drm/i915: Add eDP intermediate frequencies for CHV
[linux-2.6-block.git] / drivers / gpu / drm / i915 / intel_dp.c
index a74aaf9242b965797114c0d775171b181ffdd423..ce0bdec4b6f9071bac49a43d31106fbb563c8877 100644 (file)
@@ -84,6 +84,13 @@ static const struct dp_link_dpll chv_dpll[] = {
        { DP_LINK_BW_5_4,       /* m2_int = 27, m2_fraction = 0 */
                { .p1 = 2, .p2 = 1, .n = 1, .m1 = 2, .m2 = 0x6c00000 } }
 };
+/* Skylake supports following rates */
+static const int gen9_rates[] = { 162000, 216000, 270000,
+                                 324000, 432000, 540000 };
+static const int chv_rates[] = { 162000, 202500, 210000, 216000,
+                                243000, 270000, 324000, 405000,
+                                420000, 432000, 540000 };
+static const int default_rates[] = { 162000, 270000, 540000 };
 
 /**
  * is_edp - is the given port attached to an eDP panel (either CPU or PCH)
@@ -118,23 +125,15 @@ static void vlv_init_panel_power_sequencer(struct intel_dp *intel_dp);
 static void vlv_steal_power_sequencer(struct drm_device *dev,
                                      enum pipe pipe);
 
-int
-intel_dp_max_link_bw(struct intel_dp *intel_dp)
+static int
+intel_dp_max_link_bw(struct intel_dp  *intel_dp)
 {
        int max_link_bw = intel_dp->dpcd[DP_MAX_LINK_RATE];
-       struct drm_device *dev = intel_dp->attached_connector->base.dev;
 
        switch (max_link_bw) {
        case DP_LINK_BW_1_62:
        case DP_LINK_BW_2_7:
-               break;
-       case DP_LINK_BW_5_4: /* 1.2 capable displays may advertise higher bw */
-               if (((IS_HASWELL(dev) && !IS_HSW_ULX(dev)) ||
-                    INTEL_INFO(dev)->gen >= 8) &&
-                   intel_dp->dpcd[DP_DPCD_REV] >= 0x12)
-                       max_link_bw = DP_LINK_BW_5_4;
-               else
-                       max_link_bw = DP_LINK_BW_2_7;
+       case DP_LINK_BW_5_4:
                break;
        default:
                WARN(1, "invalid max DP link bw val %x, using 1.62Gbps\n",
@@ -210,7 +209,7 @@ intel_dp_mode_valid(struct drm_connector *connector,
                target_clock = fixed_mode->clock;
        }
 
-       max_link_clock = drm_dp_bw_code_to_link_rate(intel_dp_max_link_bw(intel_dp));
+       max_link_clock = intel_dp_max_link_rate(intel_dp);
        max_lanes = intel_dp_max_lane_count(intel_dp);
 
        max_rate = intel_dp_max_data_rate(max_link_clock, max_lanes);
@@ -240,7 +239,7 @@ uint32_t intel_dp_pack_aux(const uint8_t *src, int src_bytes)
        return v;
 }
 
-void intel_dp_unpack_aux(uint32_t src, uint8_t *dst, int dst_bytes)
+static void intel_dp_unpack_aux(uint32_t src, uint8_t *dst, int dst_bytes)
 {
        int i;
        if (dst_bytes > 4)
@@ -1075,7 +1074,7 @@ intel_dp_connector_unregister(struct intel_connector *intel_connector)
 }
 
 static void
-skl_edp_set_pll_config(struct intel_crtc_state *pipe_config, int link_bw)
+skl_edp_set_pll_config(struct intel_crtc_state *pipe_config, int link_clock)
 {
        u32 ctrl1;
 
