Re: [PATCH v3 1/2] media: uvcvideo: Support partial control reads

[Laurent Pinchart]

Hi Ricardo,

Thank you for the patch.

On Mon, Nov 18, 2024 at 05:16:51PM +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()")
> Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
> ---
>  drivers/media/usb/uvc/uvc_video.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index cd9c29532fb0..e165850397a0 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -76,8 +76,22 @@ int uvc_query_ctrl(struct uvc_device *dev, u8 query, u8 unit,
>  
>  	ret = __uvc_query_ctrl(dev, query, unit, intfnum, cs, data, size,
>  				UVC_CTRL_CONTROL_TIMEOUT);
> -	if (likely(ret == size))
> +	if (ret > 0) {
> +		if (size == ret)
> +			return 0;

Why is this within the ret > 0 block ? I would write

	if (likely(ret == size))
		return 0;

	if (ret > 0) {

> +
> +		/*
> +		 * In UVC the data is represented in little-endian by default.

By default, or always ?

> +		 * Some devices return shorter control packages that expected

What's a "control package" ?

I think you meants "than expected", not "that expected".

> +		 * if the return value can fit in less bytes.
> +		 * Zero all the bytes that the device have not written.
> +		 */

Do we want to apply this workaround to GET_INFO and GET_LEN, or can we
restrict it to GET_CUR, GET_MIN, GET_MAX and GET_RES ?

> +		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,
> 

-- 
Regards,

Laurent Pinchart