drm/bridge: Document the expected behaviour of DSI host controllers

The exact behaviour of DSI host controllers is not specified,
therefore define it.
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20221205173328.1395350-7-dave.stevenson@raspberrypi.comSigned-off-by: Maxime Ripard <maxime@cerno.tech>

Documentation/gpu/drm-kms-helpers.rst

@@ -188,6 +188,13 @@ Bridge Helper Reference
 .. kernel-doc:: drivers/gpu/drm/drm_bridge.c
    :export:
 
+MIPI-DSI bridge operation
+-------------------------
+
+.. kernel-doc:: drivers/gpu/drm/drm_bridge.c
+   :doc: dsi bridge operations
+
+
 Bridge Connector Helper Reference
 ---------------------------------
 

drivers/gpu/drm/drm_bridge.c

@@ -153,6 +153,45 @@
  * situation when probing.
  */
 
+/**
+ * DOC: dsi bridge operations
+ *
+ * DSI host interfaces are expected to be implemented as bridges rather than
+ * encoders, however there are a few aspects of their operation that need to
+ * be defined in order to provide a consistent interface.
+ *
+ * A DSI host should keep the PHY powered down until the pre_enable operation is
+ * called. All lanes are in an undefined idle state up to this point, and it
+ * must not be assumed that it is LP-11.
+ * pre_enable should initialise the PHY, set the data lanes to LP-11, and the
+ * clock lane to either LP-11 or HS depending on the mode_flag
+ * %MIPI_DSI_CLOCK_NON_CONTINUOUS.
+ *
+ * Ordinarily the downstream bridge DSI peripheral pre_enable will have been
+ * called before the DSI host. If the DSI peripheral requires LP-11 and/or
+ * the clock lane to be in HS mode prior to pre_enable, then it can set the
+ * &pre_enable_prev_first flag to request the pre_enable (and
+ * post_disable) order to be altered to enable the DSI host first.
+ *
+ * Either the CRTC being enabled, or the DSI host enable operation should switch
+ * the host to actively transmitting video on the data lanes.
+ *
+ * The reverse also applies. The DSI host disable operation or stopping the CRTC
+ * should stop transmitting video, and the data lanes should return to the LP-11
+ * state. The DSI host &post_disable operation should disable the PHY.
+ * If the &pre_enable_prev_first flag is set, then the DSI peripheral's
+ * bridge &post_disable will be called before the DSI host's post_disable.
+ *
+ * Whilst it is valid to call &host_transfer prior to pre_enable or after
+ * post_disable, the exact state of the lanes is undefined at this point. The
+ * DSI host should initialise the interface, transmit the data, and then disable
+ * the interface again.
+ *
+ * Ultra Low Power State (ULPS) is not explicitly supported by DRM. If
+ * implemented, it therefore needs to be handled entirely within the DSI Host
+ * driver.
+ */
+
 static DEFINE_MUTEX(bridge_lock);
 static LIST_HEAD(bridge_list);