@@ -1084,19 +1083,35 @@ skl_edp_set_pll_config(struct intel_crtc_state *pipe_config, int link_bw)
        pipe_config->dpll_hw_state.cfgcr2 = 0;
 
        ctrl1 = DPLL_CTRL1_OVERRIDE(SKL_DPLL0);
-       switch (link_bw) {
-       case DP_LINK_BW_1_62:
+       switch (link_clock / 2) {
+       case 81000:
                ctrl1 |= DPLL_CRTL1_LINK_RATE(DPLL_CRTL1_LINK_RATE_810,
                                              SKL_DPLL0);
                break;
-       case DP_LINK_BW_2_7:
+       case 135000:
                ctrl1 |= DPLL_CRTL1_LINK_RATE(DPLL_CRTL1_LINK_RATE_1350,
                                              SKL_DPLL0);
                break;
-       case DP_LINK_BW_5_4:
+       case 270000:
                ctrl1 |= DPLL_CRTL1_LINK_RATE(DPLL_CRTL1_LINK_RATE_2700,
                                              SKL_DPLL0);
                break;
+       case 162000:
+               ctrl1 |= DPLL_CRTL1_LINK_RATE(DPLL_CRTL1_LINK_RATE_1620,
+                                             SKL_DPLL0);
+               break;
+       /* TBD: For DP link rates 2.16 GHz and 4.32 GHz, VCO is 8640 which
+       results in CDCLK change. Need to handle the change of CDCLK by
+       disabling pipes and re-enabling them */
+       case 108000:
+               ctrl1 |= DPLL_CRTL1_LINK_RATE(DPLL_CRTL1_LINK_RATE_1080,
+                                             SKL_DPLL0);
+               break;
+       case 216000:
+               ctrl1 |= DPLL_CRTL1_LINK_RATE(DPLL_CRTL1_LINK_RATE_2160,
+                                             SKL_DPLL0);
+               break;
+
        }
        pipe_config->dpll_hw_state.ctrl1 = ctrl1;
 }
@@ -1117,6 +1132,42 @@ hsw_dp_set_ddi_pll_sel(struct intel_crtc_state *pipe_config, int link_bw)
        }
 }
 
+static int
+intel_dp_sink_rates(struct intel_dp *intel_dp, const int **sink_rates)
+{
+       if (intel_dp->num_supported_rates) {
+               *sink_rates = intel_dp->supported_rates;
+               return intel_dp->num_supported_rates;
+       }
+
+       *sink_rates = default_rates;
+
+       return (intel_dp_max_link_bw(intel_dp) >> 3) + 1;
+}
+
+static int
+intel_dp_source_rates(struct drm_device *dev, const int **source_rates)
+{
+       if (INTEL_INFO(dev)->gen >= 9) {
+               *source_rates = gen9_rates;
+               return ARRAY_SIZE(gen9_rates);
+       } else if (IS_CHERRYVIEW(dev)) {
+               *source_rates = chv_rates;
+               return ARRAY_SIZE(chv_rates);
+       }
+
+       *source_rates = default_rates;
+
+       if (IS_SKYLAKE(dev) && INTEL_REVID(dev) <= SKL_REVID_B0)
+               /* WaDisableHBR2:skl */
+               return (DP_LINK_BW_2_7 >> 3) + 1;
+       else if (INTEL_INFO(dev)->gen >= 8 ||
+           (IS_HASWELL(dev) && !IS_HSW_ULX(dev)))
+               return (DP_LINK_BW_5_4 >> 3) + 1;
+       else
+               return (DP_LINK_BW_2_7 >> 3) + 1;
+}
+
 static void
 intel_dp_set_clock(struct intel_encoder *encoder,
                   struct intel_crtc_state *pipe_config, int link_bw)
@@ -1150,6 +1201,73 @@ intel_dp_set_clock(struct intel_encoder *encoder,
        }
 }
 
