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

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>