Re: [PATCH v2] hw/arm: add versioning to sbsa-ref machine DT

[Peter Maydell <peter.maydell@linaro.org>]

On Tue, 17 May 2022 at 14:27, Rob Herring <robh@kernel.org> wrote:
>
> On Thu, May 5, 2022 at 6:41 AM Leif Lindholm <quic_llindhol@quicinc.com> wrote:
> >
> > The sbsa-ref machine is continuously evolving. Some of the changes we
> > want to make in the near future, to align with real components (e.g.
> > the GIC-700), will break compatibility for existing firmware.
> >
> > Introduce two new properties to the DT generated on machine generation:
> > - machine-version-major
> >   To be incremented when a platform change makes the machine
> >   incompatible with existing firmware.
> > - machine-version-minor
> >   To be incremented when functionality is added to the machine
> >   without causing incompatibility with existing firmware.
> >   to be reset to 0 when machine-version-major is incremented.
>
> Where's the binding documentation for this?
>
> We already have a way to version DTs and that's with compatible. I'm
> not completely opposed to a version number though, but I am opposed to
> it not being common. We've rejected vendors (QCom in fact) doing their
> own thing here.
>
>
> > This versioning scheme is *neither*:
> > - A QEMU versioned machine type; a given version of QEMU will emulate
> >   a given version of the platform.
> > - A reflection of level of SBSA (now SystemReady SR) support provided.
>
> FYI, it's planned to certify the virt machine for SR-IR which will
> include DT schema validation. Undocumented properties are a problem
> for that.

This isn't the 'virt' machine :-)

This dtb fragment is a purely private communication between
the QEMU model and the sbsa-ref EL3 firmware. We could in
theory equally replace it with a set of hardwired
"board revision" registers. There's a comment in the existing
sources about this:

/*
 * Firmware on this machine only uses ACPI table to load OS, these limited
 * device tree nodes are just to let firmware know the info which varies from
 * command line parameters, so it is not necessary to be fully compatible
 * with the kernel CPU and NUMA binding rules.
 */

Kernels running on sbsa-ref won't see a dtb (let alone one with this
version information in it), because firmware will always boot them
with ACPI.

thanks
-- PMM