Re: [PATCH v3 00/24] i.MX Media Driver

[Hans Verkuil <hverkuil@xs4all.nl>]

On 01/23/2017 12:00 PM, Philipp Zabel wrote:
> On Fri, 2017-01-20 at 21:39 +0100, Hans Verkuil wrote:
> [...]
>>> There is a VDIC entity in the i.MX IPU that performs de-interlacing with
>>> hardware filters for motion compensation. Some of the motion compensation
>>> modes ("low" and "medium" motion) require that the VDIC receive video
>>> frame fields from memory buffers (dedicated dma channels in the
>>> IPU are used to transfer those buffers into the VDIC).
>>>
>>> So one option to support those modes would be to pass the raw buffers
>>> from a camera sensor up to userspace to a capture device, and then pass
>>> them back to the VDIC for de-interlacing using a mem2mem device.
>>>
>>> Philipp and I are both in agreement that, since userland is not interested
>>> in the intermediate interlaced buffers in this case, but only the final
>>> result (motion compensated, de-interlaced frames), it is more efficient
>>> to provide a media link that allows passing those intermediate frames
>>> directly from a camera source pad to VDIC sink pad, without having
>>> to route them through userspace.
>>>
>>> So in order to support that, I've implemented a simple FIFO dma buffer
>>> queue in the driver to allow passing video buffers directly from a source
>>> to a sink. It is modeled loosely off the vb2 state machine and API, but
>>> simpler (for instance it only allows contiguous, cache-coherent buffers).
>>>
>>> This is where Philipp has an argument, that this should be done with a
>>> new API in videobuf2.
> 
> That is one part of the argument. I'm glad to understand now that we
> agree about this.
> 
>>> And I'm actually in total agreement with that. I definitely agree that there
>>> should be a mechanism in the media framework that allows passing video
>>> buffers from a source pad to a sink pad using a software queue, with no
>>> involvement from userland.
> 
> That is the other part of the argument. I do not agree that these
> software queue "links" should be presented to userspace as media pad
> links between two entities of a media device. 
> 
> First, that would limit the links to subdevices contained in the same
> media graph, while this should work between any two capture and output
> queues of different devices.
> Assume for example, we want to encode the captured, deinterlaced video
> to h.264 with the coda VPU driver. A software queue link could be
> established between the CSI capture and the VDIC deinterlacer input,
> just as between the VDIC deinterlacer output and the coda VPU input.
> Technically, there would be no difference between those two linked
> capture/output queue pairs. But the coda driver is a completely separate
> mem2mem device. And since it is not part of the i.MX media graph, there
> is no entity pad to link to.
> Or assume there is an USB analog capture device that produces interlaced
> frames. I think it should be possible to connect its capture queue to
> the VDIC deinterlacer output queue just the same way as linking the CSI
> to the VDIC (in software queue mode).
> 
> Second, the subdevice pad formats describe wire formats, not memory
> formats. The user might want to choose between 4:2:2 and 4:2:0
> subsampled YUV formats for the intermediate buffer, for example,
> depending on memory bandwidth constraints and quality requirements. This
> is impossible with the media entity / subdevice pad links.
> 
> I think an interface where userspace configures the capture and output
> queues via v4l2 API, passes dma buffers around from one to the other
> queue, and then puts both queues into a free running mode would be a
> much better fit for this mechanism.
> 
>>> My only disagreement is when this should be implemented. I think it is
>>> fine to keep my custom implementation of this in the driver for now. Once
>>> an extension of vb2 is ready to support this feature, it would be fairly
>>> straightforward to strip out my custom implementation and go with the
>>> new API.
>>
>> For a staging driver this isn't necessary, as long as it is documented in
>> the TODO file that this needs to be fixed before it can be moved out of
>> staging. The whole point of staging is that there is still work to be
>> done in the driver, after all :-)
> 
> Absolutely. The reason I am arguing against merging the mem2mem media
> control links so vehemently is that I am convinced the userspace
> interface is wrong, and I am afraid that even though in staging, it
> might become established.

As long as it is mentioned in the TODO, and ideally in the Kconfig as well,
then I'm fine with it.

The big advantage of being in the kernel is that it is much easier to start
providing fixes, improvements, etc. If you use a staging driver you know
that there is no guarantee whatsoever with respect to stable ABI/APIs.

Regards,

	Hans