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: > > 1. 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. -- Ville Syrjälä Intel