The High level Problem:
I have a custom multi-camera array that I have attached to a Raspberry Pi via some USB hubs, and I need a way to quickly identify which camera needs attention when any camera fails to respond to an image capture request.
Can "v4l2-ctl --list_devices" help me?
I am trying to determine if
"v4l2-ctl --list_devices" can help me more quickly identify which one is missing. My thought was to look at which cameras are reporting, and then notice which one is NOT reporting, and investigate the missing one.
My question is can I identify each reporting camera either for the v4l2-ctl output, or from "FFmpeg -list_formats"
Here is the output I get from v4l2-ctl:
$ v4l2-ctl --list-device
bcm2835-codec-decode (platform:bcm2835-codec):
/dev/video10
/dev/video11
/dev/video12
bcm2835-isp (platform:bcm2835-isp):
/dev/video13
/dev/video14
/dev/video15
/dev/video16
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.1.2.1.1):
/dev/video4
/dev/video5
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.1.2.1.2):
/dev/video6
/dev/video7
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.1.2.1.3):
/dev/video8
/dev/video9
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.1.2.1.4):
/dev/video17
/dev/video18
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.1.2.2):
/dev/video0
/dev/video1
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.1.2.3):
/dev/video2
/dev/video3
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.1.3.1.1):
/dev/video21
/dev/video22
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.1.3.1.2):
/dev/video25
/dev/video26
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.1.3.1.4):
/dev/video29
/dev/video30
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.1.3.2):
/dev/video19
/dev/video20
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.1.3.3):
/dev/video23
/dev/video24
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.1.3.4):
/dev/video27
/dev/video28
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.2.1.2):
/dev/video35
/dev/video36
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.2.1.3):
/dev/video39
/dev/video40
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.2.1.4):
/dev/video41
/dev/video42
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.2.2):
/dev/video31
/dev/video32
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.2.3):
/dev/video33
/dev/video34
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.2.4):
/dev/video37
/dev/video38
FHD Camera: FHD Camera (usb-0000:01:00.0-1.2.4.2):
/dev/video43
/dev/video44
All the USB cameras in my array each report the same first field (in this case "FHD Camera") so I can't use the name as a unique identifier.
Each "FHD Camera" is assigned two different /dev/videoNN ids (one will be assigned for the mpeg format, and the other for the UYV format). However these /dev/videoNN assignments change each time the computer reboots.
The sequence that cameras are listed in the output also changes each time the v4l2-ctl command runs. So that also is of no help
v4l2-ctl also reports another field, beginning "usb-0000:" followed by a series numbers (e.g. "01:00.0-1.2.2.4").
I am wondering if this number string ties in any way to the physical USB bus, and would remain permanent across reboots.
The v4l2-ctl help documentation merely says that the --list-devices flag will list the video devices, but these additional fields and their meaning is not explained.
Alternatively, for each camera that is responding, I can query the device with
"ffmpeg -hide_banner -f v4l2 -list_formats all -i /dev/videoNN", yielding a response like this:
$ ffmpeg -hide_banner -f v4l2 -list_formats all -i /dev/video8
[video4linux2,v4l2 @ 0x1a391c0] Compressed: mjpeg : Motion-JPEG : 640x480 1280x720 640x360 320x240 1920x1080
[video4linux2,v4l2 @ 0x1a391c0] Raw : yuyv422 : YUYV 4:2:2 : 640x480 1280x720 640x360 320x240 1920x1080
/dev/video8: Immediate exit requested
This ffmpeg query shows me that the device is also registered as "0x1a391c0". I am not yet sure whether these identifiers are stable across reboots, and if they are, whether they are stable with the physical camera, or if they are just a stable identifier to the USB Hub location of the camera.
My request:
Can someone explain to me how the "device name", /dev/videoNN identifier, usb-0000:01:00.0-1.2.2.4 identifier, and 0x1a391c0 are assigned?
The responding order of the items listed in v4l2-ctl changes each time it runs.
Is this reflecting that all cameras are polled simultaneously, but acquisition of the bus for response transmission is random?
Addendum
Further investigation shows me that the "0x-------" identifiers do not seem to be stable across reboots either.
The "usb-0000:01:00.0-1.2.2.2" identifier DOES seem to have a somewhat stable meaning. I have (5) 7-port usb hubs attached to my RPi 400. 4 of these are branch hubs that feed into a master hub, and the master hub feeds into the RPi.
After the 0-1. in the identifier there is either 3 or 4 digits. If there are only 3, the last digit is a port identifier 2, 3, or 4.
If there are 4 digits, then the 2nd to last digit will be a 1, and 1.1, 1.2, 1.3, and 1.4 will represent the last 4 physical ports on that hub.
Ironically, while the same port identifiers are always used in the same order on each of my (identical brand/model) hubs, they are NOT in ascending or descending sequence in terms of their physical sequence on the hubs.
I have deduced from this that each of my hubs actually uses two USB 4 port chips. So the first chip is getting the single digits, while the 1st port on that 1st chip is feeding the 2nd chip.
The 2nd digit in my identifiers is identifying which branch hub the device is on, 1,2,3 or 4. The first digit is 1, which seems to be identifying that all the branches are children of the master hub.
These addresses seem to be stable across reboots as long as all of the USB hubs are already connected at boot up. If not all are connected, those that are connected first will get the lowest hub number identifiers.
I don't know if this tree address identifier behavior would be the same on other hubs, but perhaps these observations will be of some use to anyone else trying to find stable identifiers in their own multi hub and multi-device architecture.
question from:
https://stackoverflow.com/questions/65865266/what-is-the-meaning-of-the-v4l2-ctl-list-device 
