drm/i915: Extract intel_adjusted_rate()

Extract a small helper to calculate the downscaling
adjusted pixel rate/data rate/etc.

v2: Drop the plane visibility check and add a comment explaining why
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210401154043.19466-1-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/display/intel_atomic_plane.c

@@ -133,25 +133,45 @@ intel_plane_destroy_state(struct drm_plane *plane,
 	kfree(plane_state);
 }
 
-unsigned int intel_plane_pixel_rate(const struct intel_crtc_state *crtc_state,
-				    const struct intel_plane_state *plane_state)
+static unsigned int intel_adjusted_rate(const struct drm_rect *src,
+					const struct drm_rect *dst,
+					unsigned int rate)
 {
 	unsigned int src_w, src_h, dst_w, dst_h;
-	unsigned int pixel_rate = crtc_state->pixel_rate;
 
-	src_w = drm_rect_width(&plane_state->uapi.src) >> 16;
-	src_h = drm_rect_height(&plane_state->uapi.src) >> 16;
-	dst_w = drm_rect_width(&plane_state->uapi.dst);
-	dst_h = drm_rect_height(&plane_state->uapi.dst);
+	src_w = drm_rect_width(src) >> 16;
+	src_h = drm_rect_height(src) >> 16;
+	dst_w = drm_rect_width(dst);
+	dst_h = drm_rect_height(dst);
 
 	/* Downscaling limits the maximum pixel rate */
 	dst_w = min(src_w, dst_w);
 	dst_h = min(src_h, dst_h);
 
-	return DIV_ROUND_UP_ULL(mul_u32_u32(pixel_rate, src_w * src_h),
+	return DIV_ROUND_UP_ULL(mul_u32_u32(rate, src_w * src_h),
 				dst_w * dst_h);
 }
 
+unsigned int intel_plane_pixel_rate(const struct intel_crtc_state *crtc_state,
+				    const struct intel_plane_state *plane_state)
+{
+	/*
+	 * Note we don't check for plane visibility here as
+	 * we want to use this when calculating the cursor
+	 * watermarks even if the cursor is fully offscreen.
+	 * That depends on the src/dst rectangles being
+	 * correctly populated whenever the watermark code
+	 * considers the cursor to be visible, whether or not
+	 * it is actually visible.
+	 *
+	 * See: intel_wm_plane_visible() and intel_check_cursor()
+	 */
+
+	return intel_adjusted_rate(&plane_state->uapi.src,
+				   &plane_state->uapi.dst,
+				   crtc_state->pixel_rate);
+}
+
 unsigned int intel_plane_data_rate(const struct intel_crtc_state *crtc_state,
 				   const struct intel_plane_state *plane_state)
 {