drm/dp: add drm_dp_128b132b_read_aux_rd_interval()

The DP 2.0 errata changes DP_128B132B_TRAINING_AUX_RD_INTERVAL (DPCD
0x2216) completely. Add a new function to read that. Follow-up will need
to clean up existing functions.

v2: fix reversed interpretation of bit 7 meaning (Uma)

Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/22f6637194c9edb22b6a84be82dd385550dbb958.1643878928.git.jani.nikula@intel.com

drivers/gpu/drm/drm_dp_helper.c

@@ -281,6 +281,26 @@ int drm_dp_read_channel_eq_delay(struct drm_dp_aux *aux, const u8 dpcd[DP_RECEIV
 }
 EXPORT_SYMBOL(drm_dp_read_channel_eq_delay);
 
+/* Per DP 2.0 Errata */
+int drm_dp_128b132b_read_aux_rd_interval(struct drm_dp_aux *aux)
+{
+	int unit;
+	u8 val;
+
+	if (drm_dp_dpcd_readb(aux, DP_128B132B_TRAINING_AUX_RD_INTERVAL, &val) != 1) {
+		drm_err(aux->drm_dev, "%s: failed rd interval read\n",
+			aux->name);
+		/* default to max */
+		val = DP_128B132B_TRAINING_AUX_RD_INTERVAL_MASK;
+	}
+
+	unit = (val & DP_128B132B_TRAINING_AUX_RD_INTERVAL_1MS_UNIT) ? 1 : 2;
+	val &= DP_128B132B_TRAINING_AUX_RD_INTERVAL_MASK;
+
+	return (val + 1) * unit * 1000;
+}
+EXPORT_SYMBOL(drm_dp_128b132b_read_aux_rd_interval);
+
 void drm_dp_link_train_clock_recovery_delay(const struct drm_dp_aux *aux,
 					    const u8 dpcd[DP_RECEIVER_CAP_SIZE])
 {

include/drm/drm_dp_helper.h

@@ -1115,6 +1115,7 @@ struct drm_panel;
 # define DP_UHBR13_5                           (1 << 2)
 
 #define DP_128B132B_TRAINING_AUX_RD_INTERVAL                    0x2216 /* 2.0 */
+# define DP_128B132B_TRAINING_AUX_RD_INTERVAL_1MS_UNIT          (1 << 7)
 # define DP_128B132B_TRAINING_AUX_RD_INTERVAL_MASK              0x7f
 # define DP_128B132B_TRAINING_AUX_RD_INTERVAL_400_US            0x00
 # define DP_128B132B_TRAINING_AUX_RD_INTERVAL_4_MS              0x01
@@ -1552,6 +1553,8 @@ void drm_dp_link_train_channel_eq_delay(const struct drm_dp_aux *aux,
 void drm_dp_lttpr_link_train_channel_eq_delay(const struct drm_dp_aux *aux,
 					      const u8 caps[DP_LTTPR_PHY_CAP_SIZE]);
 
+int drm_dp_128b132b_read_aux_rd_interval(struct drm_dp_aux *aux);
+
 u8 drm_dp_link_rate_to_bw_code(int link_rate);
 int drm_dp_bw_code_to_link_rate(u8 link_bw);