Re: [PATCH] hw/arm/virt: Allow additions to the generated device tree

From: Simon Glass <sjg@chromium.org>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	qemu-arm <qemu-arm@nongnu.org>,
	QEMU Developers <qemu-devel@nongnu.org>,
	Eduardo Habkost <ehabkost@redhat.com>
Subject: Re: [PATCH] hw/arm/virt: Allow additions to the generated device tree
Date: Mon, 27 Sep 2021 09:17:51 -0600	[thread overview]
Message-ID: <CAPnjgZ1AO8575LYbwmuouNR5=dgNam6EFDC1_bai=8xAHdw7-Q@mail.gmail.com> (raw)
In-Reply-To: <CAFEAcA9n+2JDFv8BezeEMA4e2Zhr0ENAvzLjBS6YXRoW9P-JXQ@mail.gmail.com>

Hi Peter,

On Mon, 27 Sept 2021 at 02:48, Peter Maydell <peter.maydell@linaro.org> wrote:
>
> On Sun, 26 Sept 2021 at 19:55, Simon Glass <sjg@chromium.org> wrote:
> > In the case of U-Boot at least, it uses the devicetree for
> > configuration (it is a boot loader, so there is no user space to
> > provide configuration). So the current setup is not sufficient to boot
> > it correctly in all cases. On the other hand, the 'virt' feature is
> > very useful for testing U-Boot, so it would be great to be able to
> > support this.
>
> So what is missing in the QEMU-provided DTB that it needs?

Quite a lot. Here are some examples:

U-Boot has limited pre-relocation memory so tries to avoid
binding/probing devices that are not used before relocation:

https://u-boot.readthedocs.io/en/latest/develop/driver-model/design.html#pre-relocation-support

There is a configuration node (which is likely to change form in
future releases, but will still be there)

https://github.com/u-boot/u-boot/blob/master/doc/device-tree-bindings/config.txt

Then there are various features which put things in U-Boot's control
dtb, such as verified boot, which adds public keys during signing:

https://github.com/u-boot/u-boot/blob/master/doc/uImage.FIT/signature.txt#L135

More generally, the U-Boot tree has hundreds of files which add
properties for each board, since we try to keep the U-Boot-specific
things out of the Linux tree:

$ find . -name *u-boot.dtsi |wc -l
398

Quite a bit of this is to do with SPL and so far it seems that QEMU
mostly runs U-Boot proper only, although I see that SPL is starting to
creep in too in the U-Boot CI.

So at present QEMU is not able to support U-Boot fully. It would be
great to add this as we use QEMU heavily in CI testing, e.g. see the
second column here:

https://source.denx.de/u-boot/custodians/u-boot-dm/-/pipelines/9260

Regards,
Simon