Re: [PATCH v2] Documentation: acpi: Add an example for PRP0001

[Thomas Preston]

On 25/03/2019 16:34, Andy Shevchenko wrote:
> On Mon, Mar 25, 2019 at 03:58:13PM +0000, Thomas Preston wrote:
> > On 25/03/2019 15:30, Andy Shevchenko wrote:
> > > On Mon, Mar 25, 2019 at 03:12:10PM +0000, Thomas Preston wrote:
> > > > Add an example for the magic PRP0001 device ID which allows matching
> > > > ACPI devices against drivers using OF Device Tree compatible property.
> > >
> > > > It wasn't clear to me that PRP0001 could be used in _CID.
> > >
> > > Yes, but it's not necessary to have it if we have defined a _HID.
> > >
> > > In that case PRP0001 is a temporary stub until corresponding driver
> > > incorporates an official _HID.
> > >
> > > On the contrary, when there is no official _HID available, PRP0001 can be used
> > > instead directly as a _HID and no _CID is needed.
> >
> > In that case, how do we uniquely identify devices in sysfs?
>
> By their class, etc.
>
> Identifying devices based in instance name is bad idea to start with.
>
> > We have a
> > case where sound/soc/soc-core.c generates an i2c codec name i2c-TDA7802:00.
> > It is useful to identify that device by part name, rather than some indexed
> > generic PRP device i2c-PRP0001:04. I can achieve this with:
>
> If TDA7802 is *official* _HID dedicated for that codec by vendor, add it to the
> driver...
>
> > 	_HID("TDA7802")
>
> > 	_CID("PRP0001")
> > 	...
> > 	compatible = "st,tda7802" // driver I want to load using OF DT
>
> ...and these lines become unneeded.
>
> The only case when both are needed is a time between one gets a case and actual
> ID appears in the upstream driver. Effectively means product developing stage.
>
> P.S. Yes, I know that sometimes the platform/BIOS vendors abuse specification
> and ACPI ID registry and made up IDs, in that case we need to support them as a
> "de facto" quirks.
>
> And yes, ASoC subsystem in ACPI case abuses Linux device hierarchy by matching
> by instance instead of matching by let say fwnode. It should be fixed there,
> not in ACPI table.

Okay this is all really useful information. Thank you.

> > > I would really recommend to look at the examples in meta-acpi repository. There
> > > are cases like described above.
> > >
> > > This one is good enough, though see below.
> >
> > I will drop the superfluous _CID - I think the example is still useful
> > to have. Even if we don't illustrate my special case for _HID/_CID.
>
> Yes, I agree with that.

I will remove _CID weirdness, and resend. I will deal with my special case
elsewhere. Thanks again