Re: [PATCH 00/10] drm/intel/display: Changes required to make vblank evasion PREEMPT_RT safe.

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Jul 02, 2026 at 09:21:43AM +0200, Maarten Lankhorst wrote:
> CONFIG_PREEMPT_RT requires code to be as deterministic as possible,
> and codes with interrupts disabled cannot use spinlocks.
> 
> In order to handle this correctly, we ensure any locking requirements
> are handled before disabling interrupts, and only start locking again
> after interrupts are re-enabled and timing sensitive path completed.
> 
> This also has the benefit of making the vblank evasion code more
> deterministic; without locking there's much less jitter.
> 
> Previously, we scheduled vblank work on completion.
> This is now handled by scheduling the vblank in advance, only arming
> it upon completion without locks.

Sashiko found a bunch of stuff that I was also thinking would be
a problem. Don't think I'll need to repeat it here.

I'm still thinking the easy way out would be to just convert
everything to raw spinlocks. The uncore lock I think should become
easier to deal with once I manage to move all of it into the
display code. I admit that the vblank locking is kinda annoying
and potentially inefficient. I've been pondering about making the
vblank locks to per-crtc to at least avoid some unnecessary contention,
but the fact that a lot of the drivers poke at the vblank internals
directly makes it a lot more work than it should be :/

> 
> Maarten Lankhorst (9):
>   drm/vblank_work: Add methods to schedule vblank_work in 2 stages
>   drm/vblank: Add a 2-stage version of drm_crtc_arm_vblank_event
>   drm/intel/display: Make intel_crtc_arm_vblank_event static
>   drm/intel/display: Convert vblank event handling to 2-stage arming
>   drm/i915/display: Move vblank put until after critical section
>   drm/i915/display: Remove locking from intel_vblank_evade critical
>     section
>   drm/i915/display: Handle vlv dsi workaround in scanline_in_safe_range
>     too
>   drm/i915/display: Make get_vblank_counter use intel_de_read_fw()
>   drm/i915/display: Do not take uncore lock in i915_get_vblank_counter
> 
> Mike Galbraith (1):
>   drm/i915: Use preempt_disable/enable_rt() where recommended
> 
>  drivers/gpu/drm/drm_vblank.c                  |  64 +++++++++-
>  drivers/gpu/drm/drm_vblank_work.c             | 110 +++++++++++++----
>  drivers/gpu/drm/i915/display/intel_crtc.c     |  86 ++++++-------
>  drivers/gpu/drm/i915/display/intel_crtc.h     |   1 -
>  drivers/gpu/drm/i915/display/intel_cursor.c   |   8 +-
>  drivers/gpu/drm/i915/display/intel_de.h       |   8 ++
>  drivers/gpu/drm/i915/display/intel_vblank.c   | 115 ++++++++++--------
>  drivers/gpu/drm/i915/display/intel_vblank.h   |   1 +
>  drivers/gpu/drm/i915/intel_uncore.h           |  26 ++--
>  .../drm/xe/compat-i915-headers/intel_uncore.h |   7 ++
>  include/drm/drm_vblank.h                      |  14 ++-
>  include/drm/drm_vblank_work.h                 |  12 ++
>  12 files changed, 324 insertions(+), 128 deletions(-)
> 
> -- 
> 2.53.0

-- 
Ville Syrjälä
Intel