drm/amdgpu: Allow vblank_disable_immediate.
With instantaneous high precision vblank timestamping that updates at leading edge of vblank, a cooked hw vblank counter which increments at leading edge of vblank, and reliable page flip execution and completion at leading edge of vblank, we should meet the requirements for fast/immediate vblank irq disable/enable. Testing on Linux-4.12-rc5 + drm-next on a Radeon R9 380 Tonga Pro (DCE 10) with timing measurement equipment indicates this works fine, so allow immediate vblank disable for power saving. For debugging in case of unexpected trouble, booting with kernel cmdline option drm.vblankoffdelay=0 (or echo 0 > /sys/module/drm/parameters/vblankoffdelay) would keep vblank irqs permanently on to approximate old behavior. Reviewed-by: NMichel Dänzer <michel.daenzer@amd.com> Signed-off-by: NMario Kleiner <mario.kleiner.de@gmail.com> Cc: Alex Deucher <alexander.deucher@amd.com> Cc: Michel Dänzer <michel.daenzer@amd.com> Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>
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