drm/i915: Zero out HOWM registers before writing new WM/HOWM register values

On VLV/CHV some of the watermark values are split across two registers:
low order bits in one, and high order bits in another. So we may not be
able to update a single watermark value atomically, and thus we must be
careful that we don't temporarily introduce out of bounds values during
the reprogramming. To prevent this we can simply zero out all the high
order bits initially, then we update the low order bits, and finally
we update the high order bits with the final value.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480354637-14209-13-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/i915/intel_pm.c

@@ -877,6 +877,17 @@ static void vlv_write_wm_values(struct intel_crtc *crtc,
 		   (wm->ddl[pipe].plane[PLANE_SPRITE0] << DDL_SPRITE_SHIFT(0)) |
 		   (wm->ddl[pipe].plane[PLANE_PRIMARY] << DDL_PLANE_SHIFT));
 
+	/*
+	 * Zero the (unused) WM1 watermarks, and also clear all the
+	 * high order bits so that there are no out of bounds values
+	 * present in the registers during the reprogramming.
+	 */
+	I915_WRITE(DSPHOWM, 0);
+	I915_WRITE(DSPHOWM1, 0);
+	I915_WRITE(DSPFW4, 0);
+	I915_WRITE(DSPFW5, 0);
+	I915_WRITE(DSPFW6, 0);
+
 	I915_WRITE(DSPFW1,
 		   FW_WM(wm->sr.plane, SR) |
 		   FW_WM(wm->pipe[PIPE_B].plane[PLANE_CURSOR], CURSORB) |
@@ -924,12 +935,6 @@ static void vlv_write_wm_values(struct intel_crtc *crtc,
 			   FW_WM(wm->pipe[PIPE_A].plane[PLANE_PRIMARY] >> 8, PLANEA_HI));
 	}
 
-	/* zero (unused) WM1 watermarks */
-	I915_WRITE(DSPFW4, 0);
-	I915_WRITE(DSPFW5, 0);
-	I915_WRITE(DSPFW6, 0);
-	I915_WRITE(DSPHOWM1, 0);
-
 	POSTING_READ(DSPFW1);
 }