drm/i915: Always enable CGM CSC on CHV

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.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230413164916.4221-5-ville.syrjala@linux.intel.com
Reviewed-by: Uma Shankar <uma.shankar@intel.com>

diff --git a/drivers/gpu/drm/i915/display/intel_color.c b/drivers/gpu/drm/i915/display/intel_color.c
index 7c14b2a..9581dc9 100644 (file)
--- 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;
 }