drm/bridge: Add the necessary bits to support bus format negotiation

drm_bridge_state is extended to describe the input and output bus
configurations. These bus configurations are exposed through the
drm_bus_cfg struct which encodes the configuration of a physical
bus between two components in an output pipeline, usually between
two bridges, an encoder and a bridge, or a bridge and a connector.

The bus configuration is stored in drm_bridge_state separately for
the input and output buses, as seen from the point of view of each
bridge. The bus configuration of a bridge output is usually identical
to the configuration of the next bridge's input, but may differ if
the signals are modified between the two bridges, for instance by an
inverter on the board. The input and output configurations of a
bridge may differ if the bridge modifies the signals internally,
for instance by performing format conversion, or*modifying signals
polarities.

Bus format negotiation is automated by the core, drivers just have
to implement the ->atomic_get_{output,input}_bus_fmts() hooks if they
want to take part to this negotiation. Negotiation happens in reverse
order, starting from the last element of the chain (the one directly
connected to the display) up to the first element of the chain (the one
connected to the encoder).
During this negotiation all supported formats are tested until we find
one that works, meaning that the formats array should be in decreasing
preference order (assuming the driver has a preference order).

Note that the bus format negotiation works even if some elements in the
chain don't implement the ->atomic_get_{output,input}_bus_fmts() hooks.
In that case, the core advertises only MEDIA_BUS_FMT_FIXED and lets
the previous bridge element decide what to do (most of the time, bridge
drivers will pick a default bus format or extract this piece of
information from somewhere else, like a FW property).

v10:
* Add changelog to the commit message

v9:
* No changes

v8:
* Fix a test in drm_atomic_bridge_chain_select_bus_fmts() (Reported by
  Jonas)

v7:
* Adapt the code to deal with the fact that not all bridges in the
  chain have a bridge state

v5 -> v6:
* No changes

v4:
* Enhance the doc
* Fix typos
* Rename some parameters/fields
* Reword the commit message

v3:
* Fix the commit message (Reported by Laurent)
* Document the fact that bus formats should not be directly modified by
  drivers (Suggested by Laurent)
* Document the fact that format order matters (Suggested by Laurent)
* Propagate bus flags by default
* Document the fact that drivers can tweak bus flags if needed
* Let ->atomic_get_{output,input}_bus_fmts() allocate the bus format
  array (Suggested by Laurent)
* Add a drm_atomic_helper_bridge_propagate_bus_fmt()
* Mandate that bridge drivers return accurate input_fmts even if they
  are known to be the first element in the bridge chain

v2:
* Rework things to support more complex use cases

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
[narmstrong: fixed doc in include/drm/drm_bridge.h:69 fmt->format]
Reviewed-by: Jernej Skrabec <jernej.skrabec@siol.net>
Tested-by: Jonas Karlman <jonas@kwiboo.se>
Link: https://patchwork.freedesktop.org/patch/msgid/20200128135514.108171-7-boris.brezillon@collabora.com

3 files changed, 132 insertions, 0 deletions

diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
index 82a888769b3d..52d65a055491 100644
--- a/include/drm/drm_atomic.h
+++ b/include/drm/drm_atomic.h
@@ -996,6 +996,38 @@ drm_atomic_crtc_effectively_active(const struct drm_crtc_state *state)
 }
 
 /**
+ * struct drm_bus_cfg - bus configuration
+ *
+ * This structure stores the configuration of a physical bus between two
+ * components in an output pipeline, usually between two bridges, an encoder
+ * and a bridge, or a bridge and a connector.
+ *
+ * The bus configuration is stored in &drm_bridge_state separately for the
+ * input and output buses, as seen from the point of view of each bridge. The
+ * bus configuration of a bridge output is usually identical to the
+ * configuration of the next bridge's input, but may differ if the signals are
+ * modified between the two bridges, for instance by an inverter on the board.
+ * The input and output configurations of a bridge may differ if the bridge
+ * modifies the signals internally, for instance by performing format
+ * conversion, or modifying signals polarities.
+ */
+struct drm_bus_cfg {
+	/**
+	 * @format: format used on this bus (one of the MEDIA_BUS_FMT_* format)
+	 *
+	 * This field should not be directly modified by drivers
+	 * (&drm_atomic_bridge_chain_select_bus_fmts() takes care of the bus
+	 * format negotiation).
+	 */
+	u32 format;
+
+	/**
+	 * @flags: DRM_BUS_* flags used on this bus
+	 */
+	u32 flags;
+};
+
+/**
  * struct drm_bridge_state - Atomic bridge state object
  */
 struct drm_bridge_state {
@@ -1008,6 +1040,16 @@ struct drm_bridge_state {
 	 * @bridge: the bridge this state refers to
 	 */
 	struct drm_bridge *bridge;
+
+	/**
+	 * @input_bus_cfg: input bus configuration
+	 */
+	struct drm_bus_cfg input_bus_cfg;
+
+	/**
+	 * @output_bus_cfg: input bus configuration
+	 */
+	struct drm_bus_cfg output_bus_cfg;
 };
 
 static inline struct drm_bridge_state *
diff --git a/include/drm/drm_atomic_helper.h b/include/drm/drm_atomic_helper.h
index 9db3cac48f4f..b268180c97eb 100644
--- a/include/drm/drm_atomic_helper.h
+++ b/include/drm/drm_atomic_helper.h
@@ -224,4 +224,12 @@ drm_atomic_plane_disabling(struct drm_plane_state *old_plane_state,
 	return old_plane_state->crtc && !new_plane_state->crtc;
 }
 
