[PATCH 00/28] Initial implementation of bootmethod/bootflow

[Mark Kettenis]

> Date: Mon, 23 Aug 2021 16:01:46 -0400
> From: Tom Rini <trini at konsulko.com>
> 
> On Mon, Aug 23, 2021 at 11:25:42AM -0600, Simon Glass wrote:
> > Hi Mark,
> > 
> > On Mon, 23 Aug 2021 at 05:54, Mark Kettenis <mark.kettenis at xs4all.nl> wrote:
> > >
> > > > From: Simon Glass <sjg at chromium.org>
> > > > Date: Wed, 18 Aug 2021 21:45:33 -0600
> > > >
> > > > Bootmethod and bootflow provide a built-in way for U-Boot to automatically boot
> > > > an Operating System without custom scripting and other customisation:
> > > >
> > > >   - bootmethod - a method to scan a device to find bootflows (owned by U-Boot)
> > > >   - bootflow - a description of how to boot (owned by the distro)
> > > >
> > > > This series provides an initial implementation of these, enable to scan
> > > > for bootflows from MMC and Ethernet. The only bootflow supported is
> > > > distro boot, i.e. an extlinux.conf file included on a filesystem or
> > > > tftp server. It works similiarly to the existing script-based approach,
> > > > but is native to U-Boot.
> > > >
> > > > With this we can boot on a Raspberry Pi 3 with just one command:
> > > >
> > > >    bootflow scan -lb
> > > >
> > > > which means to scan, listing (-l) each bootflow and trying to boot each
> > > > one (-b). The final patch shows this.
> > > >
> > > > It is intended that this approach be expanded to support mechanisms other
> > > > than distro boot, including EFI-related ones. With a standard way to
> > > > identify boot devices, these features become easier. It also should
> > > > support U-Boot scripts, for backwards compatibility only.
> > > >
> > > > The first patch of this series moves boot-related code out of common/ and
> > > > into a new boot/ directory. This helps to collect these related files
> > > > in one place, as common/ is quite large.
> > > >
> > > > Like sysboot, this feature makes use of the existing PXE implementation.
> > > > Much of this series consists of cleaning up that code and refactoring it
> > > > into something closer to a module that can be called, teasing apart its
> > > > reliance on the command-line interpreter to access filesystems and the
> > > > like. Also it now uses function arguments and its own context struct
> > > > internally rather than environment variables, which is very hard to
> > > > follow. No core functional change is included in the included PXE patches.
> > > >
> > > > For documentation, see the 'doc' patch.
> > > >
> > > > There is quite a long list of future work included in the documentation.
> > > > One question is the choice of naming. Since this is a bootloader, should
> > > > we just call this a 'method' and a 'flow' ? The 'boot' prefix is already
> > > > shared by other commands like bootm, booti, etc.
> > > >
> > > > The design is described here:
> > > >
> > > > https://drive.google.com/file/d/1ggW0KJpUOR__vBkj3l61L2dav4ZkNC12/view?usp=sharing
> > > >
> > > > The series is available at u-boot-dm/bmea-working
> > >
> > > How does the user control the order in which devices are scanned/booted?
> > 
> > That is not supported in distroboot at present, at least so far as I
> > can see. For Fedora it seems to happen in grub. Do I have that right?
> 
> Well, there's "find the next stage", which is boot_targets environment
> variable, and then "where that next stage looks for stuff" which is
> OS-dependent.  Sometimes the ESP grub.cfg file is just enough to tell
> grub to find the full grub.cfg file elsewhere, and sometimes it's a full
> grub.cfg file.  I think Mark is talking about the former, and you've
> said it's not part of this series, yet, but on the TODO list.

Right.  With the current distroboot code the order of the devices that
appears in boot_targets is determined by per-board/SOC/machine config
files and the order isn't the same for all of them.  Users can change
the order if necessary by modifying the environment variable and
saving the environment.  And for a one-off boot from a different
device they can simply run an appropriate boot command.  The
boot_targets variable in particular is documented in various install
documents so it would probably be good of the new "bootmethod" code
would respect this variable.

For OpenBSD I'm not really interested in the bootflow part.  As I
explained in the past, that part of the problem is solved in a
(mostly) uniform way across platforms by the OpenBSD bootloader which
can read an /etc/boot.conf that allows bootflow customization.  So as
long as the default of the new code still results in
\EFI\BOOT\BOOT{machine type short-name}.EFI being loaded and run if
there is no U-Boot specific bootflow configured, I'm happy.

I can't speak for the other BSDs, but my impression is that they are
pretty much in the same position.  The FreeBSD bootloader for example
supports a high-degree of "bootflow" customization and I doubt that
taking it out of the loop is a viable option for most users.

> -- 
> Tom
> 
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