drm/i915: Use a fixed N value always

Windows/BIOS always uses fixed N values. Let's match that
behaviour.

Allows us to also get rid of that constant_n quirk stuff.
Reviewed-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220907091057.11572-17-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/display/intel_display.c

@@ -2721,19 +2721,11 @@ intel_reduce_m_n_ratio(u32 *num, u32 *den)
 	}
 }
 
-static void compute_m_n(unsigned int m, unsigned int n,
-			u32 *ret_m, u32 *ret_n,
-			bool constant_n)
+static void compute_m_n(u32 *ret_m, u32 *ret_n,
+			u32 m, u32 n, u32 constant_n)
 {
-	/*
-	 * Several DP dongles in particular seem to be fussy about
-	 * too large link M/N values. Give N value as 0x8000 that
-	 * should be acceptable by specific devices. 0x8000 is the
-	 * specified fixed N value for asynchronous clock mode,
-	 * which the devices expect also in synchronous clock mode.
-	 */
 	if (constant_n)
-		*ret_n = DP_LINK_CONSTANT_N_VALUE;
+		*ret_n = constant_n;
 	else
 		*ret_n = min_t(unsigned int, roundup_pow_of_two(n), DATA_LINK_N_MAX);
 
@@ -2745,22 +2737,28 @@ void
 intel_link_compute_m_n(u16 bits_per_pixel, int nlanes,
 		       int pixel_clock, int link_clock,
 		       struct intel_link_m_n *m_n,
-		       bool constant_n, bool fec_enable)
+		       bool fec_enable)
 {
 	u32 data_clock = bits_per_pixel * pixel_clock;
 
 	if (fec_enable)
 		data_clock = intel_dp_mode_to_fec_clock(data_clock);
 
+	/*
+	 * Windows/BIOS uses fixed M/N values always. Follow suit.
+	 *
+	 * Also several DP dongles in particular seem to be fussy
+	 * about too large link M/N values. Presumably the 20bit
+	 * value used by Windows/BIOS is acceptable to everyone.
+	 */
 	m_n->tu = 64;
-	compute_m_n(data_clock,
-		    link_clock * nlanes * 8,
-		    &m_n->data_m, &m_n->data_n,
-		    constant_n);
-
-	compute_m_n(pixel_clock, link_clock,
-		    &m_n->link_m, &m_n->link_n,
-		    constant_n);
+	compute_m_n(&m_n->data_m, &m_n->data_n,
+		    data_clock, link_clock * nlanes * 8,
+		    0x8000000);
+
+	compute_m_n(&m_n->link_m, &m_n->link_n,
+		    pixel_clock, link_clock,
+		    0x80000);
 }
 
 static void intel_panel_sanitize_ssc(struct drm_i915_private *dev_priv)

drivers/gpu/drm/i915/display/intel_display.h

@@ -547,7 +547,7 @@ u8 intel_calc_active_pipes(struct intel_atomic_state *state,
 void intel_link_compute_m_n(u16 bpp, int nlanes,
 			    int pixel_clock, int link_clock,
 			    struct intel_link_m_n *m_n,
-			    bool constant_n, bool fec_enable);
+			    bool fec_enable);
 u32 intel_plane_fb_max_stride(struct drm_i915_private *dev_priv,
 			      u32 pixel_format, u64 modifier);
 enum drm_mode_status

drivers/gpu/drm/i915/display/intel_dp.c

@@ -1927,7 +1927,7 @@ static bool can_enable_drrs(struct intel_connector *connector,
 static void
 intel_dp_drrs_compute_config(struct intel_connector *connector,
 			     struct intel_crtc_state *pipe_config,
-			     int output_bpp, bool constant_n)
+			     int output_bpp)
 {
 	struct drm_i915_private *i915 = to_i915(connector->base.dev);
 	const struct drm_display_mode *downclock_mode =
@@ -1954,7 +1954,7 @@ intel_dp_drrs_compute_config(struct intel_connector *connector,
 
 	intel_link_compute_m_n(output_bpp, pipe_config->lane_count, pixel_clock,
 			       pipe_config->port_clock, &pipe_config->dp_m2_n2,
-			       constant_n, pipe_config->fec_enable);
+			       pipe_config->fec_enable);
 
 	/* FIXME: abstract this better */
 	if (pipe_config->splitter.enable)
@@ -2029,7 +2029,6 @@ intel_dp_compute_config(struct intel_encoder *encoder,
 	struct intel_dp *intel_dp = enc_to_intel_dp(encoder);
 	const struct drm_display_mode *fixed_mode;
 	struct intel_connector *connector = intel_dp->attached_connector;
-	bool constant_n = drm_dp_has_quirk(&intel_dp->desc, DP_DPCD_QUIRK_CONSTANT_N);
 	int ret = 0, output_bpp;
 
 	if (HAS_PCH_SPLIT(dev_priv) && !HAS_DDI(dev_priv) && encoder->port != PORT_A)
@@ -2108,7 +2107,7 @@ intel_dp_compute_config(struct intel_encoder *encoder,
 			       adjusted_mode->crtc_clock,
 			       pipe_config->port_clock,
 			       &pipe_config->dp_m_n,
-			       constant_n, pipe_config->fec_enable);
+			       pipe_config->fec_enable);
 
 	/* FIXME: abstract this better */
 	if (pipe_config->splitter.enable)
@@ -2119,8 +2118,7 @@ intel_dp_compute_config(struct intel_encoder *encoder,
 
 	intel_vrr_compute_config(pipe_config, conn_state);
 	intel_psr_compute_config(intel_dp, pipe_config, conn_state);
-	intel_dp_drrs_compute_config(connector, pipe_config,
-				     output_bpp, constant_n);
+	intel_dp_drrs_compute_config(connector, pipe_config, output_bpp);
 	intel_dp_compute_vsc_sdp(intel_dp, pipe_config, conn_state);
 	intel_dp_compute_hdr_metadata_infoframe_sdp(intel_dp, pipe_config, conn_state);
 

drivers/gpu/drm/i915/display/intel_dp_mst.c

@@ -57,7 +57,6 @@ static int intel_dp_mst_compute_link_config(struct intel_encoder *encoder,
 	struct drm_i915_private *i915 = to_i915(connector->base.dev);
 	const struct drm_display_mode *adjusted_mode =
 		&crtc_state->hw.adjusted_mode;
-	bool constant_n = drm_dp_has_quirk(&intel_dp->desc, DP_DPCD_QUIRK_CONSTANT_N);
 	int bpp, slots = -EINVAL;
 
 	crtc_state->lane_count = limits->max_lane_count;
@@ -93,7 +92,7 @@ static int intel_dp_mst_compute_link_config(struct intel_encoder *encoder,
 			       adjusted_mode->crtc_clock,
 			       crtc_state->port_clock,
 			       &crtc_state->dp_m_n,
-			       constant_n, crtc_state->fec_enable);
+			       crtc_state->fec_enable);
 	crtc_state->dp_m_n.tu = slots;
 
 	return 0;

drivers/gpu/drm/i915/display/intel_fdi.c

@@ -256,7 +256,7 @@ int ilk_fdi_compute_config(struct intel_crtc *crtc,
 	pipe_config->fdi_lanes = lane;
 
 	intel_link_compute_m_n(pipe_config->pipe_bpp, lane, fdi_dotclock,
-			       link_bw, &pipe_config->fdi_m_n, false, false);
+			       link_bw, &pipe_config->fdi_m_n, false);
 
 	ret = ilk_check_fdi_lanes(dev, crtc->pipe, pipe_config);
 	if (ret == -EDEADLK)