Re: [PATCH 1/3] media: uvcvideo: Support partial control reads

[Sakari Ailus]

Hi Ricardo,

On Tue, Oct 08, 2024 at 07:06:14AM +0000, Ricardo Ribalda wrote:
> Some cameras, like the ELMO MX-P3, do not return all the bytes
> requested from a control if it can fit in less bytes.
> Eg: Returning 0xab instead of 0x00ab.
> usb 3-9: Failed to query (GET_DEF) UVC control 3 on unit 2: 1 (exp. 2).
> 
> Extend the returned value from the camera and return it.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: a763b9fb58be ("media: uvcvideo: Do not return positive errors in uvc_query_ctrl()")

Is this really the patch that introduced the problem?

> Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
> ---
>  drivers/media/usb/uvc/uvc_video.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index cd9c29532fb0..853dfb7b5f7b 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -79,11 +79,16 @@ int uvc_query_ctrl(struct uvc_device *dev, u8 query, u8 unit,
>  	if (likely(ret == size))
>  		return 0;
>  
> +	if (ret > 0 && ret < size) {
> +		memset(data + ret, 0, size - ret);

It'd be nice to have a comment in the code why this is being done
(including it's little endian).

> +		return 0;
> +	}
> +
>  	if (ret != -EPIPE) {
>  		dev_err(&dev->udev->dev,
>  			"Failed to query (%s) UVC control %u on unit %u: %d (exp. %u).\n",
>  			uvc_query_name(query), cs, unit, ret, size);
> -		return ret < 0 ? ret : -EPIPE;
> +		return ret ? ret : -EPIPE;
>  	}
>  
>  	/* Reuse data[0] to request the error code. */
> 

-- 
Kind regards,

Sakari Ailus