[PATCH v3 30/31] bootstd: doc: Add documentation

Fri Jan 21 19:41:39 CET 2022

On Fri, Jan 21, 2022 at 07:22:55PM +0100, Mark Kettenis wrote:
> > From: Simon Glass <sjg at chromium.org>
> > Date: Fri, 21 Jan 2022 09:53:37 -0700
> > 
> > Hi Mark,
> > 
> > On Fri, 21 Jan 2022 at 09:03, Mark Kettenis <mark.kettenis at xs4all.nl> wrote:
> > >
> > > > From: Simon Glass <sjg at chromium.org>
> > > > Date: Fri, 21 Jan 2022 08:20:17 -0700
> > > >
> > > > Hi,
> > > >
> > > > On Fri, 21 Jan 2022 at 08:08, Tom Rini <trini at konsulko.com> wrote:
> > > > >
> > > > > On Wed, Jan 19, 2022 at 12:39:03PM +0100, Heinrich Schuchardt wrote:
> > > > > > On 1/19/22 02:43, Simon Glass wrote:
> > > > > > > Add documentation for this feature, including the commands and full
> > > > > > > devicetree bindings.
> > > > > > >
> > > > > > > Signed-off-by: Simon Glass <sjg at chromium.org>
> > > > > > > ---
> > > > > > >
> > > > > > > Changes in v3:
> > > > > > > - Update docs for "bootmeths" and "boot_targets" env vars
> > > > > > >
> > > > > > >   MAINTAINERS                           |   4 +
> > > > > > >   doc/develop/bootstd.rst               | 638 ++++++++++++++++++++++++++
> > > > > > >   doc/develop/distro.rst                |   3 +
> > > > > > >   doc/develop/index.rst                 |   1 +
> > > > > > >   doc/device-tree-bindings/bootdev.txt  |  18 +
> > > > > > >   doc/device-tree-bindings/bootmeth.txt |  31 ++
> > > > > > >   doc/device-tree-bindings/bootstd.txt  |   8 +
> > > > > > >   doc/usage/bootdev.rst                 | 135 ++++++
> > > > > > >   doc/usage/bootflow.rst                | 427 +++++++++++++++++
> > > > > > >   doc/usage/bootmeth.rst                | 108 +++++
> > > > > > >   doc/usage/index.rst                   |   3 +
> > > > > > >   11 files changed, 1376 insertions(+)
> > > > > > >   create mode 100644 doc/develop/bootstd.rst
> > > > > > >   create mode 100644 doc/device-tree-bindings/bootmeth.txt
> > > > > > >   create mode 100644 doc/usage/bootdev.rst
> > > > > > >   create mode 100644 doc/usage/bootflow.rst
> > > > > > >   create mode 100644 doc/usage/bootmeth.rst
> > > > > > >
> > > > > > > diff --git a/MAINTAINERS b/MAINTAINERS
> > > > > > > index 8ad70d3d968..c2af8ada3c9 100644
> > > > > > > --- a/MAINTAINERS
> > > > > > > +++ b/MAINTAINERS
> > > > > > > @@ -669,6 +669,10 @@ F:     boot/bootmeth*.c
> > > > > > >   F:        boot/bootstd.c
> > > > > > >   F:        cmd/bootdev.c
> > > > > > >   F:        cmd/bootflow.c
> > > > > > > +F: doc/develop/bootstd.rst
> > > > > > > +F: doc/usage/bootdev.rst
> > > > > > > +F: doc/usage/bootflow.rst
> > > > > > > +F: doc/usage/bootmeth.rst
> > > > > > >   F:        drivers/mmc/mmc_bootdev.c
> > > > > > >   F:        include/bootdev.h
> > > > > > >   F:        include/bootflow.h
> > > > > > > diff --git a/doc/develop/bootstd.rst b/doc/develop/bootstd.rst
> > > > > > > new file mode 100644
> > > > > > > index 00000000000..1b65a806efb
> > > > > > > --- /dev/null
> > > > > > > +++ b/doc/develop/bootstd.rst
> > > > > > > @@ -0,0 +1,638 @@
> > > > > > > +.. SPDX-License-Identifier: GPL-2.0+:
> > > > > > > +
> > > > > > > +U-Boot Standard Boot
> > > > > > > +====================
> > > > > > > +
> > > > > > > +Introduction
> > > > > > > +------------
> > > > > > > +
> > > > > > > +Standard boot provides a built-in way for U-Boot to automatically boot
> > > > > > > +an Operating System without custom scripting and other customisation. It
> > > > > > > +introduces the following concepts:
> > > > > > > +
> > > > > > > +   - bootdev  - a device which can hold or access a distro (e.g. MMC, Ethernet)
> > > > > > > +   - bootmeth - a method to scan a bootdev to find bootflows (e.g. distro boot)
> > > > > > > +   - bootflow - a description of how to boot (provided by the distro)
> > > > > > > +
> > > > > > > +For Linux, the distro (Linux distribution, e.g. Debian, Fedora) is responsible
> > > > > > > +for creating a bootflow for each kernel combination that it wants to offer.
> > > > > >
> > > > > > This gets it completely wrong. There is one standardized boot flow: UEFI.
> > > > > > All major distros support this. U-Boot has to offer UEFI booting out of the
> > > > > > box.
> > > > >
> > > > > I want to jump up and down and emphasize this part as well.  While I
> > > > > believe our UEFI bootmgr is still missing the normal scan code, that's
> > > > > something that has been promised to be implemented.  And that turns the
> > > > > bootcmd for platforms that just want to support modern off the shelf
> > > > > distros in to something fairly small.
> > > >
> > > > Sigh...
> > > >
> > > > UEFI is a bootflow in this model, one of many. If we don't support the
> > > > others, then U-Boot is not U-Boot anymore, it is just EFI Boot.
> > > >
> > > > If we get EFI bootmgr going, then are you saying you want to disable
> > > > everything else?
> > > >
> > > > You say 'major distros' but there are many that don't use it,
> > > > particularly in the embedded space. I'll go out on a limb and say that
> > > > the vast majority of embedded devices in the world don't use it. Are
> > > > you really saying we should drop support for everything else? Even the
> > > > distro stuff supports other options.
> > >
> > > And U-Boot supports a wide variety of CPUs and some of those don't
> > > even have official UEFI support.
> > >
> > > However, on arm64 (and possibly riscv64) even the embedded folks
> > > should seriously consider using the UEFI bootflow.  Linux now supports
> > > physical address randomization when loaded via the UEFI stub, which is
> > > something that can't really be implemented using the legacy boot path.
> > > Note that you don't have to use a separate UEFI bootloader as U-Boot
> > > can directly boot kernels with the UEFI stub.
> > 
> > 'legacy'? Isn't it just a case of relocating the kernel to a random
> > address? I'm pretty sure U-Boot can do that :-)
> 
> The problem is that the legacy boot protocol for the Linux arm64
> kernel requires a 2MB aligned kernel base, which reduces the number of
> randomized bits.  That also means that virtual addresses are not fully
> randomized as the kernel uses large mappings to map itself.  My
> understanding is that the UEFI stub can relocate the kernel to any 64K
> aligned address.  I suppose it is possible to add code to achieve the
> same thing for the legacy boot path, but I don't think the arm64
> maintainers are really interested in doing that.
> 
> But yes, U-Boot should certainly try to load arm64 kernels at a random
> address instead hardcoding the load address ;)

Well, it sounds like "bootefi" should be used over "booti" directly,
these days on aarch64 kernels.

-- 
Tom
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