Re: Using the V4L2 device kernel drivers - TC358743

Dave Stevenson
2016-01-13T20:42:56Z

On 13/01/2016 11:38, Mauro Carvalho Chehab wrote:
> Em Wed, 13 Jan 2016 08:58:08 +0100
> Hans Verkuil <hverkuil@xs4all.nl> escreveu:
>
>> Hi Dave,
>>
>> On 01/12/2016 09:38 PM, Dave Stevenson wrote:
>>> Hi All.
>>>
>>> Apologies for what feels like such a newbie question, but I've failed to
>>> find useful information elsewhere.
>>>
>>> I'm one of the ex-Broadcom developers who is still supporting Raspberry
>>> Pi, although I'm not employed by Pi Foundation or Trading.
>>> My aim is to open up that platform by exposing the CSI2 receiver block
>>> (and eventually parts of the ISP) via V4L2. The first use case would be
>>> for the Toshiba TC358743 HDMI to CSI2 converter, but it should be
>>> applicable to any of the other device drivers too.
>>> Sadly it probably won't be upstreamable as it will require the GPU to do
>>> most of the register poking to avoid potential IP issues (Broadcom not
>>> having released the docs for the relevant hardware blocks). In that
>>> regard it will be fairly similar to the existing V4L2 driver for the Pi
>>> camera.
>
> Hmm.... Broadcom wrote a new GPU driver (vc4) that was recently
> upstreamed:
> https://wiki.freedesktop.org/dri/VC4/
>
> Maybe its driver could be used/modified to cope with a V4L2 driver.

Broadcom released the IP details for the 3D graphics side
(http://www.broadcom.com/docs/support/videocore/VideoCoreIV-AG100-R.pdf). Nothing
for the imaging pipe, codecs, or other VideoCore hardware blocks.
They did make a slight whoopsie in that the register names and offsets
for all blocks were also released. Sadly the bit-usage and descriptions
weren't :-(

I'm actually waiting for the complaints of the camera not working with
that new stack - the AWB algorithm is run on the 3D processing units as
they support vector floating point operations. Having the VideoCore
processor and the ARM programming up those units is currently going to
break :-(

>>>
>>> There is now the driver for the TC358743 in mainline, but my stumbling
>>> block is finding a useful example of how to actually use it. The commit
>>> text by Mats Randgaard says it was "tested on our hardware and all the
>>> implemented features works as expected", but I don't know what that
>>> hardware was or how it was used.
>>
>> It's for Cisco video conferencing equipment, but it basically boils down to
>> capturing HDMI input over a CSI2 bus. I believe it's on an omap4.
>>
>> I know Philip Zabel also developed for the TC358743. Philip, do you have a
>> git tree available that shows how it is used?
>>
>>> The media controller API seems to be part of the answer, but that seems
>>> to be a large overhead for an application to have to connect together
>>> multiple sub-devices when it is only interested in images out the back.
>>
>> The MC is only needed if you have hardware that allows for complex and/or
>> dynamic internal video routing. For a standard linear video pipeline it
>> is not needed.
>>
>>> Is there something that sets up default connections that I'm missing?
>>> Somewhere within device tree?
>>
>> Typically the bridge driver (i.e. the platform driver that sets up the
>> pipeline and creates the video devices) will use the device tree to find
>> the v4l2-subdevice(s) it has to load and hooks them into the pipeline.
>>
>> drivers/media/platform/am437x/am437x-vpfe.c looks to be a decent example
>> of that.
>>
>> Of course, if you have more complex pipelines, then you need to support
>> the MC.
>>
>>>
>>> I have looked at the OMAP4 ISS driver as a vaguely similar device, but
>>> that seemingly covers the image processing pipe only, not hooking in to
>>> the sensor drivers.
>>
>> Sensor drivers are hooked in in function iss_register_entities(), see the
>> section for "/* Register external entities */".
>>
>>> I've also got a slight challenge in that ideally I want the GPU to
>>> allocate the memory, and ARM map that memory (we already have a service
>>> to do that), but I can't see how that would fit in with the the existing
>>> videobuf modes. Any thoughts on how I might be able to support that? The
>>> existing V4L2 driver ends up doing a full copy of every buffer from GPU
>>> memory to ARM, which isn't great for performance.
>>> There may be an option to use contiguous memory and get the GPU to map
>>> that, but it's more involved as I don't believe the supporting code is
>>> on the Pi branch.
>>
>> The proper way to do this is that the GPU can export buffers as a DMABUF file
>> descriptor, then import them in V4L2 (V4L2_MEMORY_DMABUF). The videobuf2
>> v4l2 framework will handle all the details for you, so it is trivial on
>> the v4l2 side.
>>
>> If the GPU doesn't support dmabuf, then I'm not sure if you can do this
>> without horrible hacks.
>>
>> Regards,
>>
>> Hans
>>
>>>
>>> Any help much appreciated.
>>>
>>> Thanks.
>>> Dave
>>>
>>> PS If those involved in the TC358743 driver are reading, a couple of
>>> quick emails over the possibility of bringing the audio in over CSI2
>>> rather than I2S would be appreciated. I can split out the relevant CSI2
>>> ID stream, but have no idea how I would then feed that through the
>>> kernel to appear via ALSA.
>>> --
>>> To unsubscribe from this list: send the line "unsubscribe linux-media" in
>>> the body of a message to majordomo@vger.kernel.org
>>> More majordomo info at http://vger.kernel.org/majordomo-info.html
>>>
>>
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-media" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at http://vger.kernel.org/majordomo-info.html
>