Usage of device-tree for blobs

[Mark Kettenis]

> From: Simon Glass <sjg at chromium.org>
> Date: Thu, 26 Aug 2021 13:54:49 -0600
> 
> Hi Heinrich,
> 
> 
> On Thu, 26 Aug 2021 at 01:10, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
> >
> > On 8/26/21 5:15 AM, Simon Glass wrote:
> > > 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 following boards use OF_PRIO_STAGE:
> >
> > * QEMU
> > * bcm7260_defconfig
> > * bcm7445_defconfig
> > * ae350_rv32_defconfig
> > * ae350_rv32_spl_defconfig
> > * ae350_rv64_defconfig
> > * ae350_rv64_spl_defconfig
> 
> Most of these seem OK as they have an in-tree DT. But the arm and
> riscv qemus and the bcm builds do not:
> 
> bcm7260_defconfig
> bcm7445_defconfig
> configs/qemu_arm64_defconfig
> configs/qemu_arm_defconfig
> configs/qemu-ppce500_defconfig
> configs/qemu-riscv32_defconfig
> configs/qemu-riscv32_smode_defconfig
> configs/qemu-riscv64_defconfig
> configs/qemu-riscv64_smode_defconfig
> 
> I think we are going to have to ban this. We are not really testing
> the build at all, and it presumably depends on the version of qemu
> that is used. It's OK to provide the DT to U-Boot as one flow, but not
> to completely drop it from the tree.

So you want to have a DT in the U-Boot tree that is completely unused?

> Where is the qemu source code that creates these DTs?

For ARM the code is in:

https://github.com/qemu/qemu/blob/master/hw/arm/virt.c

> > > 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? :-)
> >
> > No.
> >
> > > 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.
> >
> > The DT provided by QEMU is not hosted anywhere. It is generated on the
> > fly by QEMU in dependence of the command line arguments that are used
> > for calling QEMU. The project is hosted at
> >
> >      https://github.com/qemu/qemu.
> 
> 404 on that. Do you have a link to the code that actually generates the DT?
> 
> >
> > On RISC-V the address of the device-tree of the prior bootstage is
> > provided in register t0.
> >
> > On ARM platforms QEMU places the device-tree at 0x40000000.
> >
> > QEMU is not the only platform where the prior boot stage supplies the
> > device-tree which is to be used for booting the operating system. Any
> > platform using TF-A or OpenSBI can be setup in this way.
> >
> > Generally it would be preferable that the prior boot stage provides the
> > device-tree. But unfortunately Linux is not always backwards compatible.
> >
> > Don't expect the device-tree of the prior stage to contain any U-Boot
> > specific quirks.
> 
> This is my concern. I think every board in U-Boot needs at least a
> basic DT, even if only a skeleton, so we have places to put things.
> Any packaging at all needs a binman node, for example. U-Boot also has
> its own options for certain things.

Most of the options are for SPL though, and these platforms don't have
that as there is other firmware that does the low-level bringup.  And
these platforms typically don't need packaging.  I believe QEMU just
needs the ELF binary, and that's also the case for how we boot the
Apple M1 using m1n1.

I suppose we could have a DT for these platforms that doesn't describe
the hardware and just does the config stuff.  But then you'll be
juggling two DTs and have to make sure that the real one is passed on
to the kernel booted by U-Boot.  But I think your idea of having
everything configured in a single DT is flawed.

> Also, where does the environment come from on these boards? Having the
> env in one tree and the DT in another must make things very
> interesting.

What environment do you mean?  The U-Boot variables are defined in the
config files, not in the DT.