[RFC] [media] imx: assume MEDIA_ENT_F_ATV_DECODER entities output video on pad 1

[Steve Longerbeam]

On 04/04/2017 04:10 PM, Russell King - ARM Linux wrote:
> On Thu, Mar 30, 2017 at 07:25:49PM +0200, Philipp Zabel wrote:
>> The TVP5150 DT bindings specify a single output port (port 0) that
>> corresponds to the video output pad (pad 1, DEMOD_PAD_VID_OUT).
>>
>> Signed-off-by: Philipp Zabel <p.zabel at pengutronix.de>
>> ---
>> I'm trying to get this to work with a TVP5150 analog TV decoder, and the
>> first problem is that this device doesn't have pad 0 as its single
>> output pad. Instead, as a MEDIA_ENT_F_ATV_DECODER entity, it has for
>> pads (input, video out, vbi out, audio out), and video out is pad 1,
>> whereas the device tree only defines a single port (0).
>
> Looking at the patch, it's highlighted another review point with
> Steve's driver.
>
>> diff --git a/drivers/staging/media/imx/imx-media-dev.c b/drivers/staging/media/imx/imx-media-dev.c
>> index 17e2386a3ca3a..c52d6ca797965 100644
>> --- a/drivers/staging/media/imx/imx-media-dev.c
>> +++ b/drivers/staging/media/imx/imx-media-dev.c
>> @@ -267,6 +267,15 @@ static int imx_media_create_link(struct imx_media_dev *imxmd,
>>  	source_pad = link->local_pad;
>>  	sink_pad = link->remote_pad;
>>
>> +	/*
>> +	 * If the source subdev is an analog video decoder with a single source
>> +	 * port, assume that this port 0 corresponds to the DEMOD_PAD_VID_OUT
>> +	 * entity pad.
>> +	 */
>> +	if (source->entity.function == MEDIA_ENT_F_ATV_DECODER &&
>> +	    local_sd->num_sink_pads == 0 && local_sd->num_src_pads == 1)
>> +		source_pad = DEMOD_PAD_VID_OUT;
>> +
>>  	v4l2_info(&imxmd->v4l2_dev, "%s: %s:%d -> %s:%d\n", __func__,
>>  		  source->name, source_pad, sink->name, sink_pad);
>
> What is "local" and what is "remote" here?  It seems that, in the case of
> a link being created with the sensor(etc), it's all back to front.
>
> Eg, I see locally:
>
> imx-media: imx_media_create_link: imx219 0-0010:0 -> imx6-mipi-csi2:0
>
> So here, "source" is the imx219 (the sensor), and sink is "imx6-mipi-csi2"
> (part of the iMX6 capture.)  However, this makes "local_sd" the subdev of
> the sensor, and "remote_sd" the subdev of the CSI2 interface - which is
> totally back to front - this code is part of the iMX6 capture system,
> so "local" implies that it should be part of that, and the "remote" thing
> would be the sensor.
>
> Hence, this seems completely confused.  I'd suggest that:
>
> (a) the "pad->pad.flags & MEDIA_PAD_FL_SINK" test in imx_media_create_link()
>     is moved into imx_media_create_links(), and placed here instead:
>
> 		for (j = 0; j < num_pads; j++) {
> 			pad = &local_sd->pad[j];
>
> 			if (pad->pad.flags & MEDIA_PAD_FL_SINK)
> 				continue;
>
> 			...
> 		}
>
>     as the pad isn't going to spontaneously change this flag while we
>     consider each individual link.


Sure, I can do that. It would avoid iterating unnecessarily through the
pad's links if the pad is a sink.


>  However, maybe the test should be:
>
> 			if (!(pad->pad.flags & MEDIA_PAD_FL_SOURCE))
>
>     ?
>

maybe that is more intuitive.


> (b) the terms "local" and "remote" in imx_media_create_link() are
>     replaced with "source" and "sink" respectively, since this will
>     now be called with a guaranteed source pad.

Agreed. I'll change the arg and local var names.

>
> As for Philipp's solution, I'm not sure what the correct solution for
> something like this is.  It depends how you view "hardware interface"
> as defined by video-interfaces.txt, and whether the pads on the TVP5150
> represent the hardware interfaces.  If we take the view that the pads
> do represent hardware interfaces, then using the reg= property on the
> port node will solve this problem.

And the missing port nodes would have to actually be defined first.
According to Philipp they aren't, only a single output port 0.


>
> If not, it would mean that we would have to have the iMX capture code
> scan the pads on the source device, looking for outputs - but that
> runs into a problem - if the source device has multiple outputs, does
> the reg= property specify the output pad index or the pad number,

And how do we even know the data direction of a DT port. Is it an input,
an output, bidirectional? The OF graph parsing in imx-media-of.c can't
determine a port's direction if it encounters a device it doesn't
recognize that has multiple ports. For now that is not really a problem
because upstream from the video mux and csi-2 receiver it's expected
there will only be original sources of video with only one source port.
But it can become a limitation later. For example a device that has
multiple output bus interfaces, which would require multiple output
ports.


Steve



> and what if one binding for a device specifies it one way and another
> device's binding specifies it a different way.
>
> There's lots of scope for making things really painful here, and
> ending up with needing sensor-specific code in capture drivers to
> work around different decisions on this.
>
> I think someone needs to nail this down, if it's not already too late.
>