+u32 *
+drm_atomic_helper_bridge_propagate_bus_fmt(struct drm_bridge *bridge,
+					struct drm_bridge_state *bridge_state,
+					struct drm_crtc_state *crtc_state,
+					struct drm_connector_state *conn_state,
+					u32 output_fmt,
+					unsigned int *num_input_fmts);
+
 #endif /* DRM_ATOMIC_HELPER_H_ */
diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
index ff4df6eb2689..45626ecf20f8 100644
--- a/include/drm/drm_bridge.h
+++ b/include/drm/drm_bridge.h
@@ -371,6 +371,72 @@ struct drm_bridge_funcs {
 				     struct drm_bridge_state *state);
 
 	/**
+	 * @atomic_get_output_bus_fmts:
+	 *
+	 * Return the supported bus formats on the output end of a bridge.
+	 * The returned array must be allocated with kmalloc() and will be
+	 * freed by the caller. If the allocation fails, NULL should be
+	 * returned. num_output_fmts must be set to the returned array size.
+	 * Formats listed in the returned array should be listed in decreasing
+	 * preference order (the core will try all formats until it finds one
+	 * that works).
+	 *
+	 * This method is only called on the last element of the bridge chain
+	 * as part of the bus format negotiation process that happens in
+	 * &drm_atomic_bridge_chain_select_bus_fmts().
+	 * This method is optional. When not implemented, the core will
+	 * fall back to &drm_connector.display_info.bus_formats[0] if
+	 * &drm_connector.display_info.num_bus_formats > 0,
+	 * or to MEDIA_BUS_FMT_FIXED otherwise.
+	 */
+	u32 *(*atomic_get_output_bus_fmts)(struct drm_bridge *bridge,
+					   struct drm_bridge_state *bridge_state,
+					   struct drm_crtc_state *crtc_state,
+					   struct drm_connector_state *conn_state,
+					   unsigned int *num_output_fmts);
+
+	/**
+	 * @atomic_get_input_bus_fmts:
+	 *
+	 * Return the supported bus formats on the input end of a bridge for
+	 * a specific output bus format.
+	 *
+	 * The returned array must be allocated with kmalloc() and will be
+	 * freed by the caller. If the allocation fails, NULL should be
+	 * returned. num_output_fmts must be set to the returned array size.
+	 * Formats listed in the returned array should be listed in decreasing
+	 * preference order (the core will try all formats until it finds one
+	 * that works). When the format is not supported NULL should be
+	 * returned and *num_output_fmts should be set to 0.
+	 *
+	 * This method is called on all elements of the bridge chain as part of
+	 * the bus format negotiation process that happens in
+	 * &drm_atomic_bridge_chain_select_bus_fmts().
+	 * This method is optional. When not implemented, the core will bypass
+	 * bus format negotiation on this element of the bridge without
+	 * failing, and the previous element in the chain will be passed
+	 * MEDIA_BUS_FMT_FIXED as its output bus format.
+	 *
+	 * Bridge drivers that need to support being linked to bridges that are
+	 * not supporting bus format negotiation should handle the
+	 * output_fmt == MEDIA_BUS_FMT_FIXED case appropriately, by selecting a
+	 * sensible default value or extracting this information from somewhere
+	 * else (FW property, &drm_display_mode, &drm_display_info, ...)
+	 *
+	 * Note: Even if input format selection on the first bridge has no
+	 * impact on the negotiation process (bus format negotiation stops once
+	 * we reach the first element of the chain), drivers are expected to
+	 * return accurate input formats as the input format may be used to
+	 * configure the CRTC output appropriately.
+	 */
+	u32 *(*atomic_get_input_bus_fmts)(struct drm_bridge *bridge,
+					  struct drm_bridge_state *bridge_state,
+					  struct drm_crtc_state *crtc_state,
+					  struct drm_connector_state *conn_state,
+					  u32 output_fmt,
+					  unsigned int *num_input_fmts);
+
+	/**
 	 * @atomic_check:
 	 *
 	 * This method is responsible for checking bridge state correctness.
@@ -384,6 +450,14 @@ struct drm_bridge_funcs {
 	 * called when &drm_bridge_funcs.atomic_check() is implemented, so only
 	 * one of them should be provided.
 	 *
+	 * If drivers need to tweak &drm_bridge_state.input_bus_cfg.flags or
+	 * &drm_bridge_state.output_bus_cfg.flags it should should happen in
+	 * this function. By default the &drm_bridge_state.output_bus_cfg.flags
+	 * field is set to the next bridge
+	 * &drm_bridge_state.input_bus_cfg.flags value or
+	 * &drm_connector.display_info.bus_flags if the bridge is the last
+	 * element in the chain.
+	 *
 	 * RETURNS:
 	 * zero if the check passed, a negative error code otherwise.
 	 */
@@ -578,6 +652,14 @@ void drm_atomic_bridge_chain_pre_enable(struct drm_bridge *bridge,
 void drm_atomic_bridge_chain_enable(struct drm_bridge *bridge,
 				    struct drm_atomic_state *state);
 
+u32 *
+drm_atomic_helper_bridge_propagate_bus_fmt(struct drm_bridge *bridge,
+					struct drm_bridge_state *bridge_state,
+					struct drm_crtc_state *crtc_state,
+					struct drm_connector_state *conn_state,
+					u32 output_fmt,
+					unsigned int *num_input_fmts);
+
 #ifdef CONFIG_DRM_PANEL_BRIDGE
 struct drm_bridge *drm_panel_bridge_add(struct drm_panel *panel);
 struct drm_bridge *drm_panel_bridge_add_typed(struct drm_panel *panel,