Re: [PATCH 7/7] dt-bindings: display: add panel-timing property to sitronix,st7789v

[Maxime Ripard]

On Wed, Mar 29, 2023 at 12:08:50PM +0200, Michael Riesch wrote:
> On 3/29/23 11:16, Maxime Ripard wrote:
> > On Thu, Mar 16, 2023 at 11:29:53PM +0100, Michael Riesch wrote:
> >> Hi Rob,
> >>
> >> On 3/16/23 22:57, Rob Herring wrote:
> >>> On Tue, Mar 14, 2023 at 12:56:44PM +0100, Gerald Loacker wrote:
> >>>> The sitronix-st7789v driver now considers the panel-timing property.
> >>>
> >>> I read the patch for that and still don't know 'why'. Commit messages 
> >>> should answer why.
> >>>
> >>>> Add the property to the documentation.
> >>>
> >>> We generally don't put timings in DT for panels. Why is this one 
> >>> special?
> >>
> >> For now, having the timings in the device tree allows for setting the
> >> hsync/vsync/de polarity.
> >>
> >> As a next step, we aim to implement the partial mode feature of this
> >> panel. It is possible to use only a certain region of the panel, which
> >> is helpful e.g., when a part of the panel is occluded and should not be
> >> considered by DRM. We thought that this could be specified as timing in DT.
> >>
> >> (The hactive and vactive properties serve as dimensions of this certain
> >> region, of course. We still need to specify somehow the position of the
> >> region. Maybe with additional properties hactive-start and vactive-start?)
> >>
> >> What do you think about that?
> > 
> > I don't see why we would need the device tree to support that. What you
> > described is essentially what overscan is for HDMI/analog output, and we
> > already have everything to deal with overscan properly in KMS.
> 
> Thanks for your response, but I am afraid I don't quite follow.
> 
> How are we supposed to expose control over the hsync/vsync/data enable
> polarity? I only know that the display controller and the panel need to
> agree on a setting that works for both. What is the canonical way to do
> this?

So typically, it would come from the panel datasheet and would thus be
exposed by the panel driver. st7789v is not a panel itself but a (pretty
flexible) panel controller so it's not fixed and I don't think we have a
good answer to that (yet).

> A different question is the partial mode, for which (IIUC) you suggest
> the overscan feature. As I have never heard of this before, it would be
> very nice if you could point me to some examples. Where would the
> effective resolution be set in this case?

So, back when CRT were a thing the edges of the tube were masked by the
plastic case. HDMI inherited from that and that's why you still have
some UI on some devices (like consoles) to setup the active area of the
display.

The underlying issue is exactly what you describe: the active area is
larger than what the plastic case allows to see. I don't think anyone
ever had the usecase you have, but it would be the right solution to me
to solve essentially the same issue the same way we do on other output
types.

Maxime