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

From: Hans Verkuil
Date: Mon Mar 13 2017 - 06:45:09 EST

On 03/12/2017 06:56 PM, Steve Longerbeam wrote:
> 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
> somewhat.
> 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
> entity.
> 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.

And the reason we went into this direction is that the end-users that use
these SoCs with complex pipelines actually *need* this functionality. Which
is also part of the reason why work on improved userspace support gets
little attention: they don't need to have a plugin that allows generic V4L2
applications to work (at least with simple scenarios).

If they would need it, it would have been done (and paid for) before.

And improving userspace support for this isn't even at the top of our prio
list: getting the request API and stateless codec support in is our highest
priority. And that's a big job as well.

If you want to blame anyone for this, blame Nokia who set fire to their linux-based
phones and thus to the funding for this work.

Yes, I am very unhappy with the current state, but given the limited resources
I understand why it is as it is. I will try to get time to work on this this summer,
but there is no guarantee that that will be granted. If someone else is interested
in doing this and can get funding for it, then that would be very welcome.