[Bug 72961] New: Patch to hdmi_portclock_limit disables resolutions that worked with the 3.13.6 kernel

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=72961

            Bug ID: 72961
           Summary: Patch to hdmi_portclock_limit disables resolutions
                    that worked with the 3.13.6 kernel
           Product: Drivers
           Version: 2.5
    Kernel Version: 3.13.7
          Hardware: x86-64
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Video(DRI - Intel)
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: nvinson at comcast.net
                CC: intel-gfx-bugs at lists.freedesktop.org
        Regression: No

My computer uses the Asus Maximux VI Hero motherboard which only contains an
HDMI out, the i4771 CPU, and is connected to a Dell 2713hm using a DVI <-> HDMI
cable
(http://www.amazon.com/gp/product/B00851NVDO/ref=oh_details_o03_s00_i00?ie=UTF8&psc=1).
 The rationale for this setup is that the Dell monitor is capable of 2560x1440,
but only if the signal comes in on its DVI or Display Ports.  This setup worked
with the 3.13.6 kernel.

However, after upgrading to gentoo-sources 3.13.7 from 3.13.6 I could no longer
switch to 2560x1440 @ 60 Hz due to a patch to the hdmi_portclock_limit()
function found in drivers/gpu/drm/i915/intel_hdmi.c.  The patch appears to
limit the port clock to 165000 (while in 3.13.6 it was limited to 300000) and
this limit causes the prune the modeline for the 2560x1440 resolution.  I
discovered this by adding debugging statements to the hdmi_portclock_limit()
function for both kernels and comparing dmesg output.

Reproducible: Always

Steps to Reproduce:
1. Update 3.13.6 kernel configuration for 3.13.7 gentoo-sources
2. Build 3.13.7 kernel (and associated modules)
3. Reboot using new kernel
Actual Results:  
System reboots and starts using the 1920x1200 resolution.  All attempts to
manually set the resolution to 2560x1440 (including adding the modeline using
xrandr) fails.

Expected Results:  
System to start up using 2560x1440 resolution as it did with the 3.13.6 linux
kernel.

The patch made to hdmi_portclock_limit() seems to be related to the issues
described in https://bugzilla.kernel.org/show_bug.cgi?id=70331 and
https://bugs.freedesktop.org/show_bug.cgi?id=75345

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