Re: [PATCH 3/8] drm/i915/vblank: Add helper to get correct vblank length

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

On Thu, Oct 09, 2025 at 02:30:57PM +0530, Ankit Nautiyal wrote:
> Currently crtc_vblank_start is assumed to be the vblank_start for the fixed
> refresh rate case. That value can be different from the variable refresh
> rate case whenever always_use_vrr_tg()==false. On icl/tgl it's always
> different due to the extra vblank delay, and also on adl+ it could be
> different if we were to use an optimized guardband.
> 
> So places where crtc_vblank_start is used to compute vblank length needs
> change so as to account for cases where vrr is enabled. Specifically
> with vrr.enable the effective vblank length is actually guardband.
> 
> Add a helper to get the correct vblank length for both vrr and fixed
> refresh rate cases. Use this helper where vblank_start is used to
> compute the vblank length.
> 
> Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>

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

> ---
>  drivers/gpu/drm/i915/display/intel_vblank.c  | 10 ++++++++++
>  drivers/gpu/drm/i915/display/intel_vblank.h  |  2 ++
>  drivers/gpu/drm/i915/display/skl_watermark.c |  3 ++-
>  3 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_vblank.c b/drivers/gpu/drm/i915/display/intel_vblank.c
> index 0b7fcc05e64c..2fc0c1c0bb87 100644
> --- a/drivers/gpu/drm/i915/display/intel_vblank.c
> +++ b/drivers/gpu/drm/i915/display/intel_vblank.c
> @@ -767,3 +767,13 @@ int intel_vblank_evade(struct intel_vblank_evade_ctx *evade)
>  
>  	return scanline;
>  }
> +
> +int intel_crtc_vblank_length(const struct intel_crtc_state *crtc_state)
> +{
> +	const struct drm_display_mode *adjusted_mode = &crtc_state->hw.adjusted_mode;
> +
> +	if (crtc_state->vrr.enable)
> +		return crtc_state->vrr.guardband;
> +	else
> +		return adjusted_mode->crtc_vtotal - adjusted_mode->crtc_vblank_start;
> +}
> diff --git a/drivers/gpu/drm/i915/display/intel_vblank.h b/drivers/gpu/drm/i915/display/intel_vblank.h
> index 21fbb08d61d5..98d04cacd65f 100644
> --- a/drivers/gpu/drm/i915/display/intel_vblank.h
> +++ b/drivers/gpu/drm/i915/display/intel_vblank.h
> @@ -48,4 +48,6 @@ const struct intel_crtc_state *
>  intel_pre_commit_crtc_state(struct intel_atomic_state *state,
>  			    struct intel_crtc *crtc);
>  
> +int intel_crtc_vblank_length(const struct intel_crtc_state *crtc_state);
> +
>  #endif /* __INTEL_VBLANK_H__ */
> diff --git a/drivers/gpu/drm/i915/display/skl_watermark.c b/drivers/gpu/drm/i915/display/skl_watermark.c
> index 9df9ee137bf9..06e5e6c77d2e 100644
> --- a/drivers/gpu/drm/i915/display/skl_watermark.c
> +++ b/drivers/gpu/drm/i915/display/skl_watermark.c
> @@ -28,6 +28,7 @@
>  #include "intel_flipq.h"
>  #include "intel_pcode.h"
>  #include "intel_plane.h"
> +#include "intel_vblank.h"
>  #include "intel_wm.h"
>  #include "skl_universal_plane_regs.h"
>  #include "skl_watermark.h"
> @@ -2241,7 +2242,7 @@ skl_is_vblank_too_short(const struct intel_crtc_state *crtc_state,
>  		scaler_prefill_latency(crtc_state) +
>  		dsc_prefill_latency(crtc_state) +
>  		wm0_lines >
> -		adjusted_mode->crtc_vtotal - adjusted_mode->crtc_vblank_start;
> +		intel_crtc_vblank_length(crtc_state);
>  }
>  
>  static int skl_max_wm0_lines(const struct intel_crtc_state *crtc_state)
> -- 
> 2.45.2

-- 
Ville Syrjälä
Intel