Re: [PATCH v4 1/2] media: uvcvideo: Support partial control reads

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

On Wed, Nov 20, 2024 at 03:26:19PM +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@vger.kernel.org
> Fixes: a763b9fb58be ("media: uvcvideo: Do not return positive errors in uvc_query_ctrl()")
> Reviewed-by: Hans de Goede <hdegoede@redhat.com>
> Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
> ---
>  drivers/media/usb/uvc/uvc_video.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index cd9c29532fb0..482c4ceceaac 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -79,6 +79,22 @@ int uvc_query_ctrl(struct uvc_device *dev, u8 query, u8 unit,
>  	if (likely(ret == size))
>  		return 0;
>  
> +	/*
> +	 * In UVC the data is usually represented in little-endian.

I had a comment about this in the previous version, did you ignore it on
purpose because you disagreed, or was it an oversight ?

> +	 * Some devices return shorter USB control packets that expected if the
> +	 * returned value can fit in less bytes. Zero all the bytes that the
> +	 * device have not written.

s/have/has/

And if you meant to start a new paragraph here, a blank line is missing.
Otherwise, no need to break the line before 80 columns.

> +	 * We exclude UVC_GET_INFO from the quirk. UVC_GET_LEN does not need to
> +	 * be excluded because its size is always 1.
> +	 */
> +	if (ret > 0 && query != UVC_GET_INFO) {
> +		memset(data + ret, 0, size - ret);
> +		dev_warn_once(&dev->udev->dev,
> +			      "UVC non compliance: %s control %u on unit %u returned %d bytes when we expected %u.\n",
> +			      uvc_query_name(query), cs, unit, ret, size);
> +		return 0;
> +	}
> +
>  	if (ret != -EPIPE) {
>  		dev_err(&dev->udev->dev,
>  			"Failed to query (%s) UVC control %u on unit %u: %d (exp. %u).\n",

-- 
Regards,

Laurent Pinchart