Re: [RFC PATCH 1/3] [media] v4l2-mc.h: Add a S-Video C input PAD to demod enum

[Hans Verkuil <hverkuil@xs4all.nl>]

On 03/21/2016 07:24 PM, Javier Martinez Canillas wrote:
> Hello Hans,
> 
> On 03/21/2016 03:08 PM, Hans Verkuil wrote:
>> On 03/21/2016 06:50 PM, Mauro Carvalho Chehab wrote:
>>> Hi Hans,
>>>
>>> Em Mon, 21 Mar 2016 17:01:43 +0100
>>> Hans Verkuil <hverkuil@xs4all.nl> escreveu:
>>>
>>>>> A reasonable solution to simplify converting legacy drivers without creating
>>>>> these global ugly pad indices is to add a new video (and probably audio) op
>>>>> 'g_pad_of_type(type)' where you ask the subdev entity to return which pad carries
>>>>> signals of a certain type.  
>>>>
>>>> This basically puts a layer between the low-level pads as defined by the entity
>>>> and the 'meta-pads' that a generic MC link creator would need to handle legacy
>>>> drivers. The nice thing is that this is wholly inside the kernel so we can
>>>> modify it at will later without impacting userspace.
>>>
>>> I prepared a long answer to your email, but I guess we're not at the
>>> same page.
>>>
>>> Let be clear on my view. Please let me know where you disagree:
>>>
>>> 1) I'm not defending Javier's patchset. I have my restrictions to
>>> it too. My understanding is that he sent this as a RFC for feeding
>>> our discussions for the media summit.
>>>
>>> Javier, please correct me if I'm wrong.
>>>
> 
> That's correct. I wanted to have some patches that were aligned to what
> were discussed so far in order to have more examples to contribute in
> the media summit discussion (since I won't be there).
> 
> The patches are RFC and not meant to upstream since there are too many
> open questions. I just hoped that having more examples could help of
> them. I was specially interested in the DT bindings using OF graph to
> lookup the connectors and the level of detail there.
> 
>>> 2) I don't understand what you're calling as "meta-pads". For me, a
>>> PAD is a physical set of pins. 
>>
>> Poorly worded on my side. I'll elaborate below.
>>
>>> 3) IMO, the best is to have just one PAD for a decoder input. That makes
>>> everything simple, yet functional.
>>>
>>> In my view, the input PAD will be linked to several "input connections".
>>> So, in the case of tvp5150, it will have:
>>>
>>> 	- composite 1
>>> 	- composite 2
>>> 	- s-video
>>>
>>> 4) On that view, the input PAD is actually a set of pins. In the
>>> case of tvp5150, the pins that compose the input PADs are
>>> AIP1A and AIP1B.
>>>
>>> The output PAD is also a set of pins YOUT0 to YOUT7, plus some other
>>> pins for sync. Yet, it should, IMHO, have just one output PAD at
>>> the MC graph.
>>
>> Indeed. So a tvp5150 has three sink pads and one source pad (pixel port).
> 
> Why 3 sink pads? Are we going to model each possible connection as a PAD
> instead of an entity or are you talking about physical pins? Because if
> is the latter, then the tvp5150 has only 2 (Composite1 shares S-Video Y
> and Composite2 shares C signal).

I'd go with Mauro's proposal of a single pad. And I didn't look into detail
in the hardware specs of the tvp5150, so that's why I got it wrong.

>> Other similar devices may have different numbers of sink pads (say four
>> composite sinks and no S-Video sinks). So the pads the entity creates
>> should match what the hardware supports.
>>
>> So far, so good.
>>
> 
> I'm confused. I thought that the latest agreed approach was to model the
> actual connection signals and input pins as PADs instead of a simplied
> model that just each connection as a sink.

My opinion is to just use the simple option (one pad) if you can get away
with it. I.e. in this case adding more sink pads doesn't add any useful
information. In the case of an adv7604 it does provide useful information since
you need to program the adv7604 based on how it is hooked up.

BTW, if the tvp5150 needs to know which composite connector is connected
to which hardware pin, then you still may need to be explicit w.r.t. the
number of pads. I just thought of that.

>> If we want to create code that can more-or-less automatically create a MC
>> topology for legacy drivers, then we would like to be able to map a high-level
>> description like 'the first S-Video sink pad' into the actual pad. So you'd
>> have a 'MAP_PAD_SVID_1' define that, when passed to the g_pad_of_type() op
>> would return the actual pad index for the first S-Video sink pad (or an error
>> if there isn't one). That's what I meant with 'meta-pad' (and let's just
>> forget about that name, poor choice from my side).
>>
> 
> Can you please provide an example of a media pipeline that user-space should
> use with this approach? AFAICT whatever PADs are created when initiliazing
> the PADs for an entity, will be exposed to user-space in the media graph.
> 
> So I'm not understading how it will be used in practice. I don't mean that
> your approach is not correct, is just I'm not getting it :)

Why would userspace need to use the pads? This is for legacy drivers (right?)
where the pipeline is fixed anyway.

To quote Mauro:

"we want this for two reasons:

1) To fix the locking troubles with analog, digital and audio parts of
the driver;

2) To report userspace apps what devnodes belong to each devices."

1) is internal to the kernel and doesn't involve userspace, 2) does involve
userspace, but you won't need to know about pads to handle that.

Regards,

	Hans