drm: Add helper for conversion from signed-magnitude

CTM values are defined as signed-magnitude values. Add
a helper that converts from CTM signed-magnitude fixed
point value to the twos-complement value used by
drm_fixed.

Reviewed-by: Sebastian Wick <sebastian.wick@redhat.com>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Simon Ser <contact@emersion.fr>
Link: https://patch.msgid.link/20251115000237.3561250-2-alex.hung@amd.com

diff --git a/include/drm/drm_fixed.h b/include/drm/drm_fixed.h
index 1922188f00e89e97fd3ca5cccb5bde43a82a20c9..33de514a522129962bdbb7341fc4d843f62ca57a 100644 (file)
--- a/include/drm/drm_fixed.h
+++ b/include/drm/drm_fixed.h
@@ -78,6 +78,23 @@ static inline u32 dfixed_div(fixed20_12 A, fixed20_12 B)
 #define DRM_FIXED_EPSILON      1LL
 #define DRM_FIXED_ALMOST_ONE   (DRM_FIXED_ONE - DRM_FIXED_EPSILON)
 
+/**
+ * @drm_sm2fixp
+ *
+ * Convert a 1.31.32 signed-magnitude fixed point to 32.32
+ * 2s-complement fixed point
+ *
+ * @return s64 2s-complement fixed point
+ */
+static inline s64 drm_sm2fixp(__u64 a)
+{
+       if ((a & (1LL << 63))) {
+               return -(a & 0x7fffffffffffffffll);
+       } else {
+               return a;
+       }
+}
+
 static inline s64 drm_int2fixp(int a)
 {
        return ((s64)a) << DRM_FIXED_POINT;