hi all, I didn't change anything except rebooting and now my webcam works in both firefox and chromium on https://webcamtests.com. Cheese takes a minute or so to display "There was an error playing video from the webcam" and say:

../src/intel/isl/isl.c:2216: FINISHME: ../src/intel/isl/isl.c:isl_surf_supports_ccs: CCS for 3D textures is disabled, but a workaround is available.

(cheese:17967): cheese-WARNING **: 11:49:36.517: Internal data stream error.: ../libs/gst/base/gstbasesrc.c(3127): gst_base_src_loop (): /GstCameraBin:camerabin/GstWrapperCameraBinSrc:camera_source/GstBin:bin18/GstPipeWireSrc:pipewiresrc1:
streaming stopped, reason not-negotiated (-4)

The v4l2-relayd unit works.

"GST_DEBUG=2,v4l2src:5 gst-launch-1.0 v4l2src device=/dev/video0 ! videoconvert ! xvimagesink" works as well:

Setting pipeline to PAUSED ...
Pipeline is live and does not need PREROLL ...
Pipeline is PREROLLED ...
Setting pipeline to PLAYING ...
New clock: GstSystemClock
0:00:00.039094074 28464 0x56465e78c300 INFO                 v4l2src gstv4l2src.c:550:gst_v4l2src_query_preferred_size:<v4l2src0> Detect input 0 as `loopback`
0:00:00.039161508 28464 0x56465e78c300 DEBUG                v4l2src gstv4l2src.c:601:gst_v4l2src_negotiate:<v4l2src0> caps of src: video/x-raw, format=(string)YUY2, width=(int)1280, height=(int)720, framerate=(fraction)30/1; video/x-raw(format:Interlaced), format=(string)YUY2, width=(int)1280, height=(int)720, framerate=(fraction)30/1, interlace-mode=(string)alternate
0:00:00.039328145 28464 0x56465e78c300 DEBUG                v4l2src gstv4l2src.c:609:gst_v4l2src_negotiate:<v4l2src0> caps of peer: video/x-raw, format=(string)YV12, width=(int)[ 1, 16384 ], height=(int)[ 1, 16384 ], framerate=(fraction)[ 0/1, 2147483647/1 ]; video/x-raw, format=(string)I420, width=(int)[ 1, 16384 ], height=(int)[ 1, 16384 ], framerate=(fraction)[ 0/1, 2147483647/1 ]; video/x-raw, format=(string)NV12, width=(int)[ 1, 16384 ], height=(int)[ 1, 16384 ], framerate=(fraction)[ 0/1, 2147483647/1 ]; video/x-raw, width=(int)[ 1, 16384 ], height=(int)[ 1, 16384 ], framerate=(fraction)[ 0/1, 2147483647/1 ], format=(string){ ABGR64_LE, BGRA64_LE, AYUV64, ARGB64_LE, ARGB64, RGBA64_LE, ABGR64_BE, BGRA64_BE, ARGB64_BE, RGBA64_BE, GBRA_12LE, GBRA_12BE, Y412_LE, Y412_BE, A444_10LE, GBRA_10LE, A444_10BE, GBRA_10BE, A422_10LE, A422_10BE, A420_10LE, A420_10BE, RGB10A2_LE, BGR10A2_LE, Y410, GBRA, ABGR, VUYA, BGRA, AYUV, ARGB, RGBA, A420, AV12, Y444_16LE, Y444_16BE, v216, P016_LE, P016_BE, Y444_12LE, GBR_12LE, Y444_12BE, GBR_12BE, I422_12LE, I422_12BE, Y212_LE, Y212_BE, I420_12LE, I420_12BE, P012_LE, P012_BE, Y444_10LE, GBR_10LE, Y444_10BE, GBR_10BE, r210, I422_10LE, I422_10BE, NV16_10LE32, Y210, v210, UYVP, I420_10LE, I420_10BE, P010_10LE, NV12_10LE32, NV12_10LE40, P010_10BE, Y444, RGBP, GBR, BGRP, NV24, xBGR, BGRx, xRGB, RGBx, BGR, IYU2, v308, RGB, Y42B, NV61, NV16, VYUY, UYVY, YVYU, YUY2, I420, YV12, NV21, NV12, NV12_64Z32, NV12_4L4, NV12_32L32, Y41B, IYU1, YVU9, YUV9, RGB16, BGR16, RGB15, BGR15, RGB8P, GRAY16_LE, GRAY16_BE, GRAY10_LE32, GRAY8 }
0:00:00.039351854 28464 0x56465e78c300 DEBUG                v4l2src gstv4l2src.c:615:gst_v4l2src_negotiate:<v4l2src0> intersect: video/x-raw, width=(int)1280, height=(int)720, framerate=(fraction)30/1, format=(string)YUY2
0:00:00.039366247 28464 0x56465e78c300 DEBUG                v4l2src gstv4l2src.c:406:gst_v4l2src_fixate:<v4l2src0> Fixating caps video/x-raw, width=(int)1280, height=(int)720, framerate=(fraction)30/1, format=(string)YUY2
0:00:00.039372387 28464 0x56465e78c300 DEBUG                v4l2src gstv4l2src.c:407:gst_v4l2src_fixate:<v4l2src0> Preferred size 3840x2160
0:00:00.039384011 28464 0x56465e78c300 DEBUG                v4l2src gstv4l2src.c:430:gst_v4l2src_fixate:<v4l2src0> sorted and normalized caps video/x-raw, width=(int)1280, height=(int)720, framerate=(fraction)30/1, format=(string)YUY2
0:00:00.039411914 28464 0x56465e78c300 DEBUG                v4l2src gstv4l2src.c:488:gst_v4l2src_fixate:<v4l2src0> fixated caps video/x-raw, width=(int)1280, height=(int)720, framerate=(fraction)30/1, format=(string)YUY2, interlace-mode=(string)progressive, colorimetry=(string)2:4:7:1
0:00:00.039421239 28464 0x56465e78c300 INFO                 v4l2src gstv4l2src.c:647:gst_v4l2src_negotiate:<v4l2src0> fixated to: video/x-raw, width=(int)1280, height=(int)720, framerate=(fraction)30/1, format=(string)YUY2, interlace-mode=(string)progressive, colorimetry=(string)2:4:7:1
0:00:00.041361313 28464 0x56465e78c300 DEBUG                v4l2src gstv4l2src.c:1049:gst_v4l2src_create:<v4l2src0> ts: 0:28:43.205426000 now 0:28:43.205671897 delay 0:00:00.000245897
Redistribute latency...
0:00:00.045701409 28464 0x56465e79a460 DEBUG                v4l2src gstv4l2src.c:814:gst_v4l2src_query:<v4l2src0> report latency min 0:00:00.033333333 max 0:00:00.066666666
0:00:00.045711363 28464 0x56465e76ea10 DEBUG                v4l2src gstv4l2src.c:814:gst_v4l2src_query:<v4l2src0> report latency min 0:00:00.033333333 max 0:00:00.066666666
0:00:00.723310991 28464 0x56465e78c300 WARN                 v4l2src gstv4l2src.c:1031:gst_v4l2src_create:<v4l2src0> Timestamp going backward, ignoring driver timestamps