[PATCH v4 14/36] [media] v4l2-mc: add a function to inherit controls from a pipeline

Steve Longerbeam slongerbeam at gmail.com
Sun Mar 12 10:56:53 PDT 2017

On 03/11/2017 11:37 PM, Russell King - ARM Linux wrote:
> On Sat, Mar 11, 2017 at 07:31:18PM -0800, Steve Longerbeam wrote:
>> On 03/11/2017 10:59 AM, Russell King - ARM Linux wrote:
>>> On Sat, Mar 11, 2017 at 10:54:55AM -0800, Steve Longerbeam wrote:
>>>> On 03/11/2017 10:45 AM, Russell King - ARM Linux wrote:
>>>>> I really don't think expecting the user to understand and configure
>>>>> the pipeline is a sane way forward.  Think about it - should the
>>>>> user need to know that, because they have a bayer-only CSI data
>>>>> source, that there is only one path possible, and if they try to
>>>>> configure a different path, then things will just error out?
>>>>> For the case of imx219 connected to iMX6, it really is as simple as
>>>>> "there is only one possible path" and all the complexity of the media
>>>>> interfaces/subdevs is completely unnecessary.  Every other block in
>>>>> the graph is just noise.
>>>>> The fact is that these dot graphs show a complex picture, but reality
>>>>> is somewhat different - there's only relatively few paths available
>>>>> depending on the connected source and the rest of the paths are
>>>>> completely useless.
>>>> I totally disagree there. Raw bayer requires passthrough yes, but for
>>>> all other media bus formats on a mipi csi-2 bus, and all other media
>>>> bus formats on 8-bit parallel buses, the conersion pipelines can be
>>>> used for scaling, CSC, rotation, and motion-compensated de-interlacing.
>>> ... which only makes sense _if_ your source can produce those formats.
>>> We don't actually disagree on that.
>> ...and there are lots of those sources! You should try getting out of
>> your imx219 shell some time, and have a look around! :)
> If you think that, you are insulting me.  I've been thinking about this
> from the "big picture" point of view.  If you think I'm only thinking
> about this from only the bayer point of view, you're wrong.

No insult there, you have my utmost respect Russel. Me gives you the
Ali-G "respec!" :)

It was just a light-hearted attempt at suggesting you might be too
entangled with the imx219 (or short on hardware access, which I can
certainly understand).

> Given what Mauro has said, I'm convinced that the media controller stuff
> is a complete failure for usability, and adding further drivers using it
> is a mistake.

I do agree with you that MC places a lot of burden on the user to
attain a lot of knowledge of the system's architecture. That's really
why I included that control inheritance patch, to ease the burden

On the other hand, I also think this just requires that MC drivers have
very good user documentation.

And my other point is, I think most people who have a need to work with
the media framework on a particular platform will likely already be
quite familiar with that platform.

> I counter your accusation by saying that you are actually so focused on
> the media controller way of doing things that you can't see the bigger
> picture here.

Yeah I've been too mired in the details of this driver.

> So, tell me how the user can possibly use iMX6 video capture without
> resorting to opening up a terminal and using media-ctl to manually
> configure the pipeline.  How is the user going to control the source
> device without using media-ctl to find the subdev node, and then using
> v4l2-ctl on it.  How is the user supposed to know which /dev/video*
> node they should be opening with their capture application?

The media graph for imx6 is fairly self-explanatory in my opinion.
Yes that graph has to be generated, but just with a simple 'media-ctl
--print-dot', I don't see how that is difficult for the user.

The graph makes it quite clear which subdev node belongs to which

As for which /dev/videoX node to use, I hope I made it fairly clear
in the user doc what functions each node performs. But I will review
the doc again and make sure it's been made explicitly clear.

> If you can actually respond to the points that I've been raising about
> end user usability, then we can have a discussion.

Right, I haven't added my input to the middle-ware discussions (libv4l,
v4lconvert, and the auto-pipeline-configuration library work). I can
only say at this point that v4lconvert does indeed sound broken w.r.t
bayer formats from your description. But it also sounds like an isolated
problem and it just needs a patch to allow passing bayer through without
software conversion.

I wish I had the IMX219 to help you debug these bayer issues. I don't
have any bayer sources.

In summary, I do like the media framework, it's a good abstraction of
hardware pipelines. It does require a lot of system level knowledge to
configure, but as I said that is a matter of good documentation.


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