On Thu, 26 May 2022 at 03:28, Sean Paul <seanpaul@xxxxxxxxxxxx> wrote: > > On Wed, May 25, 2022 at 9:26 AM Daniel Vetter <daniel@xxxxxxxx> wrote: > > > > On Mon, May 23, 2022 at 05:59:02PM -0700, Doug Anderson wrote: > > > 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. :-) > > Mirroring some of my comments on that review here :-) > > IMO, this should be addressed in the kernel, or not at all. The kernel > ensures other aspects of DisplayPort implementation are compliant, so > I don't think this would be any exception. Further, the kernel is the > one creating the "safe" mode list, so it seems odd that userspace > would override that. Finally, relying on every userspace to do the > right thing is asking for trouble (we have 3 places which would need > this logic in CrOS). Oh I missed the part that this is defined in the DP spec as _the_ fallback mode. I think the probe helpers could check whether it's a DP connector and then dtrt per DP spec? I think that should have a solid chance of avoiding the regression mess, since the really shoddy stuff tends to be VGA/HDMI. Also if DP says only 640x480 should be the fallback if there's no other mode list source, then I think we should trim it down to only that. But also only for DP. Also ofc that patch should reference the right DP spec sections :-) -Daniel > > > > > Yeah it sucks a bit but I'm mildly afraid that if we muck around with the > > absolute fallback mode list in upstream we get whacked by a regression > > report :-/ > > Yeah, this seems likely (unfortunately). > > > > > There's the additional fun that on modern displays probably 720p (or maybe > > 720i) is a lot more likely to work than anything else really, so best we > > can do here maybe is to make it an uapi guarantee that if there's no > > preferred mode, then most likely the kernel invent random noise out of > > thin air, and userspace has to be careful and do its own magic heuristics. > > Or maybe we should add a flag for "this stuff is invented, buyer beware". > > > > This seems like a reasonable compromise. Perhaps marking 640x480 as > preferred would be a middle road? > > > I think clarifying that would be good. Changing defaults feels a bit too > > risky, we had some really hilarious regression reports in the past along > > the lines of the infamous xkcd. > > FWIW, I don't really have a strong opinion as to whether this should > be fixed or not. I have a hard time believing that either 1024x768 or > 640x480 would result in a happy result for the user, so we're really > just choosing a mode which is bad enough for the user to > unplug/replug. If 640x480 makes the compliance machine happy, I > suppose that's a compelling reason, but I don't really feel like this > is worth special casing each userspace. > > Sean > > > -Daniel > > -- > > Daniel Vetter > > Software Engineer, Intel Corporation > > http://blog.ffwll.ch -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
