drm/i915: workaround bad DSL readout v3

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>

drivers/gpu/drm/i915/i915_irq.c

@@ -639,6 +639,32 @@ static int __intel_get_crtc_scanline(struct intel_crtc *crtc)
 	else
 		position = __raw_i915_read32(dev_priv, PIPEDSL(pipe)) & DSL_LINEMASK_GEN3;
 
+	/*
+	 * On HSW, the DSL reg (0x70000) appears to return 0 if we
+	 * read it just before the start of vblank.  So try it again
+	 * so we don't accidentally end up spanning a vblank frame
+	 * increment, causing the pipe_update_end() code to squak at us.
+	 *
+	 * The nature of this problem means we can't simply check the ISR
+	 * bit and return the vblank start value; nor can we use the scanline
+	 * debug register in the transcoder as it appears to have the same
+	 * problem.  We may need to extend this to include other platforms,
+	 * but so far testing only shows the problem on HSW.
+	 */
+	if (IS_HASWELL(dev) && !position) {
+		int i, temp;
+
+		for (i = 0; i < 100; i++) {
+			udelay(1);
+			temp = __raw_i915_read32(dev_priv, PIPEDSL(pipe)) &
+				DSL_LINEMASK_GEN3;
+			if (temp != position) {
+				position = temp;
+				break;
+			}
+		}
+	}
+
 	/*
 	 * See update_scanline_offset() for the details on the
 	 * scanline_offset adjustment.