Hi Noralf,
Le dim. 24 mai 2020 à 19:46, Noralf Trønnes <noralf@xxxxxxxxxxx> a
écrit :
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.
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
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.
I don't think that should be much of a problem. You could have a
DBI/SPI bridge, that wraps a SPI device into a DBI host, for instance.
The panel drivers would just use the DBI API without having to know
what's done behind the scene.
Cheers,
-Paul
My initial tinydrm work used drm_panel, but I was not allowed to use
it
(at least not the way I had done it).
Noralf.
Cheers,
-Paul
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