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Message-ID: <1d1d5631-4758-46ad-a62b-8fe767e2c681@ideasonboard.com>
Date: Thu, 4 Apr 2024 16:06:43 +0300
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Subject: Re: [PATCH 2/4] media: subdev: Fix use of sd->enabled_streams in
 call_s_stream()
To: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>,
 Hans Verkuil <hverkuil@xs4all.nl>,
 Sakari Ailus <sakari.ailus@linux.intel.com>,
 Umang Jain <umang.jain@ideasonboard.com>, linux-media@vger.kernel.org,
 linux-kernel@vger.kernel.org
References: <20240404-enable-streams-impro-v1-0-1017a35bbe07@ideasonboard.com>
 <20240404-enable-streams-impro-v1-2-1017a35bbe07@ideasonboard.com>
 <20240404130022.GQ23803@pendragon.ideasonboard.com>
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From: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
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In-Reply-To: <20240404130022.GQ23803@pendragon.ideasonboard.com>
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On 04/04/2024 16:00, Laurent Pinchart wrote:
> Hi Tomi,
> 
> Thank you for the patch.
> 
> On Thu, Apr 04, 2024 at 01:50:01PM +0300, Tomi Valkeinen wrote:
>> call_s_stream() uses sd->enabled_streams to track whether streaming has
>> already been enabled. However,
>> v4l2_subdev_enable/disable_streams_fallback(), which was the original
>> user of this field, already uses it, and
>> v4l2_subdev_enable/disable_streams_fallback() will call call_s_stream().
>>
>> This leads to a conflict as both functions set the field. Afaics, both
>> functions set the field to the same value, so it won't cause a runtime
>> bug, but it's still wrong and if we, e.g., change how
>> v4l2_subdev_enable/disable_streams_fallback() operates we might easily
>> cause bugs.
>>
>> Fix this by adding a new field, 'streaming_enabled', for
>> call_s_stream().
>>
>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>> ---
>>   drivers/media/v4l2-core/v4l2-subdev.c | 8 ++------
>>   include/media/v4l2-subdev.h           | 2 ++
>>   2 files changed, 4 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/media/v4l2-core/v4l2-subdev.c b/drivers/media/v4l2-core/v4l2-subdev.c
>> index b90b5185e87f..3b3310bce5d4 100644
>> --- a/drivers/media/v4l2-core/v4l2-subdev.c
>> +++ b/drivers/media/v4l2-core/v4l2-subdev.c
>> @@ -404,12 +404,8 @@ static int call_s_stream(struct v4l2_subdev *sd, int enable)
>>   	 * The .s_stream() operation must never be called to start or stop an
>>   	 * already started or stopped subdev. Catch offenders but don't return
>>   	 * an error yet to avoid regressions.
>> -	 *
>> -	 * As .s_stream() is mutually exclusive with the .enable_streams() and
>> -	 * .disable_streams() operation, we can use the enabled_streams field
>> -	 * to store the subdev streaming state.
>>   	 */
>> -	if (WARN_ON(!!sd->enabled_streams == !!enable))
>> +	if (WARN_ON(!!sd->streaming_enabled == !!enable))
>>   		return 0;
>>   
>>   #if IS_REACHABLE(CONFIG_LEDS_CLASS)
>> @@ -429,7 +425,7 @@ static int call_s_stream(struct v4l2_subdev *sd, int enable)
>>   	}
>>   
>>   	if (!ret)
>> -		sd->enabled_streams = enable ? BIT(0) : 0;
>> +		sd->streaming_enabled = !!enable;
>>   
>>   	return ret;
>>   }
>> diff --git a/include/media/v4l2-subdev.h b/include/media/v4l2-subdev.h
>> index a9e6b8146279..8bd1e3c96d2b 100644
>> --- a/include/media/v4l2-subdev.h
>> +++ b/include/media/v4l2-subdev.h
>> @@ -1043,6 +1043,7 @@ struct v4l2_subdev_platform_data {
>>    *		     v4l2_subdev_enable_streams() and
>>    *		     v4l2_subdev_disable_streams() helper functions for fallback
>>    *		     cases.
>> + * @streaming_enabled: Tracks whether streaming has been enabled with s_stream.
> 
> Could you extend this to indicate this field can't be used by anything
> else than call_s_stream() ?

Yes, I'll add that.

> I'm also getting a bit concerned about having multiple fields with
> similar purposes. Another option would be to call the .s_stream()

I agree, it's getting slightly complex. The reason is that we support 
many ways for the subdev to implement these things.

> operation directly from the fallback handlers, instead of going through
> call_s_stream(). That may be considered as a bit of a hack though.

I thought about the same thing, but I felt it might just make things 
more complex. However, with my new unsent series, I add the privacy_led 
code also to the v4l2_subdev_enable_streams() function, so it's even 
more similar to the call_s_stream.

Maybe I'll try it out and see how it looks like if I call the op directly.

  Tomi

>>    *
>>    * Each instance of a subdev driver should create this struct, either
>>    * stand-alone or embedded in a larger struct.
>> @@ -1091,6 +1092,7 @@ struct v4l2_subdev {
>>   	 */
>>   	struct v4l2_subdev_state *active_state;
>>   	u64 enabled_streams;
>> +	bool streaming_enabled;
>>   };
>>   
>>   
>