Date:   Mon, 2 Aug 2021 14:20:35 -0400 (EDT)
From:   Philip Spencer <pspencer@...lds.utoronto.ca>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] uvcvideo: Support devices that require SET_INTERFACE(0)
 before/after streaming

(This is my first kernel-related mailing list posting; my apologies if I 
have targeted wrong maintainers and/or lists. This is posted on the Ubuntu 
launchpad bug tracker at 
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1938669 and it was 
suggested there that I post directly to the maintainers/mailing lists).

Video capture devices made by Epiphan Systems (vendor id 0x2b77) work 
once, but as soon as the video device is closed (or even if it is kept 
open but the application issues a VIDIOC_STREAMOFF ioctl) it won't work 
again - subsequent calls to VIDOC_DQBUF simply hang - until the device is 
unbound from and rebound to the uvcvideo module. (modprobe -r uvcvideo; 
modprobe uvcvideo also works).

For example:

   ffplay /dev/video0 -- works fine and shows the captured stream.

   ffplay /dev/video0 -- when run a second time: hangs and does not capture 
anything

   modprobe -r uvcvideo ; modprobe uvcvideo; ffplay /dev/video0 -- works 
fine again.

Experimenting with the device and the uvcvideo module source code reveals 
that problem is the device is expecting SET_INTERFACE(0) to be sent to 
return it to a state where it can accept control requests and start 
streaming again.

The code in uvc_video.c has several comments stating that some 
bulk-transfer devices require a SET_INTERFACE(0) call to be made before 
any control commands, even though 0 is already the default and only valid 
interface value. And, the function uvc_video_init makes such a call (which 
is why the device starts working again after rebinding to the uvcvideo 
module). But no such call is made when streaming is stopped then 
restarted.

Furthermore, SET_INTERFACE(0) is the mechanism by which isochronous 
devices are told to stop streaming, and the comments in 
uvc_video_stop_streaming state that the UVC specification is unclear on 
how a bulk-based device should be told to stop streaming, so it is 
reasonable to imagine this particular bulk-based device might be expecting 
the same SET_INTERFACE(0) call that an isochronous device would get as 
means of being told to stop streaming.

The attached patch fixes the problem for these Epiphan devices by adding a 
SET_INTERFACE(0) call in two places. Either one by itself is sufficient to 
resolve the symptoms but I think it is probably safest to include both.

The first hunk adds a SET_INTERFACE(0) call in uvc_video_start_streaming, 
but only in the bulk-based case where 0 is the only possible interface 
value (it won't mess with an isochronous device that might be set to a 
different interface).

The second hunk modifies the behaviour of uvc_video_stop_streaming to call 
SET_INTERFACE(0) unconditionally instead of only calling it for 
isochronous devices. Since interface 0 should already be set on 
non-isochronous devices, it should be safe to set it again, and this way 
devices that are expecting it as a signal to stop streaming will get it.

The patch is against 5.4.137 but also applies cleanly to 5.14-rc3.

--- a/drivers/media/usb/uvc/uvc_video.c	2021-08-01 10:19:19.343564026 -0400
+++ b/drivers/media/usb/uvc/uvc_video.c	2021-08-01 10:38:54.234311440 -0400
@@ -2108,6 +2081,15 @@ int uvc_video_start_streaming(struct uvc
  {
  	int ret;

+	/* On a bulk-based device where there is only one alternate
+	 * setting possibility, set it explicitly to 0. This should be
+	 * the default value, but some devices (e.g. Epiphan Systems
+	 * framegrabbers) freeze and won't restart streaming until they
+	 * receive a SET_INTERFACE(0) request.
+	 */
+	if (stream->intf->num_altsetting == 1) 
+  		usb_set_interface(stream->dev->udev, stream->intfnum, 0);
+
  	ret = uvc_video_clock_init(stream);
  	if (ret < 0)
  		return ret;
@@ -2135,9 +2117,17 @@ void uvc_video_stop_streaming(struct uvc
  {
  	uvc_video_stop_transfer(stream, 1);

-	if (stream->intf->num_altsetting > 1) {
-		usb_set_interface(stream->dev->udev, stream->intfnum, 0);
-	} else {
+	/* On isochronous devices, switch back to interface 0 to move
+	 * the device out of the "streaming" state.
+	 *
+	 * On bulk-based devices, this interface will already be selected
+	 * but we re-select it explicitly because some devices seem to need
+	 * a SET_INTERFACE(0) request to prepare them for receiving other
+	 * control requests and/or to tell them to stop streaming.
+	 */
+	usb_set_interface(stream->dev->udev, stream->intfnum, 0);
+
+	if (stream->intf->num_altsetting == 1) {
  		/* UVC doesn't specify how to inform a bulk-based device
  		 * when the video stream is stopped. Windows sends a
  		 * CLEAR_FEATURE(HALT) request to the video streaming

Philip

--------------------------------------------+-------------------------------
Philip Spencer  pspencer@...lds.utoronto.ca | Director of Computing Services
Room 348        (416)-348-9710  ext3048     | The Fields Institute for
222 College St, Toronto ON M5T 3J1 Canada   | Research in Mathematical Sciences

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