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

[Russell King - ARM Linux]

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.  However, maybe the test should be:

			if (!(pad->pad.flags & MEDIA_PAD_FL_SOURCE))

    ?

(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.

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.

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
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.

-- 
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