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.

[mrittler]

@popcornmix Thanks a lot for the hint. Using negative values works indeed.
What is the lowest value that can be used?

I know this is declared as a legacy option but I think it would be nice to update this in the following documentation accordingly: https://www.raspberrypi.com/documentation/computers/legacy_config_txt.html#config_hdmi_boost

[popcornmix]

The code will accept values down to 0 after the subtraction (so config_hdmi_boost=-5 is the lowest).
But this is not supported operation. The output will almost certainly not pass hdmi compliance (as the signal will be too low), and it no doubt won't work on some displays.

To fix interference issues you should focus on the shielding of your board and hdmi cable.