media: uvcvideo: Support partial control reads

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: [email protected]
Fixes: a763b9f ("media: uvcvideo: Do not return positive errors in uvc_query_ctrl()")
Reviewed-by: Hans de Goede <[email protected]>
Signed-off-by: Ricardo Ribalda <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Hans de Goede <[email protected]>
Reviewed-by: Laurent Pinchart <[email protected]>
Signed-off-by: Laurent Pinchart <[email protected]>
Signed-off-by: Mauro Carvalho Chehab <[email protected]>

Author: ribalda

drivers/media/usb/uvc/uvc_video.c

@@ -79,6 +79,27 @@ int uvc_query_ctrl(struct uvc_device *dev, u8 query, u8 unit,
 	if (likely(ret == size))
 		return 0;
 
+	/*
+	 * Some devices return shorter USB control packets than expected if the
+	 * returned value can fit in less bytes. Zero all the bytes that the
+	 * device has not written.
+	 *
+	 * This quirk is applied to all controls, regardless of their data type.
+	 * Most controls are little-endian integers, in which case the missing
+	 * bytes become 0 MSBs. For other data types, a different heuristic
+	 * could be implemented if a device is found needing it.
+	 *
+	 * 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",