Re: [RFC] Representing hardware connections via MC

[Hans Verkuil <hverkuil@xs4all.nl>]

On 03/02/2016 01:08 PM, Mauro Carvalho Chehab wrote:
> Em Wed, 02 Mar 2016 12:28:10 +0100
> Hans Verkuil <hverkuil@xs4all.nl> escreveu:
> 
>> On 03/02/16 12:16, Laurent Pinchart wrote:
>>> Hi Mauro,
>>>
>>> On Wednesday 02 March 2016 08:13:23 Mauro Carvalho Chehab wrote:  
>>>> Em Wed, 02 Mar 2016 12:34:42 +0200 Laurent Pinchart escreveu:  
>>>>> On Friday 26 February 2016 09:13:17 Mauro Carvalho Chehab wrote:  
>>>
>>> [snip]
>>>   
>>>>>> 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.  
>>>>>
>>>>> Whether to add labels to pads, and more generically how to differentiate
>>>>> them from userspace, is an interesting question. I'd like to decouple it
>>>>> from the connectors entities discussion if possible, in such a way that
>>>>> using labels wouldn't be required to leave the discussion open on that
>>>>> topic. If we foresee a dependency on labels for pads then we should open
>>>>> that discussion now.  
>>>>
>>>> We can postpone such discussion. PAD labels are not needed for
>>>> what we have so far (RF, Composite, S-Video). Still, I think that
>>>> we'll need it by the time we add connector support for more complex
>>>> connector types, like HDMI.  
>>>
>>> If we don't add pad labels now then they should be optional for future 
>>> connectors too, including HDMI. If you think that HDMI connectors will require 
>>> them then we should discuss them now.
>>>   
>>
>> Pad labels are IMHO only useful for producing human readable output. For complex
>> designs that helps a lot to understand what is going on.
>>
>> But for kernel/applications all you need are #defines with the pad numbers (e.g.
>> HDMI_PAD_TMDS, HDMI_PAD_CEC, HDMI_PAD_ARC) to use for connectors.
> 
> As we add complexity to MC graph, just hardcoding PAD numbers don't work
> fine even at the Kernel level. 
> 
> Basically, what we currently call as "PAD number", is actually a PAD
> number+type, as different PADs have different types/functions on most
> cases.
> 
> Any code that needs to connect PADs need to know which type corresponds
> to a pad number on a given entity.
> 
> See for example the code at : it only
> works because we've created a generic enum demod_pad_index that
> is been used by the analog TV demods currently supported by the
> drivers that enable the MC API.
> 
> There, we had to standardize the PAD numbers for the analog TV
> demod, as we need to be able to link the connectors for v4l2-interface
> centric devices, in order to have a generic function to build the
> links:
> 
> enum demod_pad_index {
> 	DEMOD_PAD_IF_INPUT,
> 	DEMOD_PAD_VID_OUT,
> 	DEMOD_PAD_VBI_OUT,
> 	DEMOD_NUM_PADS
> };
> 
> (I'll ommit DEMOD_NUM_PADS on the discussions below, just to make
> the enums clearer)
> 
> Due to S-Video, we'll need to add an extra input PAD there
> (and one extra PAD for audio output - currently only supported
> by au0828 driver):
> 
> enum demod_pad_index {
> 	/* Input PADs */
> 	DEMOD_PAD_IF_INPUT,	/* Composite or Y input */
> 	DEMOD_PAD_C_INPUT,
> 
> 	/* Output PADs*/
> 	DEMOD_PAD_VID_OUT,
> 	DEMOD_PAD_VBI_OUT,
> 	DEMOD_PAD_AUDIO_OUT,
> };
> 
> But, an HDMI-only demod would need, instead:
> 
> enum hdmi_demod_pad_index {
> 	/* HDMI-specific input PADs*/
> 	DEMOD_HDMI_PAD_TMDS,
> 	DEMOD_HDMI_PAD_CEC,
> 	DEMOD_HDMI_PAD_ARC,
> 
> 	/* Output PADs */
> 	DEMOD_HDMI_PAD_VID_OUT,
> 	DEMOD_HDMI_PAD_VBI_OUT,
> 	DEMOD_HDMI_PAD_AUDIO_OUT,
> };
> 
> If we do that, an extra logic to handle the "HDMI" special case
> would need at v4l2_mc_create_media_graph(), and we'll need to
> use a different function for such entity, for it to work.
> 
> A demod capable of handling both HDMI and analog TV would need a mix
> of the above enums:
> 
> enum hdmi_and_composite_demod_pad_index {
> 	/* HDMI-specific input PADs*/
> 	DEMOD2_PAD_HDMI_TMDS,
> 	DEMOD2_PAD_HDMI_CEC,
> 	DEMOD2_PAD_HDMI_ARC,
> 
> 	/* non-HDMI Input PADs */
> 	DEMOD2_PAD_IF_INPUT,	/* Composite or Y input */
> 	DEMOD2_PAD_C_INPUT,
> 
> 	/* Output PADs */
> 	DEMOD2_PAD_VID_OUT,
> 	DEMOD2_PAD_VBI_OUT,
> };
> 
> Again, we'll need an extra logic v4l2_mc_create_media_graph(), and
> a different function for the entity.
> 
> We could, instead, just add those new PADs at the existing 
> enum demod_pad_index, but, if add pad numbers to existing enums, 
> we will be breaking binary compatibility on every new release,
> and entities will be exporting PADs that the hardware don't support.
> 
> The same trouble will happen also at userspace side, as a generic
> application written to work with subdev-centric devices would need to
> know the PAD numbers for each PAD type and for each entity type.
> 
> Also, a given entity type would need to have a fixed number of PADs,
> and such pad numbers are actually part of the uAPI/ABI.
> 
> So, IMHO, the proper fix is to create, inside the Kernel, a PAD type
> field, with will be used by the Kernel generic functions and allow each
> driver to create as many PADs it needs, without needing to add
> PADs for non-supported types. So, a demod like saa7115 will never have
> DEMOD_HDMI_PAD_*. It will be driver's responsibility to fill the PAD
> type for each PAD.
> 
> The core will then use the PAD type to create the pads via
> v4l2_mc_create_media_graph().
> 
> For a generic mc-centric application, the PAD type (or PAD label?)
> will need to be exported to userspace, for the userspace logic
> that would be equivalent to what's done at v4l2_mc_create_media_graph().

I would have to think about this much more carefully. Perhaps this could be
a topic if we're having a media mini/micro summit.

I don't see the HDMI ever hooked up to a demod. It's all digital after all,
I don't think there is anything to demod. I have certainly never seen anything
like that in the various HDMI receivers/transmitters that I am familiar with.

Regards,

	Hans