Re: [PATCH v7 02/10] dt-bindings: display: simple: List hpd properties in panel-simple

From: Doug Anderson
Date: Tue May 18 2021 - 09:58:43 EST


On Tue, May 18, 2021 at 5:42 AM Rob Herring <robh+dt@xxxxxxxxxx> wrote:
> On Mon, May 17, 2021 at 3:09 PM Douglas Anderson <dianders@xxxxxxxxxxxx> wrote:
> >
> > These are described in panel-common.yaml but if I don't list them in
> > panel-simple then I get yells when running 'dt_binding_check' in a
> > future patch. List them along with other properties that seem to be
> > listed in panel-simple for similar reasons.
> If you have HPD, is it still a simple panel? I don't see this as an
> omission because the use of these properties for simple panels was
> never documented IIRC.

I would say so. It is currently supported by panel-simple in Linux. Of
course, you could make the argument that panel-simple is no longer
really "simple" because of things like this...

I guess I'd say this: I believe that the HPD properties eventually
belong in the generic "edp-panel" that I'm still planning to post (and
which will be based on this series). I justified that previously [1]
by talking about the fact that there's a single timing diagram that
(as far as I've been able to tell) is fairly universal in panel specs.
It's a complicated timing diagram showing some two dozen timings (and
includes HPD!), but if you support all the timings then you've
supported pretty much all panels. IMO the original intent of
"simple-panel" was to specify a panel that's just like all the other
panels w/ a few parameters.

NOTE: I'd also say that for nearly all eDP panels HPD is important,
but in many designs HPD is handled "magically" and not specified in
the device tree. This is because it goes to a dedicated location on
the eDP controller / bridge chip. I added the HPD GPIO support (and
no-hpd) to simple-panel because my bridge chip has a fairly useless
HPD line and we don't use it. Even though the fact that we need the
HPD specified like this is a function of our bridge chip, back in the
day I was told that the property belonged in the panel and so that's
where it lives.

> Not saying we can't add them, but justify it as an addition, not just
> fixing a warning.

Sure, I'll beef up the commit message.

> > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> > ---
> > I didn't spend tons of time digging to see if there was supposed to be
> > a better way of doing this. If there is, feel free to yell.
> That's the right way to do it unless you want to allow all common
> properties, then we'd use unevaluatedProperties instead of
> additionalProperties.

Ah, perfect. Thanks!