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

Ilias Apalodimas
2022-11-28T16:27:27Z

Hi all

On Mon, 28 Nov 2022 at 18:22, 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?
>
> 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).

I'll agree with Rob here. In fact the first version of the patchset
*did* have this as a DT node. We explicitly asked Abdellatif to
change this, so u-boot and the linux kernel can have an identical
approach in discovering FF-A

Regards
/Ilias

>
> Rob