Usage of device-tree for blobs

[Mark Kettenis]

> From: Simon Glass <sjg at chromium.org>
> Date: Wed, 25 Aug 2021 21:15:00 -0600
> 
> Hi Heinrich,
> 
> On Wed, 25 Aug 2021 at 02:05, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
> >
> > Hello Simon,
> >
> > some boards like qemu-riscv64_defconfig do not use any device-tree at
> > build time. A device-tree is only supplied at runtime by the prior boot
> > stage (CONFIG_OF_PRIOR_STAGE=y).
> >
> > In doc/develop/devicetree/intro.rst you suggest to put binary blobs into
> > the device-tree.
> >
> > Could you, please, update the documentation to explain how adding blobs
> > to the device-tree works in the different scenarios depending on the
> > values of:
> >
> > CONFIG_OF_EMBED
> > CONFIG_OF_SEPARATE
> > CONFIG_OF_BOARD
> > CONFIG_OF_HOSTFILE
> > CONFIG_OF_PRIOR_STAGE
> >
> > It would be especially important to understand how one can develop a
> > board independent feature which works for all of these settings.
> 
> OK I will take a crack at this tomorrow.
> 
> But I think there is a disconnect here, since the only options that
> matter within U-Boot are OF_SEPARATE and OF_HOSTFILE, which both use a
> u-boot.dtb file. There is nothing tricky here.
> 
> The OF_BOARD business is for when the board does special things.
> Presumably signing will do special things too. We cannot really know
> what those things are because the board as 'opted out' of the standard
> options.
> 
> >
> > Please, describe CONFIG_OF_PRIOR_STAGE in
> > doc/develop/devicetree/control.rst.
> 
> So I'm not allowed to delete that option? :-) It seems to me to be
> extremely sparse on documentation. We need an explanation of what it
> means and what effect it has on the build system, U-Boot and some
> discussion of how qemu works. It seems to have been added as part of
> an unrelated broadcom commit. The tags were incorrect so I doubt
> anyone noticed it. Since then it has apparently proved useful
> elsewhere, but no one has added more docs.
> 
> So perhaps you can help me with my doc by explaining how
> OF_PRIOR_STAGE works in qmeu, why the DT in U-Boot cannot be used, and
> which project actually hosts the DT that qemu provides? Armed with
> that information, I might be able to make sense of it all.

Well, QEMU allows configuration of (emulated/paravirtualised) devices
from the command line.  So it is pretty much impossible for U-Boot to
provide a DT that matches that configuration without adding a lot of
code the dynamically build it from some sort of configuration
specification provided by QEMU to U-Boot.  But at that point QEMU
might as well provide the DT itself, don't you agree?

Anyway, replying to this thread since for the Apple M1 support that
I'm working on we're in a somewhat similar situation.  The Asahi Linux
project has implemented m1n1 as a bootloader and plans to use U-Boot
as a "payload" to implement booting a standard Linux distribution (and
other OSes).  In this scenario m1n1 actually provides the DT since it
parses Apple's version of the DT (which isn't in the standard DT
format) and adds/updates certain properties that depend on the actual
hardware it is running on.

For this code I use CONFIG_OF_BOARD and implement
board_fdt_blob_setup() to simple return a pointer to the DT that m1n1
set up.  But that seems to be exactly what CONFIG_OF_PRIOR_STAGE
does...