+static int intersect_rates(const int *source_rates, int source_len,
+                          const int *sink_rates, int sink_len,
+                          int *supported_rates)
+{
+       int i = 0, j = 0, k = 0;
+
+       while (i < source_len && j < sink_len) {
+               if (source_rates[i] == sink_rates[j]) {
+                       if (WARN_ON(k >= DP_MAX_SUPPORTED_RATES))
+                               return k;
+                       supported_rates[k] = source_rates[i];
+                       ++k;
+                       ++i;
+                       ++j;
+               } else if (source_rates[i] < sink_rates[j]) {
+                       ++i;
+               } else {
+                       ++j;
+               }
+       }
+       return k;
+}
+
+static int intel_supported_rates(struct intel_dp *intel_dp,
+                                int *supported_rates)
+{
+       struct drm_device *dev = intel_dp_to_dev(intel_dp);
+       const int *source_rates, *sink_rates;
+       int source_len, sink_len;
+
+       sink_len = intel_dp_sink_rates(intel_dp, &sink_rates);
+       source_len = intel_dp_source_rates(dev, &source_rates);
+
+       return intersect_rates(source_rates, source_len,
+                              sink_rates, sink_len,
+                              supported_rates);
+}
+
+static int rate_to_index(int find, const int *rates)
+{
+       int i = 0;
+
+       for (i = 0; i < DP_MAX_SUPPORTED_RATES; ++i)
+               if (find == rates[i])
+                       break;
+
+       return i;
+}
+
+int
+intel_dp_max_link_rate(struct intel_dp *intel_dp)
+{
+       int rates[DP_MAX_SUPPORTED_RATES] = {};
+       int len;
+
+       len = intel_supported_rates(intel_dp, rates);
+       if (WARN_ON(len <= 0))
+               return 162000;
+
+       return rates[rate_to_index(0, rates) - 1];
+}
+
+int intel_dp_rate_select(struct intel_dp *intel_dp, int rate)
+{
+       return rate_to_index(rate, intel_dp->supported_rates);
+}
+
 bool
 intel_dp_compute_config(struct intel_encoder *encoder,
                        struct intel_crtc_state *pipe_config)
@@ -1166,10 +1284,18 @@ intel_dp_compute_config(struct intel_encoder *encoder,
        int max_lane_count = intel_dp_max_lane_count(intel_dp);
        /* Conveniently, the link BW constants become indices with a shift...*/
        int min_clock = 0;
-       int max_clock = intel_dp_max_link_bw(intel_dp) >> 3;
+       int max_clock;
        int bpp, mode_rate;
-       static int bws[] = { DP_LINK_BW_1_62, DP_LINK_BW_2_7, DP_LINK_BW_5_4 };
        int link_avail, link_clock;
+       int supported_rates[DP_MAX_SUPPORTED_RATES] = {};
+       int supported_len;
+
+       supported_len = intel_supported_rates(intel_dp, supported_rates);
+
+       /* No common link rates between source and sink */
+       WARN_ON(supported_len <= 0);
+
+       max_clock = supported_len - 1;
 
        if (HAS_PCH_SPLIT(dev) && !HAS_DDI(dev) && port != PORT_A)
                pipe_config->has_pch_encoder = true;
@@ -1193,8 +1319,8 @@ intel_dp_compute_config(struct intel_encoder *encoder,
                return false;
 
        DRM_DEBUG_KMS("DP link computation with max lane count %i "
-                     "max bw %02x pixel clock %iKHz\n",
-                     max_lane_count, bws[max_clock],
+                     "max bw %d pixel clock %iKHz\n",
+                     max_lane_count, supported_rates[max_clock],
                      adjusted_mode->crtc_clock);
 
        /* Walk through all bpp values. Luckily they're all nicely spaced with 2
@@ -1223,8 +1349,11 @@ intel_dp_compute_config(struct intel_encoder *encoder,
                                                   bpp);
 
                for (clock = min_clock; clock <= max_clock; clock++) {
-                       for (lane_count = min_lane_count; lane_count <= max_lane_count; lane_count <<= 1) {
-                               link_clock = drm_dp_bw_code_to_link_rate(bws[clock]);
+                       for (lane_count = min_lane_count;
+                               lane_count <= max_lane_count;
+                               lane_count <<= 1) {
+
+                               link_clock = supported_rates[clock];
                                link_avail = intel_dp_max_data_rate(link_clock,
                                                                    lane_count);
 
@@ -1253,10 +1382,20 @@ found:
        if (intel_dp->color_range)
                pipe_config->limited_color_range = true;
 
