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

[Simon Glass]

Hi Ilias,

On Mon, 23 Aug 2021 at 06:35, Ilias Apalodimas
<ilias.apalodimas at linaro.org> wrote:
>
> On Thu, Aug 19, 2021 at 01:27:50PM -0400, Tom Rini wrote:
> > On Thu, Aug 19, 2021 at 08:25:33AM -0600, Simon Glass wrote:
> > > Hi Tom,
> > >
> > > On Thu, 19 Aug 2021 at 07:59, Tom Rini <trini at konsulko.com> wrote:
> > > >
> > > > On Wed, Aug 18, 2021 at 09:45:33PM -0600, Simon Glass wrote:
> > > >
> > > > > 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
> > > >
> > > > My question / concern is this.  Would the next step here be to
> > > > implement the generic UEFI boot path?  Today, I can write Fedora 34 for
> > > > AArch64 to a USB stick, boot U-Boot off of uSD card and the installer
> > > > automatically boots.  I'm sure I could do the same with the BSDs.
> > > > Reading the documentation left me with the impression that every OSV
> > > > would be expected to write something, so that their installer / OS boot.
> > > > But there's already standards for that, which they do, and we should be
> > > > implementing (and do, via the current distro_boot) or making easier to
> > > > enable.  Thanks!
> > >
> > > Here you are talking about scanning for a bootflow. It is not actually
> > > OS-specific. If it were, there would be no point to this :-)
> > >
> > > If you look in the distro scripts you will see 'scan_dev_for_efi' (and
> > > also scan_dev_for_scrips). At least the first needs to be implemented
> > > a bit like the distro boot is at present. So far I have only
> > > implemented scan_dev_for_extlinux (plus pxe) as it is enough to show
> > > the concept.
> > >
> > > Adding EFI it's likely to be about the same amount of code as distro.c
> > > at present, perhaps a little less since we don't have the network
> > > case. It is used by Fedora 34, I believe, so is easy enough for me to
> > > do.  But I wanted to get something out as the concept is visible in
> > > this series.
> >
> > OK, good.  I'd certainly like to see how this looks with EFI added.
> >
>
> What would be the order preference after scanning?

(from other thread)
With distro boot this is done with an environment variable, as I
understand it. That is not implemented in this series, but is easy to
do and is in the TODO. For now it can only be done with aliases to set
the order of the bootmethods and that requires adding to the DT, so I
don't think it scales well. I'm open to ideas though.

>
> Keep in mind that our efibootmgr is pretty complete wrt to booting.
> It can even support booting multiple OS'es without GRUB2 (even loading
> different initrds is supported).  Ideally (and assuming we want to support
> EFI booting for more devices), I would map existing extlinux configs into
> efibootmgr entries.

Yes I understand. I believe distro boot supports multiple OSes too,
but perhaps only on different media? I'm not sure. Anywayt ere are
always going to be people who don't want or need to use EFI (or grub)
and it would be nice to support both seamlessly with the same
concepts. From my side, my objective here is to provide suitable
frameworks for:

- locating devices we can boot from (via 'bootmethod' drivers)
- locating instructions from OSes to boot (called 'bootflows')

There are core features for a bootloader and U-Boot should probably
have had them some time ago.

>From what I can tell, we can make substantial improvements to the
integration of the EFI code within U-Boot, particularly when the DM
migrations are done. I see bootmethod/bootflow as part of that.

BTW in the other direction, I have a prototype of U-Boot booting on
EFI and accessing EFI devices from which to boot. It is based on
Heinrich's work for iSCSI.

Regards,
Simon