MIPI DSI, DBI, and tinydrm drivers

[Emil Velikov]

On Sun, 24 May 2020 at 19:35, Daniel Vetter <daniel at ffwll.ch> wrote:
>
> On Sun, May 24, 2020 at 7:46 PM Noralf Trønnes <noralf at tronnes.org> wrote:
> >
> >
> >
> > Den 24.05.2020 18.13, skrev Paul Cercueil:
> > > Hi list,
> > >
> > > I'd like to open a discussion about the current support of MIPI DSI and
> > > DBI panels.
> > >
> > > Both are standards from the MIPI alliance, both are communication
> > > protocols between a LCD controller and a LCD panel, they generally both
> > > use the same commands (DCS), the main difference is that DSI is serial
> > > and DBI is generally parallel.
> > >
> > > In the kernel right now, DSI is pretty well implemented. All the
> > > infrastucture to register a DSI host, DSI device etc. is there. DSI
> > > panels are implemented as regular drm_panel instances, and their drivers
> > > go through the DSI API to communicate with the panel, which makes them
> > > independent of the DSI host driver.
> > >
> > > DBI, on the other hand, does not have any of this. All (?) DBI panels
> > > are implemented as tinydrm drivers, which make them impossible to use
> > > with regular DRM drivers. Writing a standard drm_panel driver is
> > > impossible, as there is no concept of host and device. All these tinydrm
> > > drivers register their own DBI host as they all do DBI over SPI.
> > >
> > > I think this needs a good cleanup. Given that DSI and DBI are so
> > > similar, it would probably make sense to fuse DBI support into the
> > > current DSI code, as trying to update DBI would result in a lot of code
> > > being duplicated. With the proper host/device registration mechanism
> > > from DSI code, it would be possible to turn most of the tinydrm drivers
> > > into regular drm_panel drivers.
>
> Do we have drivers with dbi support that actually want to reuse the
> tinydrm drivers? Good clean is all good, but we need a solid reason
> for changing stuff. Plus we need to make sure we're not just
> rediscovering all the old reasons for why we ended up where we are
> right now in the first place.
>
> > > The problem then is that these should still be available as tinydrm
> > > drivers. If the DSI/DBI panels can somehow register a .update_fb()
> > > callback, it would make it possible to have a panel-agnostic tinydrm
> > > driver, which would then probably open a lot of doors, and help a lot to
> > > clean the mess.
> > >
> > > I think I can help with that, I just need some guidance - I am fishing
> > > in exotic seas here.
> > >
> > > Thoughts, comments, are very welcome.
> >
> > I did look at this a few months back:
> >
> > drm/mipi-dbi: Support panel drivers
> > https://lists.freedesktop.org/archives/dri-devel/2019-August/228966.html
> >
Coming late to the party - the series looks like a great step forward.

> > The problem with DBI is that it has reused other busses which means we
> > don't have DBI drivers, we have SPI drivers instead (6800/8080 is not
> > avail. as busses in Linux yet). DSI and DPI on the other hand has
> > dedicated hw controller drivers not shared with other subsystems.
> >
> > My initial tinydrm work used drm_panel, but I was not allowed to use it
> > (at least not the way I had done it).
>
> Hm, do we have a summary of all the discussions/reasons from back
> then? All I remember is that it's all that simple, you've done a lot
> of work exploring all the options, I'm fairly sure I suggested
> drm_panel even back then but somehow it didn't really work. Would be
> good if we make sure we don't at least repeat history too much :-)
>
This pretty much ^^. Does anyone have a link/summary of the concerns?

>From userspace POV - having these as panel makes sense.
Currently as new tiny drm _driver_ gets added, userspace has to be
updated to deal with it ... every so often.

Additionally having both DPI and DBI code for the given panel
alongside one another makes the overall picture clearer.

-Emil
Aside: mipi_dbi API should grow a drm_ prefix.