-       intel_dp->link_bw = bws[clock];
        intel_dp->lane_count = lane_count;
+
+       if (intel_dp->num_supported_rates) {
+               intel_dp->link_bw = 0;
+               intel_dp->rate_select =
+                       intel_dp_rate_select(intel_dp, supported_rates[clock]);
+       } else {
+               intel_dp->link_bw =
+                       drm_dp_link_rate_to_bw_code(supported_rates[clock]);
+               intel_dp->rate_select = 0;
+       }
+
        pipe_config->pipe_bpp = bpp;
-       pipe_config->port_clock = drm_dp_bw_code_to_link_rate(intel_dp->link_bw);
+       pipe_config->port_clock = supported_rates[clock];
 
        DRM_DEBUG_KMS("DP link bw %02x lane count %d clock %d bpp %d\n",
                      intel_dp->link_bw, intel_dp->lane_count,
@@ -1279,7 +1418,7 @@ found:
        }
 
        if (IS_SKYLAKE(dev) && is_edp(intel_dp))
-               skl_edp_set_pll_config(pipe_config, intel_dp->link_bw);
+               skl_edp_set_pll_config(pipe_config, supported_rates[clock]);
        else if (IS_HASWELL(dev) || IS_BROADWELL(dev))
                hsw_dp_set_ddi_pll_sel(pipe_config, intel_dp->link_bw);
        else
@@ -2691,11 +2830,14 @@ static uint8_t
 intel_dp_voltage_max(struct intel_dp *intel_dp)
 {
        struct drm_device *dev = intel_dp_to_dev(intel_dp);
+       struct drm_i915_private *dev_priv = dev->dev_private;
        enum port port = dp_to_dig_port(intel_dp)->port;
 
-       if (INTEL_INFO(dev)->gen >= 9)
+       if (INTEL_INFO(dev)->gen >= 9) {
+               if (dev_priv->vbt.edp_low_vswing && port == PORT_A)
+                       return DP_TRAIN_VOLTAGE_SWING_LEVEL_3;
                return DP_TRAIN_VOLTAGE_SWING_LEVEL_2;
-       else if (IS_VALLEYVIEW(dev))
+       else if (IS_VALLEYVIEW(dev))
                return DP_TRAIN_VOLTAGE_SWING_LEVEL_3;
        else if (IS_GEN7(dev) && port == PORT_A)
                return DP_TRAIN_VOLTAGE_SWING_LEVEL_2;
@@ -2719,6 +2861,8 @@ intel_dp_pre_emphasis_max(struct intel_dp *intel_dp, uint8_t voltage_swing)
                        return DP_TRAIN_PRE_EMPH_LEVEL_2;
                case DP_TRAIN_VOLTAGE_SWING_LEVEL_2:
                        return DP_TRAIN_PRE_EMPH_LEVEL_1;
+               case DP_TRAIN_VOLTAGE_SWING_LEVEL_3:
+                       return DP_TRAIN_PRE_EMPH_LEVEL_0;
                default:
                        return DP_TRAIN_PRE_EMPH_LEVEL_0;
                }
@@ -3201,6 +3345,9 @@ intel_hsw_signal_levels(uint8_t train_set)
                return DDI_BUF_TRANS_SELECT(7);
        case DP_TRAIN_VOLTAGE_SWING_LEVEL_2 | DP_TRAIN_PRE_EMPH_LEVEL_1:
                return DDI_BUF_TRANS_SELECT(8);
+
+       case DP_TRAIN_VOLTAGE_SWING_LEVEL_3 | DP_TRAIN_PRE_EMPH_LEVEL_0:
+               return DDI_BUF_TRANS_SELECT(9);
        default:
                DRM_DEBUG_KMS("Unsupported voltage swing/pre-emphasis level:"
                              "0x%x\n", signal_levels);
@@ -3358,6 +3505,9 @@ intel_dp_start_link_train(struct intel_dp *intel_dp)
        if (drm_dp_enhanced_frame_cap(intel_dp->dpcd))
                link_config[1] |= DP_LANE_COUNT_ENHANCED_FRAME_EN;
        drm_dp_dpcd_write(&intel_dp->aux, DP_LINK_BW_SET, link_config, 2);
+       if (intel_dp->num_supported_rates)
+               drm_dp_dpcd_write(&intel_dp->aux, DP_LINK_RATE_SET,
+                               &intel_dp->rate_select, 1);
 
