MIPI DSI, DBI, and tinydrm drivers

From: Paul Cercueil <paul@crapouillou.net>
To: dri-devel <dri-devel@lists.freedesktop.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Subject: MIPI DSI, DBI, and tinydrm drivers
Date: Sun, 24 May 2020 18:13:16 +0200	[thread overview]
Message-ID: <4QFUAQ.UPWBIKSUSOG@crapouillou.net> (raw)

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.

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.

Cheers,
-Paul

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