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Date: Wed, 27 Mar 2024 13:06:42 +0200
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Subject: Re: [PATCH] media: v4l2-subdev: Support enable/disable_streams for
 single-pad subdevs
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To: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
 Mauro Carvalho Chehab <mchehab@kernel.org>, Hans Verkuil
 <hverkuil@xs4all.nl>, Umang Jain <umang.jain@ideasonboard.com>,
 linux-media@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20240325-single-pad-enable-streams-v1-1-142e19896a72@ideasonboard.com>
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From: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
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On 27/03/2024 12:46, Sakari Ailus wrote:
> Heippa,
> 
> On Mon, Mar 25, 2024 at 07:56:46PM +0200, Tomi Valkeinen wrote:
>> 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@ideasonboard.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?
> 
> No, no. Not all sub-device drivers with pads are camera sensor drivers. :-)

Hmm, alright. So we want to support enable/disable_streams for 
sub-devices with multiple source pads but no routing (so probably no 
sink pads)?

>>>>>>> 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?
> 
> I'd use V4L2_SUBDEV_FL_STREAMS flag instead of the number of pads. The
> number of pads is less related to routing.

Well, with one pad you cannot have routing =).

In this patch I used sd->enabled_streams to track the enabled streams, 
but if we need to support multiple pads, I'll have to invent something 
new for that.

  Tomi