        link_config[0] = 0;
        link_config[1] = DP_SET_ANSI_8B10B;
@@ -3570,6 +3720,7 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
        struct intel_digital_port *dig_port = dp_to_dig_port(intel_dp);
        struct drm_device *dev = dig_port->base.base.dev;
        struct drm_i915_private *dev_priv = dev->dev_private;
+       uint8_t rev;
 
        if (intel_dp_dpcd_read_wake(&intel_dp->aux, 0x000, intel_dp->dpcd,
                                    sizeof(intel_dp->dpcd)) < 0)
@@ -3601,6 +3752,29 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
        } else
                intel_dp->use_tps3 = false;
 
+       /* Intermediate frequency support */
+       if (is_edp(intel_dp) &&
+           (intel_dp->dpcd[DP_EDP_CONFIGURATION_CAP] & DP_DPCD_DISPLAY_CONTROL_CAPABLE) &&
+           (intel_dp_dpcd_read_wake(&intel_dp->aux, DP_EDP_DPCD_REV, &rev, 1) == 1) &&
+           (rev >= 0x03)) { /* eDp v1.4 or higher */
+               __le16 supported_rates[DP_MAX_SUPPORTED_RATES];
+               int i;
+
+               intel_dp_dpcd_read_wake(&intel_dp->aux,
+                               DP_SUPPORTED_LINK_RATES,
+                               supported_rates,
+                               sizeof(supported_rates));
+
+               for (i = 0; i < ARRAY_SIZE(supported_rates); i++) {
+                       int val = le16_to_cpu(supported_rates[i]);
+
+                       if (val == 0)
+                               break;
+
+                       intel_dp->supported_rates[i] = val * 200;
+               }
+               intel_dp->num_supported_rates = i;
+       }
        if (!(intel_dp->dpcd[DP_DOWNSTREAMPORT_PRESENT] &
              DP_DWN_STRM_PORT_PRESENT))
                return true; /* native DP sink */
@@ -3803,7 +3977,7 @@ go_again:
  *  3. Use Link Training from 2.5.3.3 and 3.5.1.3
  *  4. Check link status on receipt of hot-plug interrupt
  */
-void
+static void
 intel_dp_check_link_status(struct intel_dp *intel_dp)
 {
        struct drm_device *dev = intel_dp_to_dev(intel_dp);
@@ -4736,6 +4910,18 @@ intel_dp_init_panel_power_sequencer_registers(struct drm_device *dev,
                      I915_READ(pp_div_reg));
 }
 
+/**
+ * intel_dp_set_drrs_state - program registers for RR switch to take effect
+ * @dev: DRM device
+ * @refresh_rate: RR to be programmed
+ *
+ * This function gets called when refresh rate (RR) has to be changed from
+ * one frequency to another. Switches can be between high and low RR
+ * supported by the panel or to any other RR based on media playback (in
+ * this case, RR value needs to be passed from user space).
+ *
+ * The caller of this function needs to take a lock on dev_priv->drrs.
+ */
 static void intel_dp_set_drrs_state(struct drm_device *dev, int refresh_rate)
 {
        struct drm_i915_private *dev_priv = dev->dev_private;
@@ -4793,14 +4979,32 @@ static void intel_dp_set_drrs_state(struct drm_device *dev, int refresh_rate)
                return;
        }
 
