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

[Tom Rini]

On Mon, Aug 23, 2021 at 11:25:40AM -0600, Simon Glass wrote:
> 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)

Well, here's where I'm a bit curious.  I have seen at least twice "wait,
EFI stub in the linux kernel adds how much?" type commentary.  I don't
know how prevalent that type of concern will really be given just how
often I don't see the Linux kernel config shrunk down from what the
reference platform started with.  And as Ilias is pointing out, you can
do _a_lot_ with efibootmgr and without grub/whatever.  And both Arm (the
company) and RISC-V (the overarching organization) are both pushing to
standardize around the idea that if you're on something bigger than an
MCU, EFI-the-ABI should get you up and running.

-- 
Tom
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