Re: [PATCH] drm/i915/display: Simplify modular operations with vtotal

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

On Thu, 11 Sep 2025, Jonathan Cavitt <jonathan.cavitt@intel.com> wrote:
> There are a couple of modulus operations in the i915 display code with
> vtotal as the divisor that add vtotal to the dividend.  In modular
> arithmetic, adding the divisor to the dividend is equivalent to adding
> zero to the dividend, so this addition can be dropped.

The result might become negative with this?

BR,
Jani.

>
> Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Animesh Manna <animesh.manna@intel.com>
> Cc: Jani Nikula <jani.nikula@intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dsb.c    | 4 ++--
>  drivers/gpu/drm/i915/display/intel_vblank.c | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_dsb.c b/drivers/gpu/drm/i915/display/intel_dsb.c
> index dee44d45b668..67315116839b 100644
> --- a/drivers/gpu/drm/i915/display/intel_dsb.c
> +++ b/drivers/gpu/drm/i915/display/intel_dsb.c
> @@ -173,7 +173,7 @@ static int dsb_scanline_to_hw(struct intel_atomic_state *state,
>  		intel_pre_commit_crtc_state(state, crtc);
>  	int vtotal = dsb_vtotal(state, crtc);
>  
> -	return (scanline + vtotal - intel_crtc_scanline_offset(crtc_state)) % vtotal;
> +	return (scanline - intel_crtc_scanline_offset(crtc_state)) % vtotal;
>  }
>  
>  /*
> @@ -482,7 +482,7 @@ static void assert_dsl_ok(struct intel_atomic_state *state,
>  	 * Waiting for the entire frame doesn't make sense,
>  	 * (IN==don't wait, OUT=wait forever).
>  	 */
> -	drm_WARN(crtc->base.dev, (end - start + vtotal) % vtotal == vtotal - 1,
> +	drm_WARN(crtc->base.dev, (end - start) % vtotal == vtotal - 1,
>  		 "[CRTC:%d:%s] DSB %d bad scanline window wait: %d-%d (vt=%d)\n",
>  		 crtc->base.base.id, crtc->base.name, dsb->id,
>  		 start, end, vtotal);
> diff --git a/drivers/gpu/drm/i915/display/intel_vblank.c b/drivers/gpu/drm/i915/display/intel_vblank.c
> index c15234c1d96e..bcfca2fcef3c 100644
> --- a/drivers/gpu/drm/i915/display/intel_vblank.c
> +++ b/drivers/gpu/drm/i915/display/intel_vblank.c
> @@ -288,7 +288,7 @@ static int __intel_get_crtc_scanline(struct intel_crtc *crtc)
>  	 * See update_scanline_offset() for the details on the
>  	 * scanline_offset adjustment.
>  	 */
> -	return (position + vtotal + crtc->scanline_offset) % vtotal;
> +	return (position + crtc->scanline_offset) % vtotal;
>  }
>  
>  /*

-- 
Jani Nikula, Intel