[Intel-gfx] [PATCH v2 4/7] drm/i915: Always enable CGM CSC on CHV

[Shankar, Uma]

> -----Original Message-----
> From: Intel-gfx <intel-gfx-bounces at lists.freedesktop.org> On Behalf Of Ville Syrjala
> Sent: Thursday, April 13, 2023 10:19 PM
> To: intel-gfx at lists.freedesktop.org
> Cc: dri-devel at lists.freedesktop.org
> Subject: [Intel-gfx] [PATCH v2 4/7] drm/i915: Always enable CGM CSC on CHV
> 
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> On CHV toggling the CGM CSC on/off while the pipe is running leads to underruns.
> Looks like we'd have to do the toggling strictly inside the start_of_vblank-
> frame_start window to avoid this, but that window is less than a scanline so there's
> no way we can guarantee hitting it.
> 
> As a workaround let's just leave the CGM CSC permanently enabled.
> Fortunately the CGM gamma/degamma units don't seem to suffer from this malady.
> 
> I also tried turning off CGM unit clock gating, but that did not help.

Looks Good to me.
Reviewed-by: Uma Shankar <uma.shankar at intel.com>

> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_color.c | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_color.c
> b/drivers/gpu/drm/i915/display/intel_color.c
> index 63141f4ed372..7a705e7d8776 100644
> --- a/drivers/gpu/drm/i915/display/intel_color.c
> +++ b/drivers/gpu/drm/i915/display/intel_color.c
> @@ -605,6 +605,16 @@ static void chv_cgm_csc_convert_ctm(const struct
> intel_crtc_state *crtc_state,
>  		csc->coeff[i] = ctm_to_twos_complement(ctm->matrix[i], 4, 12);  }
> 
> +#define CHV_CGM_CSC_COEFF_1_0 (1 << 12)
> +
> +static const struct intel_csc_matrix chv_cgm_csc_matrix_identity = {
> +	.coeff = {
> +		CHV_CGM_CSC_COEFF_1_0, 0, 0,
> +		0, CHV_CGM_CSC_COEFF_1_0, 0,
> +		0, 0, CHV_CGM_CSC_COEFF_1_0,
> +	},
> +};
> +
>  static void chv_load_cgm_csc(struct intel_crtc *crtc,
>  			     const struct intel_csc_matrix *csc)  { @@ -667,9 +677,9
> @@ static void chv_assign_csc(struct intel_crtc_state *crtc_state)
> 
>  		chv_cgm_csc_convert_ctm(crtc_state, &crtc_state->csc);
>  	} else {
> -		drm_WARN_ON(&i915->drm, (crtc_state->cgm_mode &
> CGM_PIPE_MODE_CSC) != 0);
> +		drm_WARN_ON(&i915->drm, (crtc_state->cgm_mode &
> CGM_PIPE_MODE_CSC) ==
> +0);
> 
> -		intel_csc_clear(&crtc_state->csc);
> +		crtc_state->csc = chv_cgm_csc_matrix_identity;
>  	}
>  }
> 
> @@ -2033,6 +2043,13 @@ static u32 chv_cgm_mode(const struct intel_crtc_state
> *crtc_state)
>  	    !lut_is_legacy(crtc_state->hw.gamma_lut))
>  		cgm_mode |= CGM_PIPE_MODE_GAMMA;
> 
> +	/*
> +	 * Toggling the CGM CSC on/off outside of the tiny window
> +	 * between start of vblank and frame start causes underruns.
> +	 * Always enable the CGM CSC as a workaround.
> +	 */
> +	cgm_mode |= CGM_PIPE_MODE_CSC;
> +
>  	return cgm_mode;
>  }
> 
> --
> 2.39.2