Re: [RFC] Representing hardware connections via MC

[Hans Verkuil <hverkuil@xs4all.nl>]

On 02/26/2016 01:13 PM, Mauro Carvalho Chehab wrote:
> We had some discussions on Feb, 12 about how to represent connectors via
> the Media Controller:
> 	https://linuxtv.org/irc/irclogger_log/v4l?date=2016-02-12,Fri&sel=31#l27
> 
> We tried to finish those discussions on the last two weeks, but people
> doesn't seem to be available at the same time for the discussions. So,
> let's proceed with the discussions via e-mail.
> 
> So, I'd like to do such discussions via e-mail, as we need to close
> this question next week.
> 
> QUESTION:
> ========
> 
> How to represent the hardware connection for inputs (and outputs) like:
> 	- Composite TV video;
> 	- stereo analog audio;
> 	- S-Video;
> 	- HDMI
> 
> Problem description:
> ===================
> 
> During the MC summit last year, we decided to add an entity called
> "connector" for such things. So, we added, so far, 3 types of
> connectors:
> 
> #define MEDIA_ENT_F_CONN_RF		(MEDIA_ENT_F_BASE + 10001)
> #define MEDIA_ENT_F_CONN_SVIDEO		(MEDIA_ENT_F_BASE + 10002)
> #define MEDIA_ENT_F_CONN_COMPOSITE	(MEDIA_ENT_F_BASE + 10003)
> 
> However, while implementing it, we saw that the mapping on hardware
> is actually more complex, as one physical connector may have multiple
> signals with can eventually used on a different way.
> 
> One simple example of this is the S-Video connector. It has internally
> two video streams, one for chrominance and another one for luminance.
> 
> It is very common for vendors to ship devices with a S-Video input
> and a "S-Video to RCA" cable.
> 
> At the driver's level, drivers need to know if such cable is
> plugged, as they need to configure a different input setting to
> enable either S-Video or composite decoding.
> 
> So, the V4L2 API usually maps "S-Video" on a different input
> than "Composite over S-Video". This can be seen, for example, at the
> saa7134 driver, who gained recently support for MC.
> 
> Additionally, it is interesting to describe the physical aspects
> of the connector (color, position, label, etc).
> 
> Proposal:
> ========
> 
> It seems that there was an agreement that the physical aspects of
> the connector should be mapped via the upcoming properties API,
> with the properties present only when it is possible to find them
> in the hardware. So, it seems that all such properties should be
> optional.
> 
> However, we didn't finish the meeting, as we ran out of time. Yet,
> I guess the last proposal there fulfills the requirements. So,
> let's focus our discussions on it. So, let me formulate it as a
> proposal
> 
> We should represent the entities based on the inputs. So, for the
> already implemented entities, we'll have, instead:
> 
> #define MEDIA_ENT_F_INPUT_RF		(MEDIA_ENT_F_BASE + 10001)
> #define MEDIA_ENT_F_INPUT_SVIDEO	(MEDIA_ENT_F_BASE + 10002)
> #define MEDIA_ENT_F_INPUT_COMPOSITE	(MEDIA_ENT_F_BASE + 10003)
> 
> The MEDIA_ENT_F_INPUT_RF and MEDIA_ENT_F_INPUT_COMPOSITE will have
> just one sink PAD each, as they carry just one signal. As we're
> describing the logical input, it doesn't matter the physical
> connector type. So, except for re-naming the define, nothing
> changes for them.

What if my device has an SVIDEO output (e.g. ivtv)? 'INPUT' denotes
the direction, and I don't think that's something you want in the
define for the connector entity.

As was discussed on irc we are really talking about signals received
or transmitted by/from a connector. I still prefer F_SIG_ or F_SIGNAL_
or F_CONN_SIG_ or something along those lines.

I'm not sure where F_INPUT came from, certainly not from the irc
discussion.

> Devices with S-Video input will have one MEDIA_ENT_F_INPUT_SVIDEO
> per each different S-Video input. Each one will have two sink pads,
> one for the Y signal and another for the C signal.
> 
> So, a device like Terratec AV350, that has one Composite and one
> S-Video input[1] would be represented as:
> 	https://mchehab.fedorapeople.org/terratec_av350-modified.png
> 
> 
> [1] Physically, it has a SCART connector that could be enabled
> via a physical switch, but logically, the device will still switch
> from S-Video over SCART or composite over SCART.
> 
> More complex devices would be represented like:
> 	https://hverkuil.home.xs4all.nl/adv7604.png
> 	https://hverkuil.home.xs4all.nl/adv7842.png
> 
> NOTE:
> 
> The labels at the PADs currently can't be represented, but the
> idea is adding it as a property via the upcoming properties API.

I think this is a separate discussion. It's not needed for now.
When working on the adv7604/7842 examples I realized that pad names help
understand the topology a lot better, but they are not needed for the actual
functionality.

> 
> Anyone disagree?

I agree except for the naming.

Regards,

	Hans