-       if (INTEL_INFO(dev)->gen > 6 && INTEL_INFO(dev)->gen < 8) {
+       if (INTEL_INFO(dev)->gen >= 8 && !IS_CHERRYVIEW(dev)) {
+               switch (index) {
+               case DRRS_HIGH_RR:
+                       intel_dp_set_m_n(intel_crtc, M1_N1);
+                       break;
+               case DRRS_LOW_RR:
+                       intel_dp_set_m_n(intel_crtc, M2_N2);
+                       break;
+               case DRRS_MAX_RR:
+               default:
+                       DRM_ERROR("Unsupported refreshrate type\n");
+               }
+       } else if (INTEL_INFO(dev)->gen > 6) {
                reg = PIPECONF(intel_crtc->config->cpu_transcoder);
                val = I915_READ(reg);
+
                if (index > DRRS_HIGH_RR) {
-                       val |= PIPECONF_EDP_RR_MODE_SWITCH;
-                       intel_dp_set_m_n(intel_crtc);
+                       if (IS_VALLEYVIEW(dev))
+                               val |= PIPECONF_EDP_RR_MODE_SWITCH_VLV;
+                       else
+                               val |= PIPECONF_EDP_RR_MODE_SWITCH;
                } else {
-                       val &= ~PIPECONF_EDP_RR_MODE_SWITCH;
+                       if (IS_VALLEYVIEW(dev))
+                               val &= ~PIPECONF_EDP_RR_MODE_SWITCH_VLV;
+                       else
+                               val &= ~PIPECONF_EDP_RR_MODE_SWITCH;
                }
                I915_WRITE(reg, val);
        }
@@ -4810,6 +5014,12 @@ static void intel_dp_set_drrs_state(struct drm_device *dev, int refresh_rate)
        DRM_DEBUG_KMS("eDP Refresh Rate set to : %dHz\n", refresh_rate);
 }
 
+/**
+ * intel_edp_drrs_enable - init drrs struct if supported
+ * @intel_dp: DP struct
+ *
+ * Initializes frontbuffer_bits and drrs.dp
+ */
 void intel_edp_drrs_enable(struct intel_dp *intel_dp)
 {
        struct drm_device *dev = intel_dp_to_dev(intel_dp);
@@ -4837,6 +5047,11 @@ unlock:
        mutex_unlock(&dev_priv->drrs.mutex);
 }
 
+/**
+ * intel_edp_drrs_disable - Disable DRRS
+ * @intel_dp: DP struct
+ *
+ */
 void intel_edp_drrs_disable(struct intel_dp *intel_dp)
 {
        struct drm_device *dev = intel_dp_to_dev(intel_dp);
@@ -4896,6 +5111,17 @@ unlock:
        mutex_unlock(&dev_priv->drrs.mutex);
 }
 
+/**
+ * intel_edp_drrs_invalidate - Invalidate DRRS
+ * @dev: DRM device
+ * @frontbuffer_bits: frontbuffer plane tracking bits
+ *
+ * When there is a disturbance on screen (due to cursor movement/time
+ * update etc), DRRS needs to be invalidated, i.e. need to switch to
+ * high RR.
+ *
+ * Dirty frontbuffers relevant to DRRS are tracked in busy_frontbuffer_bits.
+ */
 void intel_edp_drrs_invalidate(struct drm_device *dev,
                unsigned frontbuffer_bits)
 {
@@ -4906,12 +5132,13 @@ void intel_edp_drrs_invalidate(struct drm_device *dev,
        if (!dev_priv->drrs.dp)
                return;
 
+       cancel_delayed_work_sync(&dev_priv->drrs.work);
+
        mutex_lock(&dev_priv->drrs.mutex);
        crtc = dp_to_dig_port(dev_priv->drrs.dp)->base.base.crtc;
        pipe = to_intel_crtc(crtc)->pipe;
 
        if (dev_priv->drrs.refresh_rate_type == DRRS_LOW_RR) {
-               cancel_delayed_work_sync(&dev_priv->drrs.work);
                intel_dp_set_drrs_state(dev_priv->dev,
                                dev_priv->drrs.dp->attached_connector->panel.
                                fixed_mode->vrefresh);
@@ -4923,6 +5150,17 @@ void intel_edp_drrs_invalidate(struct drm_device *dev,
        mutex_unlock(&dev_priv->drrs.mutex);
 }
 
