Can't switch between HDMI and composite on Pi Zero W with latest Raspbian

[GilchristT]

This is a follow on from issue #1166 https://github.com/raspberrypi/firmware/issues/1166 which has been closed.

I'm running the latest Buster build of Raspbian (as far as I can tell), have installed all patches via apt upgrade and have installed the latest firmware via rpi-update

pi@raspberrypi:~ $ uname -a
Linux raspberrypi 4.19.75+ #1271 Mon Sep 30 13:46:57 BST 2019 armv6l GNU/Linux

Describe the bug
If I disconnect the HDMI cable before powering on then I can get output via composite video but if the connector is plugged in during boot I cannot use tvservice to switch between the outputs.

To reproduce

• Power on with HDMI connected
• Run command tvservice -c "PAL 4:3"

Expected behaviour
Console is displayed via TV Out / Composite Video

Actual behaviour

pi@raspberrypi:~ $ tvservice -c "PAL 4:3"
Powering on SDTV with explicit settings (mode:2 aspect:1)
[E] Failed to power on SDTV with explicit settings (mode:2 aspect:1)

System
PiZeroW

pi@raspberrypi:~ $ cat /etc/rpi-issue
Raspberry Pi reference 2019-07-10
Generated using pi-gen, https://github.com/RPi-Distro/pi-gen, 175dfb027ffabd4b8d5080097af0e51ed9a4a56c, stage2

pi@raspberrypi:~ $ vcgencmd version
Sep 30 2019 14:40:39
Copyright (c) 2012 Broadcom
version 6820edeee4ef3891b95fc01cf02a7abd7ca52f17 (clean) (release) (start)

pi@raspberrypi:~ $ uname -a
Linux raspberrypi 4.19.75+ #1271 Mon Sep 30 13:46:57 BST 2019 armv6l GNU/Linux

Additional context

• My use case is the Pi Zero as the video source for a device used for testing video capture devices.
• These devices have both HD and SD inputs so I need to swap between the two outputs.
• The Pi is in an enclosure and permanently wired to a HDMI to SDI converter so swapping cables isn't an option
• I appreciate this is a fairly obscure scenario and I can work around it with an older version of the kernel (I actually use Buildroot and the image it builds doesn't appear to have this issue). I just thought it would be better to flag this here :-)