[RFC PATCH v4 03/42] drm: Correctly round for fixp2int_round

[Harry Wentland]

A value of 0x80000000 and higher should round up, and
below should round down. VKMS Kunit tests for lerp_u16
showed that this is not the case. Fix it.

1 << (DRM_FIXED_POINT_HALF - 1) =
1 << 15 = 0x8000

This is not 0.5, but 0.00000762939453125.

Instead of some smart math use a simple if/else to
round up or down. This helps people like me to understand
what the function does.

Signed-off-by: Harry Wentland <harry.wentland at amd.com>
---
 include/drm/drm_fixed.h | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/include/drm/drm_fixed.h b/include/drm/drm_fixed.h
index cb842ba80ddd..8ee549f68537 100644
--- a/include/drm/drm_fixed.h
+++ b/include/drm/drm_fixed.h
@@ -77,6 +77,8 @@ static inline u32 dfixed_div(fixed20_12 A, fixed20_12 B)
 #define DRM_FIXED_DIGITS_MASK	(~DRM_FIXED_DECIMAL_MASK)
 #define DRM_FIXED_EPSILON	1LL
 #define DRM_FIXED_ALMOST_ONE	(DRM_FIXED_ONE - DRM_FIXED_EPSILON)
+#define DRM_FIXED_FRACTIONAL	0xffffffffll
+#define DRM_FIXED_HALF		0x80000000ll
 
 /**
  * @drm_sm2fixp
@@ -106,11 +108,6 @@ static inline int drm_fixp2int(s64 a)
 	return ((s64)a) >> DRM_FIXED_POINT;
 }
 
-static inline int drm_fixp2int_round(s64 a)
-{
-	return drm_fixp2int(a + (1 << (DRM_FIXED_POINT_HALF - 1)));
-}
-
 static inline int drm_fixp2int_ceil(s64 a)
 {
 	if (a >= 0)
@@ -119,6 +116,14 @@ static inline int drm_fixp2int_ceil(s64 a)
 		return drm_fixp2int(a - DRM_FIXED_ALMOST_ONE);
 }
 
+static inline int drm_fixp2int_round(s64 a)
+{
+	if ((a & DRM_FIXED_FRACTIONAL) < DRM_FIXED_HALF)
+		return drm_fixp2int(a);
+	else
+		return drm_fixp2int_ceil(a);
+}
+
 static inline unsigned drm_fixp_msbset(s64 a)
 {
 	unsigned shift, sign = (a >> 63) & 1;
-- 
2.44.0