Re: [PATCH 1/2] media: uvcvideo: add quirks for per-device stream error handling

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

On Mon, Jun 29, 2026 at 06:38:02PM +0200, Pol Fernández Fernández wrote:
> The UVC_STREAM_ERR bit in the payload header signals that the device
> could not deliver a frame correctly. By default, uvcvideo delivers
> these frames to userspace with V4L2_BUF_FLAG_ERROR when uvc_no_drop_param
> is set (the default). Applications that rely on strict JPEG decoders
> (e.g. libjpeg) will display gray bands for the incomplete portion of
> the image, since libjpeg fills undecodable MCU rows with a neutral gray
> when the EOI marker is missing.
> 
> Add UVC_QUIRK_DROP_STREAM_ERR to force-drop frames flagged with
> UVC_STREAM_ERR for specific devices, regardless of the uvc_no_drop_param
> module parameter. This ensures applications never receive truncated frames
> from devices known to set UVC_STREAM_ERR on genuine encoding errors.

That completely defeats the point of uvc_no_drop_param. The issue should
be fixed in userspace, where you should ignore frames with the error
flag set if desired.

> The existing uvc_queue_to_stream() helper is used to reach the device
> quirks from within uvc_queue_buffer_complete().
> 
> Additionally, uvcvideo unconditionally overrides the camera's probed
> wCompQuality with the maximum value returned by GET_MAX. For devices
> whose firmware treats wCompQuality=0 as an adaptive encoding mode (where
> the encoder adjusts quality dynamically to fit within the available USB
> bandwidth),

The UVC 1.1 specification documents the range wCompQuality values as

"Values for this property range from 0 to 10000 (0 indicates the lowest
quality, 10000 the highest)."

While the UVC 1.5 specification documents it as

"Values for this property range from 1 to 10000 (1 indicates the lowest
quality, 10000 the highest)."

There is no mention of 0 being allowed as a magic value. Where did you
find information about the adaptive encoding mode ?

> this override increases encoding pressure and can contribute
> to UVC_STREAM_ERR events. Add UVC_QUIRK_NO_FORCE_QUALITY to preserve
> the camera's default quality value during probe negotiation.
> 
> Signed-off-by: Pol Fernández Fernández <fernandezfernandezpol@gmail.com>

Please split this patch in two as it addresses two different issues.

> ---
>  drivers/media/usb/uvc/uvc_queue.c | 3 ++-
>  drivers/media/usb/uvc/uvc_video.c | 3 ++-
>  drivers/media/usb/uvc/uvcvideo.h  | 2 ++
>  3 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
> index XXXXXXX..XXXXXXX 100644
> --- a/drivers/media/usb/uvc/uvcvideo.h
> +++ b/drivers/media/usb/uvc/uvcvideo.h
> @@ -81,6 +81,8 @@
>  #define UVC_QUIRK_INVALID_DEVICE_SOF	0x00010000
>  #define UVC_QUIRK_MJPEG_NO_EOF		0x00020000
>  #define UVC_QUIRK_MSXU_META		0x00040000
> +#define UVC_QUIRK_DROP_STREAM_ERR	0x00080000
> +#define UVC_QUIRK_NO_FORCE_QUALITY	0x00100000
> 
>  /* Format flags */
>  #define UVC_FMT_FLAG_COMPRESSED		0x00000001
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index XXXXXXX..XXXXXXX 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -451,7 +451,8 @@ int uvc_probe_video(struct uvc_streaming *stream,
>  		if (ret < 0)
>  			goto done;
> 
> -		probe->wCompQuality = probe_max.wCompQuality;
> +		if (!(stream->dev->quirks & UVC_QUIRK_NO_FORCE_QUALITY))
> +			probe->wCompQuality = probe_max.wCompQuality;
>  	}
> 
>  	for (i = 0; i < 2; ++i) {
> diff --git a/drivers/media/usb/uvc/uvc_queue.c b/drivers/media/usb/uvc/uvc_queue.c
> index XXXXXXX..XXXXXXX 100644
> --- a/drivers/media/usb/uvc/uvc_queue.c
> +++ b/drivers/media/usb/uvc/uvc_queue.c
> @@ -357,7 +357,8 @@ static void uvc_queue_buffer_complete(struct kref *ref)
>  	struct vb2_buffer *vb = &buf->buf.vb2_buf;
>  	struct uvc_video_queue *queue = vb2_get_drv_priv(vb->vb2_queue);
> 
> -	if (buf->error && !uvc_no_drop_param) {
> +	if (buf->error && (!uvc_no_drop_param ||
> +	    (uvc_queue_to_stream(queue)->dev->quirks & UVC_QUIRK_DROP_STREAM_ERR))) {
>  		uvc_queue_buffer_requeue(queue, buf);
>  		return;
>  	}

-- 
Regards,

Laurent Pinchart