On Tue, May 03, 2022 at 04:21:08PM -0700, Douglas Anderson wrote:
When doing DP AUX transfers there are two actors that need to be powered in order for the DP AUX transfer to work: the DP source and the DP sync. Commit bacbab58f09d ("drm: Mention the power state requirement on side-channel operations") added some documentation saying that the DP source is required to power itself up (if needed) to do AUX transfers. However, that commit doesn't talk anything about the DP sink.
For full fledged DP the sink isn't really a problem. It's expected that if an external DP monitor isn't plugged in that attempting to do AUX transfers won't work. It's also expected that if a DP monitor is plugged in (and thus asserting HPD) that it AUX transfers will work.
When we're looking at eDP, however, things are less obvious. Let's add some documentation about expectations. Here's what we'll say:
- We don't expect the DP AUX transfer function to power on an eDP
panel. If an eDP panel is physically connected but powered off then it makes sense for the transfer to fail.
I don't agree with this. I think the panel should just get powred up for AUX transfers. Otherwise you can't trust that eg. the /dev/aux stuff is actually usable.