Usage of device-tree for blobs

[Sean Anderson]

On 8/26/21 3:54 PM, Simon Glass wrote:
> 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.
> 
> Where is the qemu source code that creates these DTs?
> 
>>
>> >
>> > 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?

https://github.com/qemu/qemu/blob/master/hw/riscv/virt.c#L179

> 
>>
>> 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.
> 
> 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.
> 
> Regards,
> Simon
>