+/**
+ * intel_edp_drrs_flush - Flush DRRS
+ * @dev: DRM device
+ * @frontbuffer_bits: frontbuffer plane tracking bits
+ *
+ * When there is no movement on screen, DRRS work can be scheduled.
+ * This DRRS work is responsible for setting relevant registers after a
+ * timeout of 1 second.
+ *
+ * Dirty frontbuffers relevant to DRRS are tracked in busy_frontbuffer_bits.
+ */
 void intel_edp_drrs_flush(struct drm_device *dev,
                unsigned frontbuffer_bits)
 {
@@ -4933,13 +5171,13 @@ void intel_edp_drrs_flush(struct drm_device *dev,
        if (!dev_priv->drrs.dp)
                return;
 
+       cancel_delayed_work_sync(&dev_priv->drrs.work);
+
        mutex_lock(&dev_priv->drrs.mutex);
        crtc = dp_to_dig_port(dev_priv->drrs.dp)->base.base.crtc;
        pipe = to_intel_crtc(crtc)->pipe;
        dev_priv->drrs.busy_frontbuffer_bits &= ~frontbuffer_bits;
 
-       cancel_delayed_work_sync(&dev_priv->drrs.work);
-
        if (dev_priv->drrs.refresh_rate_type != DRRS_LOW_RR &&
                        !dev_priv->drrs.busy_frontbuffer_bits)
                schedule_delayed_work(&dev_priv->drrs.work,
@@ -4947,6 +5185,56 @@ void intel_edp_drrs_flush(struct drm_device *dev,
        mutex_unlock(&dev_priv->drrs.mutex);
 }
 
+/**
+ * DOC: Display Refresh Rate Switching (DRRS)
+ *
+ * Display Refresh Rate Switching (DRRS) is a power conservation feature
+ * which enables swtching between low and high refresh rates,
+ * dynamically, based on the usage scenario. This feature is applicable
+ * for internal panels.
+ *
+ * Indication that the panel supports DRRS is given by the panel EDID, which
+ * would list multiple refresh rates for one resolution.
+ *
+ * DRRS is of 2 types - static and seamless.
+ * Static DRRS involves changing refresh rate (RR) by doing a full modeset
+ * (may appear as a blink on screen) and is used in dock-undock scenario.
+ * Seamless DRRS involves changing RR without any visual effect to the user
+ * and can be used during normal system usage. This is done by programming
+ * certain registers.
+ *
+ * Support for static/seamless DRRS may be indicated in the VBT based on
+ * inputs from the panel spec.
+ *
+ * DRRS saves power by switching to low RR based on usage scenarios.
+ *
+ * eDP DRRS:-
+ *        The implementation is based on frontbuffer tracking implementation.
+ * When there is a disturbance on the screen triggered by user activity or a
+ * periodic system activity, DRRS is disabled (RR is changed to high RR).
+ * When there is no movement on screen, after a timeout of 1 second, a switch
+ * to low RR is made.
+ *        For integration with frontbuffer tracking code,
+ * intel_edp_drrs_invalidate() and intel_edp_drrs_flush() are called.
+ *
+ * DRRS can be further extended to support other internal panels and also
+ * the scenario of video playback wherein RR is set based on the rate
+ * requested by userspace.
+ */
+
+/**
+ * intel_dp_drrs_init - Init basic DRRS work and mutex.
+ * @intel_connector: eDP connector
+ * @fixed_mode: preferred mode of panel
+ *
+ * This function is  called only once at driver load to initialize basic
+ * DRRS stuff.
+ *
+ * Returns:
+ * Downclock mode if panel supports it, else return NULL.
+ * DRRS support is determined by the presence of downclock mode (apart
+ * from VBT setting).
+ */
 static struct drm_display_mode *
 intel_dp_drrs_init(struct intel_connector *intel_connector,
                struct drm_display_mode *fixed_mode)
@@ -4970,7 +5258,7 @@ intel_dp_drrs_init(struct intel_connector *intel_connector,
                                        (dev, fixed_mode, connector);
 
        if (!downclock_mode) {
-               DRM_DEBUG_KMS("DRRS not supported\n");
+               DRM_DEBUG_KMS("Downclock mode is not found. DRRS not supported\n");
                return NULL;
        }