drm/i915/display: Fix NV12 ceiling division for bigjoiner case

Commit 16df4cc63c58 ("drm/i915/display: Use ceiling division for NV12
UV surface offset calculation") computes the UV (chroma) surface
start/size as ceiling(half of Y plane start/size) directly from the
U16.16 fixed-point source rectangle:

        x = fp_16_16_to_int_ceil(fp_16_16_div2(src.x1));

For a single pipe the source coordinates are integers, so this is
correct.
(UV start = ceiling(half of Y plane start)).

With bigjoiner + a plane scaler the picture changes. The pipe boundary
is a fixed integer destination pixel, but the plane's position and the
scaler ratio are arbitrary, so drm_rect_clip_scaled() maps the seam back
to a *fractional* per-pipe source. For a 1280->2407 upscaled NV12 plane
crossing the seam:

        master src: width = 1204 * 1280/2407 = 640.265899, x1 = 0
        joiner src: width = 1203 * 1280/2407 = 639.734115, x1 = 640.265884

The luma path floors this to an integer (src.x1 >> 16 = 640), but the
UV path takes ceiling(640.265884 / 2) = ceil(320.13) = 321. The Y plane
then starts at column 640 while the UV plane starts at 321*2 = 642,
pushing the chroma read one column past the 640-wide chroma surface on
the joiner secondary:

        [CRTC:382:pipe C] PLANE ATS fault
        [CRTC:382:pipe C][PLANE:267:plane 1C] fault (CTL=0x81009400, ...)

The spec "Y plane start" is the integer pixel the luma surface actually
programs (640), not the pre-floor fixed-point value (640.27). Convert
the Y plane start/size to integer first - matching skl_check_main_surface()
- and then apply the ceiling. This is a no-op for the integer (non-joiner)
case and yields the correct, in-bounds chroma offset for the fractional
joiner seam:

                     before fix      after fix
        master 1B:   x=0  w=321      x=0   w=320   -> [0, 320)
        slave  1C:   x=321 w=320     x=320 w=320   -> [320, 640)

The two halves now tile the 640-wide chroma plane exactly and the ATS
fault is gone.

Assisted-by: GitHub-Copilot:Claude-Opus-4.8
Fixes: 16df4cc63c58 ("drm/i915/display: Use ceiling division for NV12 UV surface offset calculation")
Signed-off-by: Vidya Srinivas <vidya.srinivas@intel.com>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patch.msgid.link/20260618181837.687302-1-vidya.srinivas@intel.com
(cherry picked from commit 0c59cc78241c10e5f02d92b28d811b0435e706a7)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

1 files changed, 13 insertions, 20 deletions

diff --git a/drivers/gpu/drm/i915/display/skl_universal_plane.c b/drivers/gpu/drm/i915/display/skl_universal_plane.c
index ad4bfff6903d..164b7d61c9a3 100644
--- a/drivers/gpu/drm/i915/display/skl_universal_plane.c
+++ b/drivers/gpu/drm/i915/display/skl_universal_plane.c
@@ -2126,19 +2126,6 @@ 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);
@@ -2154,14 +2141,20 @@ static int skl_check_nv12_aux_surface(struct intel_plane_state *plane_state)
 	int max_height = intel_plane_max_height(plane, fb, uv_plane, rotation);
 
 	/*
-	 * LNL+ UV surface start/size =
-	 * ceiling(half of Y plane start/size). Use ceiling division
-	 * unconditionally; it is a no-op for even values.
+	 * UV (chroma) start/size = ceiling(half of the *integer* Y plane
+	 * start/size), i.e. the value the luma surface programs (src >> 16),
+	 * not the raw U16.16. A bigjoiner seam mapped through the scaler can
+	 * give a fractional luma src; ceiling that directly would round the
+	 * chroma one column too far and read past the chroma surface.
 	 */
-	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)));
+	int luma_x = plane_state->uapi.src.x1 >> 16;
+	int luma_y = plane_state->uapi.src.y1 >> 16;
+	int luma_w = drm_rect_width(&plane_state->uapi.src) >> 16;
+	int luma_h = drm_rect_height(&plane_state->uapi.src) >> 16;
+	int x = DIV_ROUND_UP(luma_x, 2);
+	int y = DIV_ROUND_UP(luma_y, 2);
+	int w = DIV_ROUND_UP(luma_x + luma_w, 2) - x;
+	int h = DIV_ROUND_UP(luma_y + luma_h, 2) - y;
 	u32 offset;
 
 	/* FIXME not quite sure how/if these apply to the chroma plane */