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Srinivasan Shanmugam 437cbca5ea drm/amd/display: Fix unsigned underflow in SubVP vblank schedulability check v2

subvp_vblank_schedulable() checks whether the SubVP active region can
fit the prefetch time, vblank frame time, and the larger of the vblank
blanking time and MALL region.

Commit 11236ac905 ("drm/amd/display: Fix dml2_0 narrowing boundaries")
changed the intermediate timing variables from u16 to u32. After that
change, the schedulability test:

	subvp_active_us - prefetch_us - vblank_frame_us -
	max_vblank_mallregion > 0

is evaluated in unsigned arithmetic. If the required time exceeds the
available active time, the subtraction can underflow and wrap, producing
a large positive value instead of a negative result.

Fix this by comparing the available time against the required time
directly, using u64 for the accumulated required duration.

v2:
- Use uint64_t instead of u64 for consistency with DC style (Dillon)
- Make all terms explicitly uint64_t in the accumulated sum & use
  single-definition form for required_us (Gaghik)

Fixes: 11236ac905 ("drm/amd/display: Fix dml2_0 narrowing boundaries")
Reported-by: Dan Carpenter <error27@gmail.com>
Cc: Roman Li <roman.li@amd.com>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Tom Chung <chiahsuan.chung@amd.com>
Cc: Dillon Varone <dillon.varone@amd.com>
Cc: Gaghik Khachatrian <gaghik.khachatrian@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Chenyu Chen <chen-yu.chen@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>