Re: [PATCH] drm/i915/display: Make intel_crtc_get_vblank_counter safe on PREEMPT_RT

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

On Fri, Aug 29, 2025 at 03:17:02PM +0200, Maarten Lankhorst wrote:
> drm_crtc_accurate_vblank_count takes a spinlock, which we should avoid
> in tracepoints and debug functions.
> 
> This also prevents taking the spinlock 2x during the critical
> section of pipe updates for DSI updates.
> 
> Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>
> ---
>  drivers/gpu/drm/i915/display/intel_crtc.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_crtc.c b/drivers/gpu/drm/i915/display/intel_crtc.c
> index a187db6df2d36..cee00aa2c79de 100644
> --- a/drivers/gpu/drm/i915/display/intel_crtc.c
> +++ b/drivers/gpu/drm/i915/display/intel_crtc.c
> @@ -84,8 +84,12 @@ u32 intel_crtc_get_vblank_counter(struct intel_crtc *crtc)
>  	if (!crtc->active)
>  		return 0;
>  
> -	if (!vblank->max_vblank_count)
> -		return (u32)drm_crtc_accurate_vblank_count(&crtc->base);
> +	if (!vblank->max_vblank_count) {
> +		if (!IS_ENABLED(CONFIG_PREEMPT_RT))

The ! seems rather pointless here when you could just swap the two
statements instead.

Could also use a comment to remind us why we're avoiding the use
of the correct function with RT kernels.

With that
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> +			return (u32)drm_crtc_accurate_vblank_count(&crtc->base);
> +		else
> +			return (u32)drm_crtc_vblank_count(&crtc->base);
> +	}
>  
>  	return crtc->base.funcs->get_vblank_counter(&crtc->base);
>  }
> -- 
> 2.50.0

-- 
Ville Syrjälä
Intel