Re: [FOR CI 2/5] drm/i915: Use preempt_disable/enable_rt() where recommended

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 29 Oct 2025, Maarten Lankhorst <dev@lankhorst.se> wrote:
> From: Mike Galbraith <umgwanakikbuti@gmail.com>
>
> Mario Kleiner suggest in commit
>   ad3543ede630f ("drm/intel: Push get_scanout_position() timestamping into kms driver.")
>
> a spots where preemption should be disabled on PREEMPT_RT. The
> difference is that on PREEMPT_RT the intel_uncore::lock disables neither
> preemption nor interrupts and so region remains preemptible.
>
> The area covers only register reads and writes. The part that worries me
> is:
> - __intel_get_crtc_scanline() the worst case is 100us if no match is
>   found.
>
> - intel_crtc_scanlines_since_frame_timestamp() not sure how long this
>   may take in the worst case.
>
> It was in the RT queue for a while and nobody complained.
> Disable preemption on PREEPMPT_RT during timestamping.
>
> [bigeasy: patch description.]
>
> Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
> Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>
> ---
>  drivers/gpu/drm/i915/display/intel_vblank.c | 43 ++++++++++++++++-----
>  1 file changed, 33 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_vblank.c b/drivers/gpu/drm/i915/display/intel_vblank.c
> index 44a7242658da4..cd7f2c1ef21af 100644
> --- a/drivers/gpu/drm/i915/display/intel_vblank.c
> +++ b/drivers/gpu/drm/i915/display/intel_vblank.c
> @@ -315,6 +315,20 @@ static void intel_vblank_section_exit(struct intel_display *display)
>  	struct drm_i915_private *i915 = to_i915(display->drm);
>  	spin_unlock(&i915->uncore.lock);
>  }
> +
> +static void intel_vblank_section_enter_irqf(struct intel_display *display, unsigned long *flags)
> +	__acquires(i915->uncore.lock)
> +{
> +	struct drm_i915_private *i915 = to_i915(display->drm);
> +	spin_lock_irqsave(&i915->uncore.lock, *flags);
> +}
> +
> +static void intel_vblank_section_exit_irqf(struct intel_display *display, unsigned long flags)
> +	__releases(i915->uncore.lock)
> +{
> +	struct drm_i915_private *i915 = to_i915(display->drm);
> +	spin_unlock_irqrestore(&i915->uncore.lock, flags);
> +}

The whole #ifdef I915 conditional stuff needs to go sooner rather than
later. This needs to become independent of i915 vs xe.

The comment says this is about *platforms* not *drivers*, so the check
should be about platforms, not drivers.

In any case, adding more stuff to the #ifdef I915 branches is
counter-productive.


BR,
Jani.


>  #else
>  static void intel_vblank_section_enter(struct intel_display *display)
>  {
> @@ -323,6 +337,17 @@ static void intel_vblank_section_enter(struct intel_display *display)
>  static void intel_vblank_section_exit(struct intel_display *display)
>  {
>  }
> +
> +static void intel_vblank_section_enter_irqf(struct intel_display *display, unsigned long *flags)
> +{
> +	*flags = 0;
> +}
> +
> +static void intel_vblank_section_exit_irqf(struct intel_display *display, unsigned long flags)
> +{
> +	if (flags)
> +		return;
> +}
>  #endif
>  
>  static bool i915_get_crtc_scanoutpos(struct drm_crtc *_crtc,
> @@ -359,10 +384,10 @@ static bool i915_get_crtc_scanoutpos(struct drm_crtc *_crtc,
>  	 * timing critical raw register reads, potentially with
>  	 * preemption disabled, so the following code must not block.
>  	 */
> -	local_irq_save(irqflags);
> -	intel_vblank_section_enter(display);
> +	intel_vblank_section_enter_irqf(display, &irqflags);
>  
> -	/* preempt_disable_rt() should go right here in PREEMPT_RT patchset. */
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +		preempt_disable();
>  
>  	/* Get optional system timestamp before query. */
>  	if (stime)
> @@ -426,10 +451,10 @@ static bool i915_get_crtc_scanoutpos(struct drm_crtc *_crtc,
>  	if (etime)
>  		*etime = ktime_get();
>  
> -	/* preempt_enable_rt() should go right here in PREEMPT_RT patchset. */
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +		preempt_enable();
>  
> -	intel_vblank_section_exit(display);
> -	local_irq_restore(irqflags);
> +	intel_vblank_section_exit_irqf(display, irqflags);
>  
>  	/*
>  	 * While in vblank, position will be negative
> @@ -467,13 +492,11 @@ int intel_get_crtc_scanline(struct intel_crtc *crtc)
>  	unsigned long irqflags;
>  	int position;
>  
> -	local_irq_save(irqflags);
> -	intel_vblank_section_enter(display);
> +	intel_vblank_section_enter_irqf(display, &irqflags);
>  
>  	position = __intel_get_crtc_scanline(crtc);
>  
> -	intel_vblank_section_exit(display);
> -	local_irq_restore(irqflags);
> +	intel_vblank_section_exit_irqf(display, irqflags);
>  
>  	return position;
>  }

-- 
Jani Nikula, Intel