HDMI monitor can be detected even without power

I tried using an HDMI cable to connect Jetson Nano Developer Kit to a monitor, which doesn’t have a power chord connected. But the monitor can still be detected. It is listed with “xrandr --listmonitors”, and its EDID can be correctly read from /sys/kernel/debug/tegradc.0/edid.

From https://en.wikipedia.org/wiki/Extended_Display_Identification_Data
“The EDID PROM can often be read by the host PC even if the display itself is turned off.”

Ideally I would like the monitor to be considered as non-exist unless it is powered on. Is it possible?

EDID query uses the i2c protocol on the DDC wire of the cable. All monitors which follow the HDMI and DisplayPort specs do not power their own i2c circuitry, this is powered by the video card through the cable, and this allows not erasing settings just because the monitor is off. This is how it should behave.

There are some specialty touchscreens which power their own i2c, but that is technically a violation of standards.

If you want to avoid i2c being powered by the Jetson, then you will need to unplug the cable. EDIT: The i2c is not powered at “all times” though, the i2c only powers at certain boot or connect/disconnect events.

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Thank you, linuxdev.