[Bug 76211] New: BISECTED REGRESSION: i915 resume latency 1200ms

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=76211

            Bug ID: 76211
           Summary: BISECTED REGRESSION: i915 resume latency 1200ms
           Product: Drivers
           Version: 2.5
    Kernel Version: 3.15-rc1
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Video(DRI - Intel)
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: lenb at kernel.org
                CC: intel-gfx-bugs at lists.freedesktop.org
        Regression: No

Created attachment 136101
  --> https://bugzilla.kernel.org/attachment.cgi?id=136101&action=edit
0001-drm-i915-don-t-wait-for-power-cycle-when-waiting-for.patch

On a Dell XPS11 convertable (Intel HSW-YLT),

i915 resume from S3 got slower in 3.15-rc1
Linux 3.14: 400ms 
Linux 3.15-rc1: 1200ms

If the attached patch is reverted, 400ms resume time is restored.

Unfortunately, the attached patch also had the dramatic
_benefit_ of reducing i915 suspend latency on the same XPS11
to under 150ms from over 1300ms.

The numbers are different when measured on a Lenovo Yoga2 Pro HSW-ULT,
but the trend is the same.  The patch increases resume time
to about 700ms from under 400ms, and the patch reduces
suspend time  to under 300ms from over 1100ms.

Obviously, it would be ideal to capture the suspend speed
benefit of this patch without having the resume cost.  However,
if we have to pick between suspend and resume performance,
resume performance is more important.

Note that it is easy to graphically measure resume latency with this tool:
https://01.org/suspendresume

if analyze_suspend.py is invoked with the -f argument, it also
produces an ftrace that can show exactly where the time goes.
In the case of 3.15-rc1 on the Dell XPS11:

intel_edp_panel_on takes > 1000ms
  wait_panel_power_cycle takes 800ms
  wait_panel_status takes 200ms

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