Performance regression when using vcsm-cma instead of vcsm, with zero copy MMAL

Reproduce the regression by applying the linked patch to RaspiVid.c and running the resulting raspivid binary with '-o foo.h264'. Compare the first-20-row timings and CPU usage with vcsm and vcsm-cma, using the stated firmware and kernel versions; done means explaining or correcting the slowdown while preserving zero-copy MMAL behavior.

Describe the bug

When using MMAL_PARAMETER_ZERO_COPY in combination with vcsm-cma, using the cpu to read from frames captured by a camera in MMAL_ENCODING_I420 format slows down significantly -- taking over 30x longer in a particular benchmark, compared to using the previous vcsm implementation.

In addition to the greater time taken, the CPU usage reported by top is higher (~20% instead of ~2%). Removing the calls to set MMAL_PARAMETER_ZERO_COPY regains the faster timing, but CPU usage remains high.

Steps to reproduce the behaviour

The problem can be seen by applying the patch at https://gist.github.com/malc0/0a9ee21fd92ecc1e37a18fa6507b069e to RaspiVid.c from the current (54fd97ae4066a10b6b02089bc769ceed328737e0) userland repository, and comparing the output of running the resulting raspivid binary with '-o foo.h264'. Using the final 5.4 kernel from the firmware repository (8cd76653b88939baf25c3f9d9ce90657bcc19b76) the time to read the first 20 rows of a 1920 pixels-wide image is ~200 microseconds. Renaming /dev/vcsm to something else results in the vcsm-cma mechanism being used, and the same delay is typically greater than 6000 microseconds. Using the most recent 5.15.x firmware commit (494eb71e5adfca31ec65dd535fce73de3c7c2efa) shows similar times for vcsm-cma, but comparison to vcsm isn't possible.

Device (s)

Raspberry Pi 3 Mod. B+

System

OS: Raspbian GNU/Linux 11, dist-upgraded from jessie originally installed using https://github.com/debian-pi/raspbian-ua-netinst

vcgencmd version: Dec 12 2022 12:00:07
Copyright (c) 2012 Broadcom
version ed6f6b8fcdc6476410b9cf75d141633461d34bdd (clean) (release) (start_x)

uname -a: Linux localhost 5.15.83-v7+ #1607 SMP Thu Dec 15 12:55:05 GMT 2022 armv7l GNU/Linux

Logs

No response

Additional context

cmdline.txt:
dwc_otg.lpm_enable=0 dwc_otg.fiq_enable=0 dwc_otg.fiq_fsm_enable=0 console=ttyAMA0,115200 root=/dev/mmcblk0p2 rootfstype=ext4 rootwait

config.txt:
start_x=1
gpu_mem=256
dtoverlay=cma,cma-96
dtoverlay=disable-bt