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

Fri Jan 13 11:44:15 CET 2023

On Thu, Jan 12, 2023 at 04:43:32PM -0700, Simon Glass wrote:
> Hi Rob,
> 
> On Wed, 11 Jan 2023 at 19:10, Rob Herring <robh at kernel.org> wrote:
> >
> > On Mon, Dec 19, 2022 at 1:21 PM Simon Glass <sjg at chromium.org> wrote:
> > >
> > > Hi Abdellatif,
> > >
> > > On Mon, 19 Dec 2022 at 04:12, Abdellatif El Khlifi
> > > <abdellatif.elkhlifi at 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 at chromium.org> wrote:
> > > > > >
> > > > > > Hi Rob,
> > > > > >
> > > > > > On Tue, 29 Nov 2022 at 05:22, Rob Herring <robh at kernel.org> wrote:
> > > > > > >
> > > > > > > On Fri, Nov 25, 2022 at 3:18 PM Simon Glass <sjg at chromium.org> wrote:
> > > > > > > >
> > > > > > > > Hi Abdellatif,
> > > > > > > >
> > > > > > > > On Thu, 24 Nov 2022 at 06:21, Abdellatif El Khlifi <abdellatif.elkhlifi at 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.
> 
> OK I see. It is certainly an edge case.
> 
> >
> > 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.
> 
> I understand the background here, but if we don't take a stand on
> this, this sort of thing will continue. Just because something works
> in Linux does not mean that the binding (or lack of it) is good.
> 
> The reasons to do this are:
> - avoids needing to manually call device_bind()
> - avoids extra plumbing in U-Boot
> - provides visibility into what is in the system, by looking at the
> DT, like documentation
> - DT is how devices are bound in U-Boot
> 
> You can see the problem if you look at ffa_device_get(). It is called
> from ffa_bus_discover() which is a new addition into the board_init
> list. We are trying to remove this list and certainly don't want new
> things added!!

Hi Simon,

As stated in the v8 patchset cover letter [1] and readme [2], the FF-A bus is discoverable on demand at runtime.

Clients (such as EFI) can discover the FF-A bus using ffa_bus_discover() API which triggers the
discovery process.

We no longer use the board_init list to discover the FF-A bus.

Please refer to the v8 patchset for the review.

Cheers

[1]: https://lore.kernel.org/all/20221122131751.22747-1-abdellatif.elkhlifi@arm.com/
[2]: https://lore.kernel.org/all/20221122131751.22747-4-abdellatif.elkhlifi@arm.com/#Z31doc:arch:arm64.ffa.rst

> 
> We don't need to change this in the Linux implementation, just add a
> top-level DT node for U-Boot. I don't understand why that is such a
> big problem?
> 
> Regards,
> Simon