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

[Rob Herring <robh@kernel.org>]

On Mon, Dec 19, 2022 at 1:21 PM Simon Glass <sjg@chromium.org> wrote:
>
> Hi Abdellatif,
>
> On Mon, 19 Dec 2022 at 04:12, Abdellatif El Khlifi
> <abdellatif.elkhlifi@arm.com> wrote:
> >
> > On Mon, Dec 05, 2022 at 09:49:30AM -0600, Rob Herring wrote:
> > > On Sun, Dec 4, 2022 at 1:22 PM Simon Glass <sjg@chromium.org> wrote:
> > > >
> > > > Hi Rob,
> > > >
> > > > On Tue, 29 Nov 2022 at 05:22, Rob Herring <robh@kernel.org> wrote:
> > > > >
> > > > > On Fri, Nov 25, 2022 at 3:18 PM Simon Glass <sjg@chromium.org> wrote:
> > > > > >
> > > > > > Hi Abdellatif,
> > > > > >
> > > > > > On Thu, 24 Nov 2022 at 06:21, Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com> wrote:
> > > > > > >
> > > > > > > On Tue, Nov 22, 2022 at 07:09:16PM -0700, Simon Glass wrote:
> > > > > > > >  should be called 'priov' and should beHi Abdellatif,
> > > > > > > >
> > > > > >
> > > > > > [..]
> > > > > >
> > > > > > > > > +/**
> > > > > > > > > + * ffa_device_get - create, bind and probe the arm_ffa device
> > > > > > > > > + * @pdev: the address of a device pointer (to be filled when the arm_ffa bus device is created
> > > > > > > > > + *       successfully)
> > > > > > > > > + *
> > > > > > > > > + * This function makes sure the arm_ffa device is
> > > > > > > > > + * created, bound to this driver, probed and ready to use.
> > > > > > > > > + * Arm FF-A transport is implemented through a single U-Boot
> > > > > > > > > + * device managing the FF-A bus (arm_ffa).
> > > > > > > > > + *
> > > > > > > > > + * Return:
> > > > > > > > > + *
> > > > > > > > > + * 0 on success. Otherwise, failure
> > > > > > > > > + */
> > > > > > > > > +int ffa_device_get(struct udevice **pdev)
> > > > > > > > > +{
> > > > > > > > > +       int ret;
> > > > > > > > > +       struct udevice *dev = NULL;
> > > > > > > > > +
> > > > > > > > > +       ret = device_bind(dm_root(), DM_DRIVER_GET(arm_ffa), FFA_DRV_NAME, NULL, ofnode_null(),
> > > > > > > > > +                         &dev);
> > > > > > > >
> > > > > > > > Please add a DT binding. Even if only temporary, we need something for this.
> > > > > > >
> > > > > > > Thanks for the feedback. I'm happy to address all the comments.
> > > > > > >
> > > > > > > Regarding DT binding and FF-A discovery. We agreed with Linaro and Rob Herring
> > > > > > > about the following:
> > > > > > >
> > > > > > > - DT is only for what we failed to make discoverable. For hardware, we're stuck
> > > > > > >   with it. We shouldn't repeat that for software interfaces. This approach is
> > > > > > >   already applied in the FF-A kernel driver which comes with no DT support and
> > > > > > >   discovers the bus with bus_register() API [1].
> > > > > >
> > > > > > This may be the UEFI view, but it is not how U-Boot works. This is not something we are 'stuck' with. It is how we define what is present on a device. This is how the PCI bus works in U-Boot. It is best practice in U-Boot to use the device tree to make this things visible and configurable. Unlike with Linux there is no other way to provide configuration needed by these devices.
> > > > >
> > > > > Where do you get UEFI out of this?
> > > >
> > > > I assume it was UEFI as there was no discussion about this in U-Boot.
> > > > Which firmware project was consulted about this?
> > > >
> > > > >
> > > > > It is the discoverability of hardware that is fixed (and we are stuck
> > > > > with). We can't change hardware. The disoverability may be PCI
> > > > > VID/PID, USB device descriptors, or nothing. We only use DT when those
> > > > > are not sufficient. For a software interface, there is no reason to
> > > > > make them non-discoverable as the interface can be fixed (at least for
> > > > > new things like FF-A).
> > > >
> > > > Here I am talking about the controller itself, the top-level node in
> > > > the device tree. For PCI this is a device tree node and it should be
> > > > the same here. So I am not saying that the devices on the bus need to
> > > > be in the device tree (that can be optional, but may be useful in some
> > > > situations where it is status and known).
> > >
> > > Sure, the PCI host bridges are not discoverable, have a bunch of
> > > resources, and do need to be in DT. The downstream devices only do if
> > > they have extra resources such as when a device is soldered down on a
> > > board rather than a standard slot.
> > >
> > > > We need something like:
> > > >
> > > > ff-a {
> > > >     compatible = "something";
> > > > };
> > > >
> > > > I don't know what mechanism is actually used to communicate with it,
> > > > but that will be enough to get the top-level driver started.
> > >
> > > There's discovery of FF-A itself and then discovery of FF-A features
> > > (e.g. partitions). Both of those are discoverable without DT. The
> > > first is done by checking the SMCCC version, then checking for FF-A
> > > presence and features. Putting this into DT is redundant. Worse, what
> > > if they disagree?
> >
> > Hi Simon,
> >
> > Do you agree with Rob, Ilias and myself that it makes more sense
> > FF-A bus is discovered without a DT node and following the same approach as
> > Linux ? (FF-A bus doesn't have a HW controller and is a purely SW bus,
> > no configuration/description needed at DT level).
> >
> > Your suggestions are always welcome.
>
> I'm sorry I don't agree with that. It does need a compatible string,
> like PCI has. You can just add it in U-Boot if Linux won't accept the
> binding.

It's not like PCI as the host side of PCI has non-discoverable resources.

This all could have been designed better, but hindsight is 20/20 and
things evolved step by step. There are a bunch of firmware services
that are all behind SMCCC. The first (upstream) was PSCI. IIRC, SMCCC
was invented a bit after that, but generalized PSCI for other
services. Since then more have been added. More services get added one
by one and yes we added bindings for them. Because what's one more...
But that really needs to stop. We're stuck with h/w that's not
discoverable, there's zero reason to do that with s/w interfaces. If
we could redo everything, we'd have a node for SMCCC and that's it
unless there's h/w resources provided to the rest of DT. But we can't,
so SMCCC is discovered by the presence of PSCI.

Rob