Logitech C270 support

[sorig]

Hey Folk maintainers!

I had some problems getting my camera (Logitech C270) to work. Other cameras could also hit these code paths, so I'm sharing the fix here.

Problem and identified causes

When I was setting up my folk instance, I got stuck at the calibration step. Although the device was showing up in the cameras list, the camera preview was not showing anything on the /setup and /calibrate pages. Looking at the Folk log I could see that camera selection was failing:

folk-esben make[9387]: camera/usb: Loaded /dev/v4l/... -> (Camera*) 0x75c4440f7a90
folk-esben make[9387]: camera/usb: VIDIOC_S_FMT: ret = 0 (0) (Success)
folk-esben make[9387]: Error in builtin-programs/camera/usb.folk: selection failed of fd 7

After a bit of experimentation, it looks like my camera was too slow to start up. Extending the timeout to 2 seconds and adding a retry mechanism fixed this issue for me. The C270 often only starts streaming on the second attempt with my fix:

folk-esben make[3561]: camera/usb: select timeout on fd 18, retry 1/4

At this point I could now see the camera preview on /setup and /calibrate, but when I started the calibration process, the AprilTags weren't being recognised at all. The tag grid was showing but nothing happened when I positioned the calibration board correctly.

Looking at the logs I was now seeing lots of these messages:

camera/usb: Error cameraDecompressGray: Decompression failed: Corrupt JPEG data: 3 extraneous bytes before marker 0xd6

Turns out that this Turbo jpeg message was a warning rather than a fatal error. I guess my particular camera streams slightly weird data. Folk treated the warning as a fatal error hence no AprilTag detection.

[osnr]

Can you guys test this? If it works on 2 different tabletop systems (I can test here too but only have gadget) I think fine to merge

[ppkn]

The /setup and /camera pages work fine with nothing spooky in the logs 👍

I tried it with both a Logitech C922 and a IPEVO V4K.