Hi, On Fri, May 20, 2022 at 5:01 PM Doug Anderson <dianders@xxxxxxxxxxxx> wrote: > > Hi, > > On Mon, May 16, 2022 at 3:28 AM Thomas Zimmermann <tzimmermann@xxxxxxx> wrote: > > > > Hi Douglas, > > > > I understand that you're trying to tell userspace that the modelist has > > been made up, but it's not something that should be done via fragile > > heuristics IMHO. > > > > I looked at the Chromium source code that you linked, but I cannot say > > whether it's doing the correct thing. It all depends on what your > > program needs. > > > > In that function, you could also search for 'DRM_MODE_TYPE_USERDEF'. > > It's the mode that the user specified on the kernel command line. If > > Chromium's automatic mode selection fails, you'd give your users direct > > control over it. > > That doesn't really work for Chrome OS. Certainly a kernel hacker > could do this, but it's not something I could imagine us exposing to > an average user of a Chromebook. > > > > When there's no flagged mode or if > > /sys/class/drm/card<...>/status contains "unconnected", you can assume > > that the modelist is artificial and try the modes in an appropriate order. > > So "no flagged" means that nothing is marked as preferred, correct? > > ...so I guess what you're suggesting is that the order that the kernel > is presenting the modes to userspace is not ABI. If there are no > preferred modes then userspace shouldn't necessarily assume that the > first mode returned is the best mode. Instead it should assume that if > there is no preferred mode then the mode list is made up and it should > make its own decisions about the best mode to start with. If this is > the ABI from the kernel then plausibly I could convince people to > change userspace to pick 640x480 first in this case. > > > If we really want the kernel to give additional guarantees, we should > > have a broader discussion about this topic IMHO. > > Sure. I've added Stéphane Marchesin to this thread in case he wants to > chime in about anything. > > Overall, my take on the matter: > > * Mostly I got involved because, apparently, a DP compliance test was > failing. The compliance test was upset that when it presented us with > no EDID that we didn't default to 640x480. There was a push to make a > fix for this in the Qualcomm specific driver but that didn't sit right > with me. > > * On all devices I'm currently working with (laptops), the DP is a > secondary display. If a user was trying to plug in a display with a > bad EDID and the max mode (1024x768) didn't work, they could just use > the primary display to choose a different resolution. It seems > unlikely a user would truly be upset and would probably be happy they > could get their broken display to work at all. Even if this is a > primary display, I believe there are documented key combos to change > the resolution of the primary display even if you can't see anything. > > * That all being said, defaulting to 640x480 when there's no EDID made > sense to me, especially since it's actually defined in the DP spec. So > I'm trying to do the right thing and solve this corner case. That > being said, if it's truly controversial I can just drop it. > > > So I guess my plan will be to give Stéphane a little while in case he > wants to chime in. If not then I guess I'll try a Chrome patch... > ...and if that doesn't work, I'll just drop it. OK, this userspace code seems to work: https://crrev.com/c/3662501 - ozone/drm: Try 640x480 before picking the first mode if no EDID ...so we'll see how review of that goes. :-)
