Re: [PATCH v8 03/10] arm_ffa: introduce Arm FF-A low-level driver

Simon Glass
2023-01-20T22:04:53Z

Hi Rob,

On Thu, 19 Jan 2023 at 11:11, Rob Herring <robh@kernel.org> wrote:
>
> On Thu, Jan 19, 2023 at 10:41 AM Simon Glass <sjg@chromium.org> wrote:
> >
> > Hi Abdellatif,
> >
> > On Thu, 19 Jan 2023 at 09:32, Abdellatif El Khlifi
> > <abdellatif.elkhlifi@arm.com> wrote:
> > >
> > > On Wed, Jan 18, 2023 at 08:59:32AM -0500, Tom Rini wrote:
> > > > On Wed, Jan 18, 2023 at 01:46:54PM +0000, Sudeep Holla wrote:
> > > > > On Wed, Jan 18, 2023 at 12:49 PM Tom Rini <trini@konsulko.com> wrote:
> > > > >
> > > > > >
> > > > > > I guess the problem comes down to, can we have one discovery method that
> > > > > > everyone shares, or do we have to let everyone invent a new discovery
> > > > > > method every time?
> > > > >
> > > > >
> > > > > No one needs to invent any discovery method every time if the firmware
> > > > > specification
> > > > > provides one and as Rob mentioned many times in the thread, all new firmware
> > > > > specification must provide one and we are trying to make sure that is
> > > > > the case with all new
> > > > > specs from Arm.
> > > > >
> > > > >
> > > > > > FF-A, Op-tee, U-Boot, coreboot, barebox (and
> > > > > > everyone else I'm unintentionally forgetting) could just discover these
> > > > > > things via device tree.
> > > > >
> > > > >
> > > > > I leave that to the individual projects to decide and agree but
> > > > > fundamentally if
> > > > > the specification provides a way to discover, not sure why we are even
> > > > > discussing
> > > > > an alternative method here.
> > > > >
> > > > >
> > > > > > Or, we could all write our own code to perform
> > > > > > the discovery.
> > > > >
> > > > >
> > > > > For what reason ? I can understand if there is no discovery mechanism but
> > > > > that's not the
> > > > > case in $subject.
> > > > >
> > > > >
> > > > > > And when RISC-V comes along with similar functionality,
> > > > > > we could probe their device tree and see they've implemented the same
> > > > > > concept, but a little differently, but still have the discovery portion
> > > > > > be in the device tree. To which it sounds like your answer is "not in
> > > > > > the device tree".
> > > > > >
> > > > >
> > > > > I see U-boot seem to have made a decision to create DT node for each and
> > > > > everything
> > > > > that needs to be added to DM which seems bit unfortunate but I don't
> > > > > understand the
> > > > > history/motive/background for it but I respect the decision if it is
> > > > > already made.
> > > > >
> > > > > These firmware interfaces are standard on all Arm platforms and can be
> > > > > discovered
> > > > > based on PSCI/SMCCC. Not using the same and use DT node needs unnecessary
> > > > > addition of DT nodes for all the f/w i/f on all the platforms that need the
> > > > > support when
> > > > > one can be just discovered.
> > > > >
> > > > > Sorry for the sudden appearance on this thread, I was avoiding getting into
> > > > > this but thought
> > > > > I will at least express my opinion and also the way the firmware
> > > > > specifications from Arm is
> > > > > expected to be evolved from now on. With that I will leave it to you and
> > > > > other U-boot
> > > > > maintainers and the community in general to decide the right course in this
> > > > > case.
> > > >
> > > > To be clear, if the position is that "this is what everyone else will
> > > > use, really" then yes, we'll follow this in U-Boot.
> > >
> > > Hi Simon, Tom,
> > >
> > > The FF-A transport is a SW bus and is not associated to any HW peripheral or
> > > undiscoverable base address.
> > >
> > > There is only 1 way of discovering the FF-A bus and it's through the FF-A SW
> > > interfaces. The FF-A spec [1] describes this in details.
> >
> > Can you add a DT node for the 'FF-A SW interfaces' and attach some
> > sort of top-level driver to that? Perhaps simple-bus, or your own
> > thing? You don't need to add compatible strings for subnodes (devices
> > that are discoverable within that).
>
> We already have that. It's just called 'arm,psci'. FF-A is not the
> top-level thing. SMCCC is. That's unfortunately called PSCI in DT
> because SMCCC grew out of PSCI. Evolution is ugly...
>
> It's like this:
>
> SMCCC
> +--PSCI
> +--TRNG
> +--FF-A
> +--SCMI (sometimes)
> +--OP-TEE
> +--...Whatever Arm comes up with next...

OK well that sounds OK.

So what is the problem here? We have an SMCCC top-level thing in the
DT and everything else can be bound from that, right? Are people on
this thread not aware of this...or am I still missing something?

Can you point to the SMCCC driver in U-Boot? Is this
bind_smccc_features(), i.w.c. it looks like it does what I want...why
does this thread exist?

So confused...:-)

Regards,
Simon