Dell U2711 detection command wakes up the display

[w8jcik]

Two quick questions:

• Is it expected that some displays wake up when ddcutil detect is called?
• Can it be prevented with some switches?

Normally I wouldn't be bothered by it, but it is causing following practical issue:

A project called ddcutil-service detects connection/disconnection events by polling every 30 seconds (some fallback behavior that happens in practice). Polling calls logic of ddcutil detect resulting in sleeping displays waking up every 30 seconds.

This happens only with my two Dell U2711. Other displays do not wake up. It seems display-specific.

[rockowitz]

The answer, as in so many of this situations, is "it depends" on how the monitor implements I²C and DDC. In addition to reading the EDID, the detect command makes DDC requests as part of display detection. You'd have to check if the monitor is asleep before calling detect. Unfortunately, on Wayland this check is not cheap - it has to be performed by reading from the DRM connector node in /sys.