drm/i915/gen9: fix plane_blocks_per_line on watermarks calculations

The confusing thing is that plane_blocks_per_line is listed as part of
the method 2 calculation but is also used for other things. We
calculated it in two different places and different ways: one inside
skl_wm_method2() and the other inside skl_compute_plane_wm(). The
skl_wm_method2() implementation is the one that matches the
specification.

With this patch we fix the skl_compute_plane_wm() calculation and just
pass it as a parameter to skl_wm_method2(). We also take care to not
modify the value of plane_bytes_per_line since we're going to rely on
it having a correct value in later patches.

This should affect the watermarks for Linear and Y-tiled.

From my analysis, it looks like the two plane_blocks_per_line
variables got out of sync on 0fda6568, but we can't really say
that commit was a regression, it looks like just an incomplete fix.
There's always the possibility that 0fda6568 matched our
specification at that time, and then later the specification changed.

v2: Try to add a "Fixes" tag (Maarten).

Fixes: 0fda6568 ("drm/i915/skl: Update watermarks for Y tiling")
Cc: stable@vger.kernel.org
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Lyude <cpaul@redhat.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474578035-424-7-git-send-email-paulo.r.zanoni@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -3494,30 +3494,14 @@ static uint32_t skl_wm_method1(uint32_t pixel_rate, uint8_t cpp, uint32_t latenc
 }
 
 static uint32_t skl_wm_method2(uint32_t pixel_rate, uint32_t pipe_htotal,
-			       uint32_t horiz_pixels, uint8_t cpp,
-			       uint64_t tiling, uint32_t latency,
-			       uint32_t y_min_scanlines)
+			       uint32_t latency, uint32_t plane_blocks_per_line)
 {
 	uint32_t ret;
-	uint32_t plane_bytes_per_line, plane_blocks_per_line;
 	uint32_t wm_intermediate_val;
 
 	if (latency == 0)
 		return UINT_MAX;
 
-	plane_bytes_per_line = horiz_pixels * cpp;
-
-	if (tiling == I915_FORMAT_MOD_Y_TILED ||
-	    tiling == I915_FORMAT_MOD_Yf_TILED) {
-		plane_bytes_per_line *= y_min_scanlines;
-		plane_blocks_per_line = DIV_ROUND_UP(plane_bytes_per_line, 512);
-		plane_blocks_per_line /= y_min_scanlines;
-	} else if (tiling == DRM_FORMAT_MOD_NONE) {
-		plane_blocks_per_line = DIV_ROUND_UP(plane_bytes_per_line, 512) + 1;
-	} else {
-		plane_blocks_per_line = DIV_ROUND_UP(plane_bytes_per_line, 512);
-	}
-
 	wm_intermediate_val = latency * pixel_rate;
 	ret = DIV_ROUND_UP(wm_intermediate_val, pipe_htotal * 1000) *
 				plane_blocks_per_line;
@@ -3606,17 +3590,24 @@ static int skl_compute_plane_wm(const struct drm_i915_private *dev_priv,
 		y_min_scanlines = 4;
 	}
 
+	plane_bytes_per_line = width * cpp;
+	if (fb->modifier[0] == I915_FORMAT_MOD_Y_TILED ||
+	    fb->modifier[0] == I915_FORMAT_MOD_Yf_TILED) {
+		plane_blocks_per_line =
+		      DIV_ROUND_UP(plane_bytes_per_line * y_min_scanlines, 512);
+		plane_blocks_per_line /= y_min_scanlines;
+	} else if (fb->modifier[0] == DRM_FORMAT_MOD_NONE) {
+		plane_blocks_per_line = DIV_ROUND_UP(plane_bytes_per_line, 512)
+					+ 1;
+	} else {
+		plane_blocks_per_line = DIV_ROUND_UP(plane_bytes_per_line, 512);
+	}
+
 	method1 = skl_wm_method1(plane_pixel_rate, cpp, latency);
 	method2 = skl_wm_method2(plane_pixel_rate,
 				 cstate->base.adjusted_mode.crtc_htotal,
-				 width,
-				 cpp,
-				 fb->modifier[0],
 				 latency,
-				 y_min_scanlines);
-
-	plane_bytes_per_line = width * cpp;
-	plane_blocks_per_line = DIV_ROUND_UP(plane_bytes_per_line, 512);
+				 plane_blocks_per_line);
 
 	if (fb->modifier[0] == I915_FORMAT_MOD_Y_TILED ||
 	    fb->modifier[0] == I915_FORMAT_MOD_Yf_TILED) {