Hi Neil,
Le lun. 25 mai 2020 à 16:58, Neil Armstrong narmstrong@baylibre.com a écrit :
Hi,
On 24/05/2020 21:50, Paul Cercueil wrote:
Hi Daniel,
Le dim. 24 mai 2020 à 20:35, Daniel Vetter daniel@ffwll.ch a écrit :
On Sun, May 24, 2020 at 7:46 PM Noralf Trønnes noralf@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.
I'm trying to interface a ILI9331 based panel that has a DBI/8080 interface. The ILI9331 is very similar to the ILI9341 which already has a tinydrm driver. My SoC has a dedicated DBI/DSI controller, and I have currently no way to make it work with the ingenic-drm driver.
The idea of a generic drm_panel tinydrm driver was to avoid duplicating code between regular panel and tinydrm drivers, but the focus of my email was more to point that right now there is no way to interface a DBI panel with a regular DRM driver. Unlike DSI, there are currently no drivers with DBI support as there is no API to register a host DBI driver or a DBI panel driver. This is what's really missing here.
Did you have a look at "Enable ili9341 and l3gd20 on stm32f429-disco" (http://lkml.kernel.org/r/1590378062-7965-1-git-send-email-dillon.minfei@gmai...) from dillon.minfei@gmail.com, it uses the STM32 DPI engine to feed a ili9341. Seems it would match your issue.
Note that DBI and DPI are different things. Here the ILI9341 uses SPI directly instead of a DBI API for sending its commands, which means the driver won't work on e.g. a 8080 bus.
-Paul
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.
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 :-)
Cheers, Daniel
Noralf.
Cheers, -Paul
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