Re: Using the V4L2 device kernel drivers - TC358743

Hans Verkuil
2016-01-13T07:58:14Z

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.
>
> 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.
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