MIPI DSI, DBI, and tinydrm drivers
Paul Cercueil
paul at crapouillou.net
Wed May 27 12:10:37 UTC 2020
Hi Neil,
Le lun. 25 mai 2020 à 16:58, Neil Armstrong <narmstrong at 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 at ffwll.ch> a
>> écrit :
>>> 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.
>>
>> 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@gmail.com)
> from dillon.minfei at 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
>>>> >
>>>> >
>>>> _______________________________________________
>>>> dri-devel mailing list
>>>> dri-devel at lists.freedesktop.org
>>>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>>>
>>>
>>>
>>> --
>>> Daniel Vetter
>>> Software Engineer, Intel Corporation
>>> +41 (0) 79 365 57 48 - http://blog.ffwll.ch
>>
>>
>> _______________________________________________
>> dri-devel mailing list
>> dri-devel at lists.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
More information about the dri-devel
mailing list