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Date: Thu, 4 Apr 2024 16:00:22 +0300
From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Tomi Valkeinen <tomi.valkeinen@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
Subject: Re: [PATCH 2/4] media: subdev: Fix use of sd->enabled_streams in
 call_s_stream()
Message-ID: <20240404130022.GQ23803@pendragon.ideasonboard.com>
References: <20240404-enable-streams-impro-v1-0-1017a35bbe07@ideasonboard.com>
 <20240404-enable-streams-impro-v1-2-1017a35bbe07@ideasonboard.com>
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In-Reply-To: <20240404-enable-streams-impro-v1-2-1017a35bbe07@ideasonboard.com>

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() ?

I'm also getting a bit concerned about having multiple fields with
similar purposes. Another option would be to call the .s_stream()
operation directly from the fallback handlers, instead of going through
call_s_stream(). That may be considered as a bit of a hack though.

>   *
>   * 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;
>  };
>  
>  

-- 
Regards,

Laurent Pinchart