config_hdmi_boost below 5 is not working

[mrittler]

Describe the bug
I would like to reduce the signal strength of the HDMI interface via config_hdmi_boost to reduce EMC emissions. According to the documentation the minimum value for config_hdmi_boost is 0 and the maximum is 11. But values below 5 seem to be ignored and instead the default value of 5 is used.

To reproduce

• add config_hdmi_boost=0 to /boot/config.txt
• reboot
• run vcgencmd get_config int and check the value of config_hdmi_boost

Expected behaviour
config_hdmi_boost=0

Actual behaviour
config_hdmi_boost=5

System

• Raspberry Pi CM3+
• Custom Linux distribution
• firmware version: Aug 26 2022 14:04:36 102f1e848393c2112206fadffaaf86db04e98326
• Linux kernel: 5.15.34-v7

[popcornmix]

I've had a look at the code. On CM3+ the default config_hdmi_boost is 5.

You can increase it by specifying a new positive config_hdmi_boost value.

But reducing it requires a negative setting.
config_hdmi_boost=-1
should get you 4.