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

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Date: Mon, 25 Mar 2024 19:56:46 +0200
From: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To: Sakari Ailus <sakari.ailus@...ux.intel.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

On 25/03/2024 19:52, Sakari Ailus wrote:
> 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, you think this patch should be dropped?

>>>>> 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.

Hmm, sorry, now I'm confused =). What do you mean with that?

  Tomi