Le dim. 24 mai 2020 à 22:14, Noralf Trønnes <noralf@xxxxxxxxxxx> a
écrit :
Den 24.05.2020 21.54, skrev Paul Cercueil:
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.
This will be a bridge implemented in software, are we allowed to have
software devices in the Device Tree? I though it was just allowed to
describe hardware.
It wouldn't appear in devicetree. If the panel is connected over SPI,
then DBI is just the protocol it uses.
If probed as a SPI device driver, the panel's spi_driver would register
an instance of the DBI/SPI host driver, then register itself as a
dbi_driver. If probed from a DBI host it would just register itself as
a dbi_driver.
-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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