[PATCH] [RFC v3]: drm/i915/display: Use ceiling division for NV12 UV surface offset calculation

[Vidya Srinivas <vidya.srinivas@intel.com>]

For LNL+, odd source size and panning for YUV 422/420 surfaces is
supported. However, it requires the UV (chroma) surface Start X/Y and
width/height to be calculated as ceiling(half of Y plane value) rather
than floor.

The current code uses (>> 17) which combines the U16.16 fixed-point to
integer conversion (>> 16) with a divide-by-2 for chroma subsampling
(>> 1) into a single floor division. For odd Y plane values this
produces an off-by-one error in the UV plane offset.

On Android systems we see PLANE ATS fault when NV12 overlays are
used with odd source dimensions:

[  126.854200] xe 0000:00:02.0: [drm:intel_atomic_setup_scaler [xe]] [CRTC:148:pipe A] attached scaler id 0.0 to PLANE:33
[  126.854617] xe 0000:00:02.0: [drm:skl_update_scaler [xe]] [CRTC:148:pipe A] scaler_user index 0.0: staged scaling request for 1279x719->1340x753
[  126.854837] xe 0000:00:02.0: [drm:intel_plane_atomic_check [xe]] UV plane [PLANE:33:plane 1A] using Y plane [PLANE:123:plane 4A]
[  126.854926] xe 0000:00:02.0: [drm] *ERROR* [CRTC:148:pipe A] PLANE ATS fault

With Y plane width 1279:
  floor(1279/2) = 639 (current)
  ceil(1279/2)  = 640 (required)

Introduce fp_16_16_div2() and fp_16_16_to_int_ceil() helpers to cleanly
separate the two operations: first halve the U16.16 fixed-point value
for chroma subsampling (staying in fixed-point domain), then convert
to integer with ceiling rounding.

v2: Use DIV_ROUND_UP(value, 1 << 17) to preserve sub-pixel precision
    while making the ceiling division readable (Jani, Uma)

v3: Split into two helpers - fp_16_16_div2() for fixed-point division
    by 2 and fp_16_16_to_int_ceil() for ceiling conversion to integer,
    cleanly separating chroma subsampling from fixed-point to integer
    conversion (Jani)

Signed-off-by: Vidya Srinivas <vidya.srinivas@intel.com>
---
 .../drm/i915/display/skl_universal_plane.c    | 27 ++++++++++++++++---
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/skl_universal_plane.c b/drivers/gpu/drm/i915/display/skl_universal_plane.c
index 7a9d494334b5..e772b0d716c7 100644
--- a/drivers/gpu/drm/i915/display/skl_universal_plane.c
+++ b/drivers/gpu/drm/i915/display/skl_universal_plane.c
@@ -2126,6 +2126,19 @@ static int skl_check_main_surface(struct intel_plane_state *plane_state)
 	return 0;
 }
 
+
+/* Divide a U16.16 fixed-point value by 2, staying in fixed-point domain */
+static inline u32 fp_16_16_div2(u32 fp)
+{
+	return fp >> 1;
+}
+
+/* Convert a U16.16 fixed-point value to integer, rounding up */
+static inline int fp_16_16_to_int_ceil(u32 fp)
+{
+	return DIV_ROUND_UP(fp, 1 << 16);
+}
+
 static int skl_check_nv12_aux_surface(struct intel_plane_state *plane_state)
 {
 	struct intel_display *display = to_intel_display(plane_state);
@@ -2139,10 +2152,16 @@ static int skl_check_nv12_aux_surface(struct intel_plane_state *plane_state)
 	int min_height = intel_plane_min_height(plane, fb, uv_plane, rotation);
 	int max_width = intel_plane_max_width(plane, fb, uv_plane, rotation);
 	int max_height = intel_plane_max_height(plane, fb, uv_plane, rotation);
-	int x = plane_state->uapi.src.x1 >> 17;
-	int y = plane_state->uapi.src.y1 >> 17;
-	int w = drm_rect_width(&plane_state->uapi.src) >> 17;
-	int h = drm_rect_height(&plane_state->uapi.src) >> 17;
+
+	/*
+	 * LNL+ UV surface start/size =
+	 * ceiling(half of Y plane start/size). Use ceiling division
+	 * unconditionally; it is a no-op for even values.
+	 */
+	int x = fp_16_16_to_int_ceil(fp_16_16_div2(plane_state->uapi.src.x1));
+	int y = fp_16_16_to_int_ceil(fp_16_16_div2(plane_state->uapi.src.y1));
+	int w = fp_16_16_to_int_ceil(fp_16_16_div2(drm_rect_width(&plane_state->uapi.src)));
+	int h = fp_16_16_to_int_ceil(fp_16_16_div2(drm_rect_height(&plane_state->uapi.src)));
 	u32 offset;
 
 	/* FIXME not quite sure how/if these apply to the chroma plane */
-- 
2.45.2