drm/i915: Simplify cursor register write sequence

It looks like simply writing all the cursor register every single
time might be slightly faster than checking to see of each of
them need to be written. So if any other register apart from
CURPOS needs to be written let's just write all the registers.

CURPOS is left as a special case mainly for 845/865 where we have to
disable the cursor to change many of the cursor parameters. This
introduces a slight chance of the cursor flickering when things get
updated (since we're not currently doing the vblank evade for cursor
updates). If we write CURPOS alone then that obviously can't happen.
And let's follow the same pattern in the i9xx code just for symmetry.
I wasn't able to see a singificant performance difference between
this and just writing all the registers unconditionally.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170327185546.2977-16-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -9332,36 +9332,28 @@ static void i845_update_cursor(struct intel_plane *plane,
 
 	spin_lock_irqsave(&dev_priv->uncore.lock, irqflags);
 
-	if (plane->cursor.cntl != 0 &&
-	    (plane->cursor.base != base ||
-	     plane->cursor.size != size ||
-	     plane->cursor.cntl != cntl)) {
-		/* On these chipsets we can only modify the base/size/stride
-		 * whilst the cursor is disabled.
-		 */
+	/* On these chipsets we can only modify the base/size/stride
+	 * whilst the cursor is disabled.
+	 */
+	if (plane->cursor.base != base ||
+	    plane->cursor.size != size ||
+	    plane->cursor.cntl != cntl) {
 		I915_WRITE_FW(CURCNTR(PIPE_A), 0);
-		plane->cursor.cntl = 0;
-	}
-
-	if (plane->cursor.base != base)
 		I915_WRITE_FW(CURBASE(PIPE_A), base);
-
-	if (plane->cursor.size != size)
 		I915_WRITE_FW(CURSIZE, size);
-
-	if (cntl)
 		I915_WRITE_FW(CURPOS(PIPE_A), pos);
-
-	if (plane->cursor.cntl != cntl)
 		I915_WRITE_FW(CURCNTR(PIPE_A), cntl);
 
+		plane->cursor.base = base;
+		plane->cursor.size = size;
+		plane->cursor.cntl = cntl;
+	} else {
+		I915_WRITE_FW(CURPOS(PIPE_A), pos);
+	}
+
 	POSTING_READ_FW(CURCNTR(PIPE_A));
 
 	spin_unlock_irqrestore(&dev_priv->uncore.lock, irqflags);
-
-	plane->cursor.cntl = cntl;
-	plane->cursor.base = base;
-	plane->cursor.size = size;
 }
 
 static void i845_disable_cursor(struct intel_plane *plane,
@@ -9517,27 +9509,34 @@ static void i9xx_update_cursor(struct intel_plane *plane,
 
 	spin_lock_irqsave(&dev_priv->uncore.lock, irqflags);
 
-	if (plane->cursor.cntl != cntl)
+	/*
+	 * On some platforms writing CURCNTR first will also
+	 * cause CURPOS to be armed by the CURBASE write.
+	 * Without the CURCNTR write the CURPOS write would
+	 * arm itself.
+	 *
+	 * CURCNTR and CUR_FBC_CTL are always
+	 * armed by the CURBASE write only.
+	 */
+	if (plane->cursor.base != base ||
+	    plane->cursor.size != fbc_ctl ||
+	    plane->cursor.cntl != cntl) {
 		I915_WRITE_FW(CURCNTR(pipe), cntl);
-
-	if (plane->cursor.size != fbc_ctl)
-		I915_WRITE_FW(CUR_FBC_CTL(pipe), fbc_ctl);
-
-	if (cntl)
+		if (HAS_CUR_FBC(dev_priv))
+			I915_WRITE_FW(CUR_FBC_CTL(pipe), fbc_ctl);
 		I915_WRITE_FW(CURPOS(pipe), pos);
-
-	if (plane->cursor.cntl != cntl ||
-	    plane->cursor.size != fbc_ctl ||
-	    plane->cursor.base != base)
 		I915_WRITE_FW(CURBASE(pipe), base);
 
+		plane->cursor.base = base;
+		plane->cursor.size = fbc_ctl;
+		plane->cursor.cntl = cntl;
+	} else {
+		I915_WRITE_FW(CURPOS(pipe), pos);
+	}
+
 	POSTING_READ_FW(CURBASE(pipe));
 
 	spin_unlock_irqrestore(&dev_priv->uncore.lock, irqflags);
-
-	plane->cursor.cntl = cntl;
-	plane->cursor.base = base;
-	plane->cursor.size = fbc_ctl;
 }
 
 static void i9xx_disable_cursor(struct intel_plane *plane,