drm/i915: Account for scale factor when calculating initial phase

To get the initial phase correct we need to account for the scale
factor as well. I forgot this initially and was mostly looking at
heavily upscaled content where the minor difference between -0.5
and the proper initial phase was not readily apparent.

And let's toss in a comment that tries to explain the formula
a little bit.

v2: The initial phase upper limit is 1.5, not 24.0!

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Fixes: 0a59952 ("drm/i915: Configure SKL+ scaler initial phase correctly")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181029181820.21956-1-ville.syrjala@linux.intel.com
Tested-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Tested-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> #irc
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> #irc

drivers/gpu/drm/i915/intel_display.c

@@ -4777,15 +4777,47 @@ static void cpt_verify_modeset(struct drm_device *dev, int pipe)
  * chroma samples for both of the luma samples, and thus we don't
  * actually get the expected MPEG2 chroma siting convention :(
  * The same behaviour is observed on pre-SKL platforms as well.
+ *
+ * Theory behind the formula (note that we ignore sub-pixel
+ * source coordinates):
+ * s = source sample position
+ * d = destination sample position
+ *
+ * Downscaling 4:1:
+ * -0.5
+ * | 0.0
+ * | |     1.5 (initial phase)
+ * | |     |
+ * v v     v
+ * | s | s | s | s |
+ * |       d       |
+ *
+ * Upscaling 1:4:
+ * -0.5
+ * | -0.375 (initial phase)
+ * | |     0.0
+ * | |     |
+ * v v     v
+ * |       s       |
+ * | d | d | d | d |
  */
-u16 skl_scaler_calc_phase(int sub, bool chroma_cosited)
+u16 skl_scaler_calc_phase(int sub, int scale, bool chroma_cosited)
 {
 	int phase = -0x8000;
 	u16 trip = 0;
 
 	if (chroma_cosited)
 		phase += (sub - 1) * 0x8000 / sub;
 
+	phase += scale / (2 * sub);
+
+	/*
+	 * Hardware initial phase limited to [-0.5:1.5].
+	 * Since the max hardware scale factor is 3.0, we
+	 * should never actually excdeed 1.0 here.
+	 */
+	WARN_ON(phase < -0x8000 || phase > 0x18000);
+
 	if (phase < 0)
 		phase = 0x10000 + phase;
 	else
@@ -4994,13 +5026,20 @@ static void skylake_pfit_enable(const struct intel_crtc_state *crtc_state)
 
 	if (crtc_state->pch_pfit.enabled) {
 		u16 uv_rgb_hphase, uv_rgb_vphase;
+		int pfit_w, pfit_h, hscale, vscale;
 		int id;
 
 		if (WARN_ON(crtc_state->scaler_state.scaler_id < 0))
 			return;
 
-		uv_rgb_hphase = skl_scaler_calc_phase(1, false);
-		uv_rgb_vphase = skl_scaler_calc_phase(1, false);
+		pfit_w = (crtc_state->pch_pfit.size >> 16) & 0xFFFF;
+		pfit_h = crtc_state->pch_pfit.size & 0xFFFF;
+
+		hscale = (crtc_state->pipe_src_w << 16) / pfit_w;
+		vscale = (crtc_state->pipe_src_h << 16) / pfit_h;
+
+		uv_rgb_hphase = skl_scaler_calc_phase(1, hscale, false);
+		uv_rgb_vphase = skl_scaler_calc_phase(1, vscale, false);
 
 		id = scaler_state->scaler_id;
 		I915_WRITE(SKL_PS_CTRL(pipe, id), PS_SCALER_EN |

drivers/gpu/drm/i915/intel_drv.h

@@ -1720,7 +1720,7 @@ void intel_mode_from_pipe_config(struct drm_display_mode *mode,
 void intel_crtc_arm_fifo_underrun(struct intel_crtc *crtc,
 				  struct intel_crtc_state *crtc_state);
 
-u16 skl_scaler_calc_phase(int sub, bool chroma_center);
+u16 skl_scaler_calc_phase(int sub, int scale, bool chroma_center);
 int skl_update_scaler_crtc(struct intel_crtc_state *crtc_state);
 int skl_max_scale(const struct intel_crtc_state *crtc_state,
 		  u32 pixel_format);

drivers/gpu/drm/i915/intel_sprite.c

@@ -326,23 +326,31 @@ skl_program_scaler(struct intel_plane *plane,
 	uint32_t crtc_h = drm_rect_height(&plane_state->base.dst);
 	u16 y_hphase, uv_rgb_hphase;
 	u16 y_vphase, uv_rgb_vphase;
+	int hscale, vscale;
+
+	hscale = drm_rect_calc_hscale(&plane_state->base.src,
+				      &plane_state->base.dst,
+				      0, INT_MAX);
+	vscale = drm_rect_calc_vscale(&plane_state->base.src,
+				      &plane_state->base.dst,
+				      0, INT_MAX);
 
 	/* TODO: handle sub-pixel coordinates */
 	if (plane_state->base.fb->format->format == DRM_FORMAT_NV12 &&
 	    !icl_is_hdr_plane(plane)) {
-		y_hphase = skl_scaler_calc_phase(1, false);
-		y_vphase = skl_scaler_calc_phase(1, false);
+		y_hphase = skl_scaler_calc_phase(1, hscale, false);
+		y_vphase = skl_scaler_calc_phase(1, vscale, false);
 
 		/* MPEG2 chroma siting convention */
-		uv_rgb_hphase = skl_scaler_calc_phase(2, true);
-		uv_rgb_vphase = skl_scaler_calc_phase(2, false);
+		uv_rgb_hphase = skl_scaler_calc_phase(2, hscale, true);
+		uv_rgb_vphase = skl_scaler_calc_phase(2, vscale, false);
 	} else {
 		/* not used */
 		y_hphase = 0;
 		y_vphase = 0;
 
-		uv_rgb_hphase = skl_scaler_calc_phase(1, false);
-		uv_rgb_vphase = skl_scaler_calc_phase(1, false);
+		uv_rgb_hphase = skl_scaler_calc_phase(1, hscale, false);
+		uv_rgb_vphase = skl_scaler_calc_phase(1, vscale, false);
 	}
 
 	I915_WRITE_FW(SKL_PS_CTRL(pipe, scaler_id),