drm/rect: Handle rounding errors in drm_rect_clip_scaled, v3.

Instead of relying on a scale which may increase rounding errors,
clip src by doing: src * (dst - clip) / dst and rounding the result
away from 1, so the new coordinates get closer to 1. We won't need
to fix up with a magic macro afterwards, because our scaling factor
will never go to the other side of 1.

Changes since v1:
- Adjust dst immediately, else drm_rect_width/height on dst gives bogus
  results.
Change since v2:
- Get rid of macros and use 64-bits math.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[mlankhorst: Add Villes comment, and rename newsrc to tmp. (Ville)]
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180503112217.37292-3-maarten.lankhorst@linux.intel.com

1 files changed, 36 insertions, 13 deletions

diff --git a/drivers/gpu/drm/drm_rect.c b/drivers/gpu/drm/drm_rect.c
index a8e934795c7d..8c057829b804 100644
--- a/drivers/gpu/drm/drm_rect.c
+++ b/drivers/gpu/drm/drm_rect.c
@@ -50,13 +50,25 @@ bool drm_rect_intersect(struct drm_rect *r1, const struct drm_rect *r2)
 }
 EXPORT_SYMBOL(drm_rect_intersect);
 
+static u32 clip_scaled(u32 src, u32 dst, u32 clip)
+{
+	u64 tmp = mul_u32_u32(src, dst - clip);
+
+	/*
+	 * Round toward 1.0 when clipping so that we don't accidentally
+	 * change upscaling to downscaling or vice versa.
+	 */
+	if (src < (dst << 16))
+		return DIV_ROUND_UP_ULL(tmp, dst);
+	else
+		return DIV_ROUND_DOWN_ULL(tmp, dst);
+}
+
 /**
  * drm_rect_clip_scaled - perform a scaled clip operation
  * @src: source window rectangle
  * @dst: destination window rectangle
  * @clip: clip rectangle
- * @hscale: horizontal scaling factor
- * @vscale: vertical scaling factor
  *
  * Clip rectangle @dst by rectangle @clip. Clip rectangle @src by the
  * same amounts multiplied by @hscale and @vscale.
@@ -66,33 +78,44 @@ EXPORT_SYMBOL(drm_rect_intersect);
  * %false otherwise
  */
 bool drm_rect_clip_scaled(struct drm_rect *src, struct drm_rect *dst,
-			  const struct drm_rect *clip,
-			  int hscale, int vscale)
+			  const struct drm_rect *clip)
 {
 	int diff;
 
 	diff = clip->x1 - dst->x1;
 	if (diff > 0) {
-		int64_t tmp = src->x1 + (int64_t) diff * hscale;
-		src->x1 = clamp_t(int64_t, tmp, INT_MIN, INT_MAX);
+		u32 new_src_w = clip_scaled(drm_rect_width(src),
+					    drm_rect_width(dst), diff);
+
+		src->x1 = clamp_t(int64_t, src->x2 - new_src_w, INT_MIN, INT_MAX);
+		dst->x1 = clip->x1;
 	}
 	diff = clip->y1 - dst->y1;
 	if (diff > 0) {
-		int64_t tmp = src->y1 + (int64_t) diff * vscale;
-		src->y1 = clamp_t(int64_t, tmp, INT_MIN, INT_MAX);
+		u32 new_src_h = clip_scaled(drm_rect_height(src),
+					    drm_rect_height(dst), diff);
+
+		src->y1 = clamp_t(int64_t, src->y2 - new_src_h, INT_MIN, INT_MAX);
+		dst->y1 = clip->y1;
 	}
 	diff = dst->x2 - clip->x2;
 	if (diff > 0) {
-		int64_t tmp = src->x2 - (int64_t) diff * hscale;
-		src->x2 = clamp_t(int64_t, tmp, INT_MIN, INT_MAX);
+		u32 new_src_w = clip_scaled(drm_rect_width(src),
+					    drm_rect_width(dst), diff);
+
+		src->x2 = clamp_t(int64_t, src->x1 + new_src_w, INT_MIN, INT_MAX);
+		dst->x2 = clip->x2;
 	}
 	diff = dst->y2 - clip->y2;
 	if (diff > 0) {
-		int64_t tmp = src->y2 - (int64_t) diff * vscale;
-		src->y2 = clamp_t(int64_t, tmp, INT_MIN, INT_MAX);
+		u32 new_src_h = clip_scaled(drm_rect_height(src),
+					    drm_rect_height(dst), diff);
+
+		src->y2 = clamp_t(int64_t, src->y1 + new_src_h, INT_MIN, INT_MAX);
+		dst->y2 = clip->y2;
 	}
 
-	return drm_rect_intersect(dst, clip);
+	return drm_rect_visible(dst);
 }
 EXPORT_SYMBOL(drm_rect_clip_scaled);