Re: [PATCH 6/9] drm/i915/display: Wait for scl start instead of dsb_wait_vblanks

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

On Wed, Sep 24, 2025 at 04:21:26PM +0530, Ankit Nautiyal wrote:
> Until LNL, intel_dsb_wait_vblanks() used to wait for the undelayed vblank
> start. However, from PTL onwards, it waits for the start of the
> safe-window defined by the number of lines programmed in the register
> TRANS_SET_CONTEXT_LATENCY. This change was introduced to move the SCL
> window out of the vblank region, supporting modes with higher refresh
> rates and smaller vblanks. This change introduces a "safe window" a
> scanline range from (undelayed vblank - SCL) to (delayed vblank - SCL).
> 
> As a result, on PTL+ platforms, the DSB wait for vblank completes exactly
> SCL lines earlier than the undelayed vblank start (safe window start).
> If the flip occurs in the active region and the push happens before the
> vmin decision boundary, the DSB wait fires early, and the push is sent
> inside this safe window. In such cases, the push bit is cleared at the
> delayed vblank, but our wait logic does not account for the early trigger,
> leading to DSB poll errors.
> 
> To fix this, we add an explicit wait for the end of the safe window i.e.,
> the scanline range from (undelayed vblank - SCL) to (delayed vblank - SCL).
> Once past this window, we are exactly SCL lines away from the delayed
> vblank, and our existing wait logic works as intended.
> 
> This additional wait is only effective if the push occurs before the vmin
> decision boundary. If the push happens after the boundary, the hardware
> already guarantees we're SCL lines away from the delayed vblank, and the
> extra wait becomes a no-op.
> 
> v2:
> - Use helpers for safe window start/end. (Ville)
> - Move the extra wait inside the helper to wait for delayed vblank. (Ville)
> - Update the commit message.
> 
> v3:
> - Add more documentation for explanation for the wait. (Ville)
> - Rename intel_vrr_vmin_safe_window_start/end as this is vmin safe
>   window. (Ville)
> - Minor refactoring to align with the code. (Ville)
> - Update the commit message for more clarity.
> 
> Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dsb.c | 16 ++++++++++++++++
>  drivers/gpu/drm/i915/display/intel_vrr.c | 17 +++++++++++++++++
>  drivers/gpu/drm/i915/display/intel_vrr.h |  2 ++
>  3 files changed, 35 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dsb.c b/drivers/gpu/drm/i915/display/intel_dsb.c
> index 135d40852e4c..3cb4c9be146f 100644
> --- a/drivers/gpu/drm/i915/display/intel_dsb.c
> +++ b/drivers/gpu/drm/i915/display/intel_dsb.c
> @@ -824,6 +824,22 @@ void intel_dsb_wait_for_delayed_vblank(struct intel_atomic_state *state,
>  	int usecs = intel_scanlines_to_usecs(&crtc_state->hw.adjusted_mode,
>  					     dsb_vblank_delay(state, crtc));
>  
> +	/*
> +	 * If the push happened before the vmin decision boundary
> +	 * we don't know how far we are from the undelayed vblank.
> +	 * Wait until we're past the vmin safe window, at which
> +	 * point we're SCL lines away from the delayed vblank.
> +	 *
> +	 * If the push happened after the vmin decision boundary
> +	 * the hardware itself guarantees that we're SCL lines
> +	 * away from the delayed vblank, and we won't be inside
> +	 * the vmin safe window so this extra wait does nothing.
> +	 */
> +	if (pre_commit_is_vrr_active(state, crtc))
> +		intel_dsb_wait_scanline_out(state, dsb,
> +					    intel_vrr_vmin_safe_window_start(crtc_state),
> +					    intel_vrr_vmin_safe_window_end(crtc_state));

Hmm, I thought we already had a 'if (vrr)' check here. But I guess that
was in dsb_vblank_delay(). Hmm, yeah I think what you did here is fine
for the moment.

I'm thinking we should follow up with inlining dsb_vblank_delay()
directly into intel_dsb_wait_for_delayed_vblank(), just to keep all
the VRR related wait magic in one place. I don't think there are any
other users of dsb_vblank_delay().

> +
>  	intel_dsb_wait_usec(dsb, usecs);
>  }
>  
> diff --git a/drivers/gpu/drm/i915/display/intel_vrr.c b/drivers/gpu/drm/i915/display/intel_vrr.c
> index 1bb9db06f43d..26c5c32a9a58 100644
> --- a/drivers/gpu/drm/i915/display/intel_vrr.c
> +++ b/drivers/gpu/drm/i915/display/intel_vrr.c
> @@ -800,3 +800,20 @@ void intel_vrr_get_config(struct intel_crtc_state *crtc_state)
>  	if (crtc_state->vrr.enable)
>  		crtc_state->mode_flags |= I915_MODE_FLAG_VRR;
>  }
> +
> +int intel_vrr_vmin_safe_window_start(const struct intel_crtc_state *crtc_state)

I only wanted you to rename the safe_window_end(). The safe window
start doesn't change for vmin/vmax/etc. So should drop the "vmin"
again from this one.

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

> +{
> +	struct intel_display *display = to_intel_display(crtc_state);
> +
> +	if (DISPLAY_VER(display) >= 30)
> +		return crtc_state->hw.adjusted_mode.crtc_vdisplay -
> +		       crtc_state->set_context_latency;
> +	else
> +		return crtc_state->hw.adjusted_mode.crtc_vdisplay;
> +}
> +
> +int intel_vrr_vmin_safe_window_end(const struct intel_crtc_state *crtc_state)
> +{
> +	return intel_vrr_vmin_vblank_start(crtc_state) -
> +	       crtc_state->set_context_latency;
> +}
> diff --git a/drivers/gpu/drm/i915/display/intel_vrr.h b/drivers/gpu/drm/i915/display/intel_vrr.h
> index 38bf9996b883..239e4f94725c 100644
> --- a/drivers/gpu/drm/i915/display/intel_vrr.h
> +++ b/drivers/gpu/drm/i915/display/intel_vrr.h
> @@ -41,5 +41,7 @@ void intel_vrr_transcoder_enable(const struct intel_crtc_state *crtc_state);
>  void intel_vrr_transcoder_disable(const struct intel_crtc_state *crtc_state);
>  void intel_vrr_set_fixed_rr_timings(const struct intel_crtc_state *crtc_state);
>  bool intel_vrr_always_use_vrr_tg(struct intel_display *display);
> +int intel_vrr_vmin_safe_window_start(const struct intel_crtc_state *crtc_state);
> +int intel_vrr_vmin_safe_window_end(const struct intel_crtc_state *crtc_state);
>  
>  #endif /* __INTEL_VRR_H__ */
> -- 
> 2.45.2

-- 
Ville Syrjälä
Intel