Documentation/drm: DocBook integration for DRRS

Adding an overview of DRRS in general and the implementation for eDP DRRS.
Also, describing the functions related to eDP DRRS.

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Documentation/DocBook/drm.tmpl

@@ -4051,6 +4051,17 @@ int num_ioctls;</synopsis>
 	<title>Frame Buffer Compression (FBC)</title>
 !Pdrivers/gpu/drm/i915/intel_fbc.c Frame Buffer Compression (FBC)
 !Idrivers/gpu/drm/i915/intel_fbc.c
+      </sect2>
+      <sect2>
+        <title>Display Refresh Rate Switching (DRRS)</title>
+!Pdrivers/gpu/drm/i915/intel_dp.c Display Refresh Rate Switching (DRRS)
+!Fdrivers/gpu/drm/i915/intel_dp.c intel_dp_set_drrs_state
+!Fdrivers/gpu/drm/i915/intel_dp.c intel_edp_drrs_enable
+!Fdrivers/gpu/drm/i915/intel_dp.c intel_edp_drrs_disable
+!Fdrivers/gpu/drm/i915/intel_dp.c intel_edp_drrs_invalidate
+!Fdrivers/gpu/drm/i915/intel_dp.c intel_edp_drrs_flush
+!Fdrivers/gpu/drm/i915/intel_dp.c intel_dp_drrs_init
+
       </sect2>
       <sect2>
         <title>DPIO</title>

drivers/gpu/drm/i915/intel_dp.c

@@ -4751,6 +4751,18 @@ 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;
@@ -4843,6 +4855,12 @@ 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);
@@ -4870,6 +4888,11 @@ void intel_edp_drrs_enable(struct intel_dp *intel_dp)
 	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);
@@ -4929,6 +4952,17 @@ static void intel_edp_drrs_downclock_work(struct work_struct *work)
 	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)
 {
@@ -4956,6 +4990,17 @@ 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)
 {
@@ -4980,6 +5025,56 @@ 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)