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    SubjectRe: [BUG/REGRESSION] Screen flickering
    On Tue, May 12, 2015 at 09:54:07PM +0200, Thomas Gummerer wrote:
    >
    > Hi,
    >
    > I noticed that on my machine the screen starts to flicker after I
    > suspend and resume my machine, on the main laptop display if an external
    > display is attached with kernel v4.1-rc1. I tracked the regression down
    > to commit c9f038a1a592 ("drm/i915: Don't assume primary & cursor are
    > always on for wm calculation (v4)"), and sent a patch that fixes that
    > behavior at [1] about two weeks ago, although I'm not sure it's the
    > right thing to do, as I'm not very familiar with the code. The same bug
    > still exists in vv4.1-rc3.
    >
    > Jan Niehusmann confirmed the behavior, but there has been no further
    > discussion on the topic. I also forgot to cc the people that were
    > involved in the patch that caused the regression (sorry about that).
    >
    > Is there anything else that I can do to help fixing this issue?
    >
    > Thanks,
    > Thomas
    >
    > [1] http://lists.freedesktop.org/archives/intel-gfx/2015-April/065494.html

    Sorry, I missed your patch when you first sent it. That type of fix
    looks like an okay workaround while we do a more in-depth rework of the
    watermark system. However I think your patch could cause a crash if we
    disable the primary plane via the universal plane interface; if we do
    that, p->pri.bytes_per_pixel is set to 0, but since we're now pretending
    the primary plane is always enabled, ilk_wm_fbc() can eventually get
    called and use that 0 in the denominator of a division operation.

    If you just squash the following change into your patch, I think it should be
    safe:

    diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
    index eb97cbe..99fa8ee 100644
    --- a/drivers/gpu/drm/i915/intel_pm.c
    +++ b/drivers/gpu/drm/i915/intel_pm.c
    @@ -2076,7 +2076,7 @@ static void ilk_compute_wm_parameters(struct drm_crtc *crtc,
    p->pri.bytes_per_pixel =
    crtc->primary->state->fb->bits_per_pixel / 8;
    else
    - p->pri.bytes_per_pixel = 0;
    + p->pri.bytes_per_pixel = 4;

    p->cur.bytes_per_pixel = 4;
    /*


    Matt

    --
    Matt Roper
    Graphics Software Engineer
    IoTG Platform Enabling & Development
    Intel Corporation
    (916) 356-2795


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