Changes between Version 2 and Version 3 of linux/persistent_device_naming

Timestamp:

06/05/2026 07:16:42 PM (8 days ago)

Author:

Tim Harvey

Comment:

add example of using udev rule to consistently name a webcam dev

linux/persistent_device_naming

@@ -118 +118 @@
 }}}
 
-== Audio 
+
+[=#video]
+== Video Devices (ie USB Webcams)
+The Linux kernel does not guarantee the enumeration order of devices but at the user space level you can use systemd with udev rules to name devices in a persistent way. It can happen that devices are moving around or flip flopping between boots.
+
+Take for example video devices on a modern platform like Venice which has two onboard video codec devices. When you add a USB webcam it will add another device. If these are all kernel modules they could end up enumerating in random ways.
+
+You can use {{{v4l2-ctl --list-devices}}} to see them (or cat /sys/class/video4linux/video*/names):
+{{{#!bash
+root@venice:~# v4l2-ctl --list-devices
+nxp,imx8mm-vpu-g1-dec (platform:38300000.video-codec):
+        /dev/video2
+        /dev/media1
+
+nxp,imx8mq-vpu-g2-dec (platform:38310000.video-codec):
+        /dev/video3
+        /dev/media2
+
+Logitech Webcam C930e (usb-xhci-hcd.1.auto-1):
+        /dev/video0
+        /dev/video1
+        /dev/media0
+}}}
+ - above the webcam is on /dev/video0 and /dev/video1 but if you look at the devices capabilities via {{{v4l2-ctl --device /dev/video0 --all}}} you'll see that the first device is the capture device and the 2nd one is a UVC control device. On any other boot you may find the webcam is /dev/video2 and /dev/video3.
+
+Use {{{udevadm info}}} to get attributes that you can put in a udev rule:
+{{{#!bash
+root@venice:~# udevadm info -a -p $(udevadm info -q path -n /dev/video0) | grep -E "idVendor|idProduct|serial"
+    ATTRS{idProduct}=="0843"
+    ATTRS{idVendor}=="046d"
+    ATTRS{serial}=="2FBAA8BE"
+    ATTRS{idProduct}=="0002"
+    ATTRS{idVendor}=="1d6b"
+    ATTRS{serial}=="xhci-hcd.1.auto"
+    ATTRS{dbc_idProduct}=="0010"
+    ATTRS{dbc_idVendor}=="1d6b"
+}}}
+
+Here we see that the webcam capture device /dev/video0 has a VID=046d and a PID=0843. Add a udev rule to symlink this to /dev/video_webcam:
+{{{#!bash
+# add udev rule for webcam
+cat << EOF > /etc/udev/rules.d/99-webcam.rules
+# External USB Webcam - Target only the video stream (index 0)
+SUBSYSTEM=="video4linux", ATTRS{idVendor}=="046d", ATTRS{idProduct}=="0843", ATTR{index}=="0", SYMLINK+="video_webcam"
+EOF
+# apply changes
+sudo udevadm control --reload-rules
+sudo udevadm trigger
+}}}
+ - note that we also filter on stream index as uvcvideo provides 2 devs and we want the first one with the video streams
+
+
+[=#audio]
+== Audio Devices
 
 The Linux kernel does not guarantee the enumeration order of devices but at the user space level you can use systemd with udev rules to name devices in a persistent way. It can happen that devices are moving around or flip flopping between boots.