From: Kirill A. Korinsky <kirill@korins.ky>
Subject: Re: sys/uvideo: fill v4l2_capability the right way
To: Marcus Glocker <marcus@nazgul.ch>
Cc: tech@openbsd.org
Date: Sat, 28 Dec 2024 20:54:03 +0100

On Sat, 28 Dec 2024 20:05:36 +0100,
Marcus Glocker <marcus@nazgul.ch> wrote:
> 
> > Don't you think that this is enough to for the bus?
> 
> Well, maybe, it's more a question about what one considers a bus
> information.  I'm considering "uvideo0" to be a device indentifier, not
> necessarily a bus information.
>

After some thinking, let put an actual usb bus here, see attached diff.

> > About libwebrtc: it assumes that caps->bus_info is unique, and uses strncmp.
> 
> OK.  Just to understand, since I don't know how libwebrtc operates;
> With what will it compare the caps->bus_info string (lets say it's
> "uvideo0") with to make a decision?

It works like this:

1. An application gets device names and UniqueId which is caps->bus_info by
   iterating over /dev/videoX where X between 0 to 63;

2. User decided which device should be used

3. It getscCapability map and fd to the desired device. To fine the right
   device it iterates over /dev/videoX and comparing caps->bus_info with
   expected one, or device name, when it is empty string.

Original patch fixes an issue, but here I construct the right bus I think.

Ok?

Index: sys/dev/usb/uvideo.c
===================================================================
RCS file: /home/cvs/src/sys/dev/usb/uvideo.c,v
diff -u -p -u -p -r1.231 uvideo.c
--- sys/dev/usb/uvideo.c	22 Dec 2024 20:30:04 -0000	1.231
+++ sys/dev/usb/uvideo.c	28 Dec 2024 19:44:35 -0000
@@ -2928,12 +2928,27 @@ uvideo_debug_file_write_frame(void *arg)
 int
 uvideo_querycap(void *v, struct v4l2_capability *caps)
 {
+	int res;
+	uint32_t route;
+	uint8_t bus, ifaceno;
 	struct uvideo_softc *sc = v;
 
 	bzero(caps, sizeof(*caps));
-	strlcpy(caps->driver, DEVNAME(sc), sizeof(caps->driver));
+	strlcpy(caps->driver, "uvideo", sizeof(caps->driver));
 	strlcpy(caps->card, sc->sc_udev->product, sizeof(caps->card));
-	strlcpy(caps->bus_info, "usb", sizeof(caps->bus_info));
+	strlcpy(caps->bus_info, DEVNAME(sc), sizeof(caps->bus_info));
+
+	res = usbd_get_location(sc->sc_udev, sc->sc_vs_cur->ifaceh,
+	    &bus, &route, &ifaceno);
+
+	if (!res)
+		res =
+		    snprintf(caps->bus_info, sizeof(caps->bus_info),
+			"usb%u.%u.%x.%u", bus, route, route, ifaceno)
+			>= sizeof(caps->bus_info);
+
+	if (res && sizeof(caps->bus_info) > 0)
+		caps->bus_info[0] = '\0';
 
 	caps->version = 1;
 	caps->device_caps = V4L2_CAP_VIDEO_CAPTURE


-- 
wbr, Kirill