[Bug 51821] New: Ivy Bridge triple monitor and non-matching preferred modes

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=51821

           Summary: Ivy Bridge triple monitor and non-matching preferred
                    modes
           Product: Drivers
           Version: 2.5
    Kernel Version: 3.7
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Video(DRI - Intel)
        AssignedTo: intel-gfx-bugs at lists.freedesktop.org
        ReportedBy: haagch.christoph at googlemail.com
                CC: intel-gfx-bugs at lists.freedesktop.org
        Regression: No


So as you probably know the trouble with ivy bridge is that it has two PLLs and
it supports three monitors which I want to use. (sort of related:
https://bugzilla.redhat.com/show_bug.cgi?id=836765)
The same problem applies to randr but I don't know if this should be solved
together with the kernel driver or separately.

I think in my scenario I encounter several closely related issues:

I have two "identical" monitors (Flatron W2442PE) connected to my notebook. One
monitor is connected with HDMI and the other is connected with DVI, and the
third monitor is the LVDS of the notebook.

The trouble is that these external monitors have two very similar modes and it
seems to randomly switch which of those is the preferred mode.

According to xrandr they look like this:

  1920x1080 (0x53)  138.5MHz +HSync -VSync +preferred
        h: width  1920 start 1968 end 2000 total 2080 skew    0 clock   66.6KHz
        v: height 1080 start 1083 end 1088 total 1111           clock   59.9Hz
  1920x1080 (0x54)  148.5MHz +HSync +VSync *current
        h: width  1920 start 2008 end 2052 total 2200 skew    0 clock   67.5KHz
        v: height 1080 start 1084 end 1089 total 1125           clock   60.0Hz

This time the 0x53 mode is the preferred one for both monitors. Next time it
could as well be the 0x54 one for one or both of them. I do not know if the
monitors actually provide the different preferred modes or if linux assigns the
preferred modes, but in both cases I think a workaround would be useful.

The workaround could be: For ivy bridge, if there are three monitors of which
two have the same modes but different preferred modes, then just override the
preferred settings of one. It would be preferrable to use the fastest refresh
rate anyway, right?

First reason:
linux will spam the console with messages like
    [drm:intel_crtc_set_config] *ERROR* failed to set mode on [CRTC:7]
on boot if the preferred modes of the two external monitors don't match. It
will continue to spam it randomly in the console once every few minutes.

Second reason: This is debateable but I think out of the box working of all
three monitors is worth more than the slightly unexpected behaviour if the
monitors do indeed have different preferred modes and just happen to have a
matching mode.

The alternative would be disabling the third output "somehow" until the user
"selects" at least two matching modes. I am not really in favor of that because
the user probably won't understand why the third output doesn't show up, maybe
even randomly like in my scenario.

-- 
Configure bugmail: https://bugzilla.kernel.org/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
You are on the CC list for the bug.
You are the assignee for the bug.