From 41b578fb0e8b930f2470d3f673b0fa279e77a7b8 Mon Sep 17 00:00:00 2001
From: Jesse Barnes <jbarnes@virtuousgeek.org>
Date: Tue, 22 Sep 2015 12:15:54 -0700
Subject: [PATCH] drm/i915: workaround bad DSL readout v3
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On HSW at least (still testing other platforms, but should be harmless
elsewhere), the DSL reg reads back as 0 when read around vblank start
time.  This ends up confusing the atomic start/end checking code, since
it causes the update to appear as if it crossed a frame count boundary.
Avoid the problem by making sure we don't return scanline_offset from
the get_crtc_scanline function.  In moving the code there, I add to add
an additional delay since it could be called and have a legitimate 0
result for some time (depending on the pixel clock).

v2: move hsw dsl read hack to get_crtc_scanline (Ville)
v3: use break instead of goto (Ville)
    update comment with workaround details (Ville)

References: https://bugs.freedesktop.org/show_bug.cgi?id=91579
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
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