
> > > 
> > > Is there a way to change the bootflow to look for any boot payload on
> > > the *boot* media first? So when I boot from SD, it checks this first for
> > > EFI, boot.scr, extlinux.conf, then goes to eMMC and USB. Similarly, when
> > > booting from eMMC, check there first before considering SD and USB.
> > > Maybe I am missing something obvious here?  
> > 
> > The issue here is that the EFI bootmanager by its very nature
> > considers all possible devices for which boot options have been
> > included in the BootOptions EFI variable.  So if you have enabled
> > bootmeth_efi_mgr (and you probably should), and you have an active
> > boot option (that is a boot option that is included in the BootOptions
> > EFI variable) that points at eMMC or USB, it will try too boot using
> 
> I think that's the point: sunxi is one of the platforms that doesn't
> feature persistent UEFI variables, at least not when set via UEFI RT
> services. So we don't typically have any UEFI variables to help us
> point at the right boot location, which leaves us only with ...

Right.  One of the suggested solutions for this was to store EFI
variables on the ESP in a well-defined format and have the OS update
that file directly.  Ilias posted a diff for that some time ago, but I
don't think this was committed.  I did some testing of that approach
on OpenBSD and I'm fairly confident I could make it work.  But you'd
probably need support from at least a few Linux distros to make this
feasable.  Anyway...

> > that boot option.  So you'd think things would be fine as long as you
> > don't add boot options for eMMC or USB to the BootOrder variable.
> > 
> > Unfortunately the EFI_LOADER code will automatically generate media
> > device boot options for all the block devices that get added and adds
> > those to the BootOrder variables in an attempt to implement the
> > requirement to boot from removable media if everything else fails.
> 
> ... exactly removable media. So UEFI boot so far was relying on
> efi/boot/bootaa64.efi, though I think you can construct a more specific
> entry by manually setting (and storing!) this on the U-Boot shell.

You can.  But I don't think that is something you could expect the
average user to do.

> Another complication is that very often microSD cards are either the
> only or the preferred "hard disk" for those boards, and they are surely
> removable, even though in those scenarios they are treated more like
> "fixed" disks.

Well "removable media" is a bit of a misnomer here.  The current UEFI
spec says:

  Searching for a default application to boot happens on both
  removable and fixed media types.

So the fallback actually needs to happen on "fixed" disks as well.  At
least that is what most UEFI implementations do (and is what EDK2
does).  At this point OpenBSD (on all architectures, include amd64)
pretty much relies on this as we don't configure any boot options at
the moment.  And it works on 99% of the machines out there.

> The Allwinner BootROM is writing a boot source ID into SRAM, so we know
> whether we were booted from SD card, eMMC, or SPI flash, and set
> $mmc_bootdev accordingly. I need to check whether and how this is
> reflected in bootstd.
> 
> > But I think that's a mistake.  I can't find a requirement to add boot
> > options for these in the spec, let alone a requirement to add those to
> > BootOrder.  I suppose this fallback could be implemented as a separate
> > bootmeth that can be low priority.
> 
> Yeah, I also feel like splitting this up might help. So one bootmeth for
> removable media, and one BootOrder variable based.
> 
> But I think the culprit in this particular situation is this "global" flag,
> which makes it live outside of the priority scheme, IIUC:
> "When global bootmeths are available, these are typically checked before
> the above bootdev scanning."
> 
> Not sure if there is a good solution to this problem? Maybe check at least
> the boot device first, for all bootmeth's, and only then consult global
> bootmeth's? But then again we want to current behaviour for FEL script
> booting, I wonder if this could be modelled as some pseudo bootdev instead
> of marked as global (because we are booting via the BootROM's USB-OTG
> routines in this case).

To some extent, we already have a solution.  We have
BOOTMETH_EFILOADER, which is a per-device boot method which loads the
default application.  So that pretty much does what's needed for the
fallback.

So if it was possible to disabled the autogeneration of the default
media boot options you'd probably be closer to what you want for
sunxi.  I believe that'd make you lose the EFI boot menu options for
the different devices though.  So maybe the solution is for
BOOTMETH_EFI_MGR to simply skip the autogenerated options and let
BOOTMETH_EFILOADER take care of those?

Cheers,

Mark

> > > So while I am personally happy with EFI being a prominent citizen, I
> > > think many sunxi users would still expect more traditional boot methods
> > > to at least work - at the moment they might be permanently "shadowed" by
> > > some bootaa64.efi sitting *somewhere*. That actually bites me at the
> > > moment when working on a new SoC port, where I use an extlinux.conf as
> > > an override, to load a custom dev kernel and DTB, but bootstd still
> > > finds that grub on that SD card and uses that first :-(
> > > 
> > > So I feel like EFI should still be the preferred boot method, but the
> > > more custom ways should be allowed to override that.
> > > 
> > > Do you have any ideas how to solve that, or am I holding it wrong?
> > > 
> > > Cheers,
> > > Andre
> > >   
> 
>