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

[Maxime Ripard]

On Wed, Jul 05, 2023 at 11:09:40PM +0300, Dmitry Baryshkov wrote:
> On 05/07/2023 19:53, Maxime Ripard wrote:
> > On Wed, Jul 05, 2023 at 06:20:13PM +0300, Dmitry Baryshkov wrote:
> > > On Wed, 5 Jul 2023 at 17:24, Maxime Ripard <mripard at 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).
> > 
> > I don't think we can get away without getting access to the atomic_state
> > from the panel at least.
> > 
> > Choosing one setup over another is likely going to depend on the mode,
> > and that's only available in the state.
> > 
> > We don't have to go the whole way though and create the sub-classes of
> > drm_connector_state, but I think we should at least provide it to the
> > panel.
> > 
> > What do you think of creating a new set of atomic_* callbacks for
> > panels, call that new set of functions from msm and start from there?
> 
> We are (somewhat) bound by the panel_bridge, but yeah, it seems possible.

Bridges have access to the atomic state already, so it's another place
to plumb this through but I guess it would still be doable?

Maxime
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