GM104 (GeForce 840m) with many errors "fifo: SCHED_ERROR 20"

Laptop HP Envy 15 with i7-4700mq and GeForce 840m on Ubuntu 18.04 and 20.04.

nouveau outputs many megabytes into system logs (kern.log and syslog) with

nouveau 0000:07:00.0: fifo: SCHED_ERROR 20 []

full log is in attachment some relevant output before SCHED_ERROR

nouveau 0000:07:00.0: NVIDIA GM108 (118010a2)
...
nouveau 0000:07:00.0: bios: version 82.08.14.00.0e
...
nouveau 0000:07:00.0: fb: 2048 MiB DDR3
nouveau 0000:07:00.0: bus: MMIO read of 00000000 FAULT at 6013d4 [ IBUS ]
...
nouveau 0000:07:00.0: bus: MMIO read of 00000000 FAULT at 10ac08 [ IBUS ]
nouveau 0000:07:00.0: fifo: fault 01 [WRITE] at 0000000000150000 engine 05 [BAR2] client 08 [HUB/HOST_CPU_NB] reason 02 [PTE] on channel -1 [007fd38000 unknown]
nouveau 0000:07:00.0: fifo: fault 01 [WRITE] at 0000000000000000 engine 05 [BAR2] client 08 [HUB/HOST_CPU_NB] reason 0a [UNSUPPORTED_APERTURE] on channel -1 [007fd38000 unknown]
vga_switcheroo: enabled
[TTM] Zone  kernel: Available graphics memory: 8164024 KiB
nouveau 0000:07:00.0: fifo: SCHED_ERROR 20 []
nouveau 0000:07:00.0: fifo: SCHED_ERROR 20 []
nouveau 0000:07:00.0: fifo: SCHED_ERROR 20 []

dmesg