Re: RFC: DSI host capabilities (was: [PATCH RFC 03/10] drm/panel: Add LGD panel driver for Sony Xperia XZ3)

[Dmitry Baryshkov <dmitry.baryshkov@linaro.org>]

On Wed, 5 Jul 2023 at 17:24, Maxime Ripard <mripard@kernel.org> wrote:
>
> On Wed, Jul 05, 2023 at 04:37:57PM +0300, Dmitry Baryshkov wrote:
> > > > >
> > > > > Either way, I'm not really sure it's a good idea to multiply the
> > > > > capabilities flags of the DSI host, and we should just stick to the
> > > > > spec. If the spec says that we have to support DSC while video is
> > > > > output, then that's what the panels should expect.
> > > >
> > > > Except some panels supports DSC & non-DSC, Video and Command mode, and
> > > > all that is runtime configurable. How do you handle that ?
> > >
> > > In this case, most of the constraints are going to be on the encoder
> > > still so it should be the one driving it. The panel will only care about
> > > which mode has been selected, but it shouldn't be the one driving it,
> > > and thus we still don't really need to expose the host capabilities.
> >
> > This is an interesting perspective. This means that we can and actually have
> > to extend the drm_display_mode with the DSI data and compression
> > information.
>
> I wouldn't extend drm_display_mode, but extending one of the state
> structures definitely.
>
> We already have some extra variables in drm_connector_state for HDMI,
> I don't think it would be a big deal to add a few for MIPI-DSI.
>
> We also floated the idea for a while to create bus-specific states, with
> helpers to match. Maybe it would be a good occasion to start doing it?
>
> > For example, the panel that supports all four types for the 1080p should
> > export several modes:
> >
> > 1920x1080-command
> > 1920x1080-command-DSC
> > 1920x1080-video
> > 1920x1080-video-DSC
> >
> > where video/command and DSC are some kinds of flags and/or information in
> > the drm_display_mode? Ideally DSC also has several sub-flags, which denote
> > what kind of configuration is supported by the DSC sink (e.g. bpp, yuv,
> > etc).
>
> So we have two things to do, right? We need to expose what the panel can
> take (ie, EDID for HDMI), and then we need to tell it what we picked
> (infoframes).
>
> We already express the former in mipi_dsi_device, so we could extend the
> flags stored there.
>
> And then, we need to tie what the DSI host chose to a given atomic state
> so the panel knows what was picked and how it should set everything up.

This is definitely something we need. Marijn has been stuck with the
panels that support different models ([1]).

Would you prefer a separate API for this kind of information or
abusing atomic_enable() is fine from your point of view?

My vote would be for going with existing operations, with the slight
fear of ending up with another DSI-specific hack (like
pre_enable_prev_first).

>
> > Another option would be to get this handled via the bus format negotiation,
> > but that sounds like worse idea to me.
>
> Yeah, I'm not really fond of the format negociation stuff either.


[1] https://lore.kernel.org/linux-arm-msm/20230521-drm-panels-sony-v1-8-541c341d6bee@somainline.org/

-- 
With best wishes
Dmitry