linux-kernel - Re: [PATCH] media: v4l2-subdev: Support enable/disable

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Date: Mon, 25 Mar 2024 17:52:06 +0000
From: Sakari Ailus <sakari.ailus@...ux.intel.com>
To: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Hans Verkuil <hverkuil@...all.nl>,
	Umang Jain <umang.jain@...asonboard.com>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: v4l2-subdev: Support enable/disable_streams for
 single-pad subdevs

Moi,

On Mon, Mar 25, 2024 at 03:43:01PM +0200, Tomi Valkeinen wrote:
> On 25/03/2024 15:02, Sakari Ailus wrote:
> > Moi,
> > 
> > Thanks for the patch.
> > 
> > On Mon, Mar 25, 2024 at 02:50:55PM +0200, Laurent Pinchart wrote:
> > > Hi Tomi,
> > > 
> > > On Mon, Mar 25, 2024 at 02:43:23PM +0200, Tomi Valkeinen wrote:
> > > > Currently a subdevice with a single pad, e.g. a sensor subdevice, must
> > > > use the v4l2_subdev_video_ops.s_stream op, instead of
> > > > v4l2_subdev_pad_ops.enable/disable_streams. This is because the
> > > > enable/disable_streams machinery requires a routing table which a subdev
> > > > cannot have with a single pad.
> > > > 
> > > > Implement enable/disable_streams support for these single-pad subdevices
> > > > by assuming an implicit stream 0 when the subdevice has only one pad.
> > > > 
> > > > Signed-off-by: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
> > > > ---
> > > > Even though I did send this patch, I'm not sure if this is necessary.
> > > > s_stream works fine for the subdevs with a single pad. With the upcoming
> > > > internal pads, adding an internal pad to the subdev will create a
> > > > routing table, and enable/disable_streams would get "fixed" that way.
> > 
> > I'd like to get rid of a redundant way to control streaming.
> 
> We can't get rid of it anyway, can we? We're not going to convert old
> drivers to streams.

I'd expect to do that but it'd take a long time. That being said, I think
we need to consider devices without pads (VCMs) so it may well be this
would remain after all.

> 
> For new drivers, yes, we shouldn't use s_stream. But is the answer for new
> sensor drivers this patch, or requiring an internal pad?

For new drivers I'd like to see an internal pad in fact.
{enable,disable}_streams is still internal to the kernel.

> 
> > > > So perhaps the question is, do we want to support single-pad subdevs in
> > > > the future, in which case something like this patch is necessary, or
> > > > will all modern source subdev drivers have internal pads, in which
> > > > case this is not needed...
> > > 
> > > I think the latter would be best. I however can't guarantee we won't
> > > have valid use cases for (enable|disable)_streams on single-pad subdevs
> > > though, so you patch could still be interesting.
> > 
> > Instead of the number of pads, could we use instead the
> > V4L2_SUBDEV_FL_STREAMS flag or whether g_routing op is supported to
> > determine the need for this?
> 
> Maybe, but are they better? Do you see some issue with checking for the
> number of pads? I considered a few options, but then thought that the most
> safest test for this case is 1) one pad 2) enable/disable_streams
> implemented.

I think I'd actually prefer {enable,disable}_streams in fact.

-- 
Terveisin,

Sakari Ailus