Usage of device-tree for blobs

[Heinrich Schuchardt]

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

>
> 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.

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.

Best regards

Heinrich