Re: [PATCH v7 06/12] drm/bridge: Add the necessary bits to support bus format negotiation

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On 2020-01-22 12:16, Boris Brezillon wrote:
> 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).
> 
> Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
> Signed-off-by: Neil Armstrong <narmstrong@xxxxxxxxxxxx>
> [narmstrong: fixed doc in include/drm/drm_bridge.h:69 fmt->format]
> Reviewed by: Jernej Skrabec <jernej.skrabec@xxxxxxxx>
> Tested-by: Jonas Karlman <jonas@xxxxxxxxx>
> ---
> Changes in v7:
> * Adapt the code to deal with the fact that not all bridges in the
>   chain have a bridge state
> ---
>  drivers/gpu/drm/drm_atomic_helper.c |  41 +++++
>  drivers/gpu/drm/drm_bridge.c        | 253 +++++++++++++++++++++++++++-
>  include/drm/drm_atomic.h            |  42 +++++
>  include/drm/drm_atomic_helper.h     |   8 +
>  include/drm/drm_bridge.h            |  82 +++++++++
>  5 files changed, 425 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index afe14f72a824..ea1926b5bb80 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -3528,3 +3528,44 @@ int drm_atomic_helper_legacy_gamma_set(struct drm_crtc *crtc,
>  	return ret;
>  }
>  EXPORT_SYMBOL(drm_atomic_helper_legacy_gamma_set);
> +
> +/**
> + * drm_atomic_helper_bridge_propagate_bus_fmt() - Propagate output format to
> + *						  the input end of a bridge
> + * @bridge: bridge control structure
> + * @bridge_state: new bridge state
> + * @crtc_state: new CRTC state
> + * @conn_state: new connector state
> + * @output_fmt: tested output bus format
> + * @num_input_fmts: will contain the size of the returned array
> + *
> + * This helper is a pluggable implementation of the
> + * &drm_bridge_funcs.atomic_get_input_bus_fmts operation for bridges that don't
> + * modify the bus configuration between their input and their output. It
> + * returns an array of input formats with a single element set to @output_fmt.
> + *
> + * RETURNS:
> + * a valid format array of size @num_input_fmts, or NULL if the allocation
> + * failed
> + */
> +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)
> +{
> +	u32 *input_fmts;
> +
> +	input_fmts = kzalloc(sizeof(*input_fmts), GFP_KERNEL);
> +	if (!input_fmts) {
> +		*num_input_fmts = 0;
> +		return NULL;
> +	}
> +
> +	*num_input_fmts = 1;
> +	input_fmts[0] = output_fmt;
> +	return input_fmts;
> +}
> +EXPORT_SYMBOL(drm_atomic_helper_bridge_propagate_bus_fmt);
> diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
> index dd3a2a9f87d3..0c28816146ba 100644
> --- a/drivers/gpu/drm/drm_bridge.c
> +++ b/drivers/gpu/drm/drm_bridge.c
> @@ -598,13 +598,247 @@ static int drm_atomic_bridge_check(struct drm_bridge *bridge,
>  	return 0;
>  }
>  
> +static int select_bus_fmt_recursive(struct drm_bridge *first_bridge,
> +				    struct drm_bridge *cur_bridge,
> +				    struct drm_crtc_state *crtc_state,
> +				    struct drm_connector_state *conn_state,
> +				    u32 out_bus_fmt)
> +{
> +	struct drm_bridge_state *cur_state;
> +	unsigned int num_in_bus_fmts, i;
> +	struct drm_bridge *prev_bridge;
> +	u32 *in_bus_fmts;
> +	int ret;
> +
> +	prev_bridge = drm_bridge_get_prev_bridge(cur_bridge);
> +	cur_state = drm_atomic_get_new_bridge_state(crtc_state->state,
> +						    cur_bridge);
> +
> +	/*
> +	 * If bus format negotiation is not supported by this bridge, let's
> +	 * pass MEDIA_BUS_FMT_FIXED to the previous bridge in the chain and
> +	 * hope that it can handle this situation gracefully (by providing
> +	 * appropriate default values).
> +	 */
> +	if (!cur_bridge->funcs->atomic_get_input_bus_fmts) {
> +		if (cur_bridge != first_bridge) {
> +			ret = select_bus_fmt_recursive(first_bridge,
> +						       prev_bridge, crtc_state,
> +						       conn_state,
> +						       MEDIA_BUS_FMT_FIXED);
> +			if (ret)
> +				return ret;
> +		}
> +
> +		/*
> +		 * Driver does not implement the atomic state hooks, but that's
> +		 * fine, as long as it does not access the bridge state.
> +		 */
> +		if (cur_state) {
> +			cur_state->input_bus_cfg.format = MEDIA_BUS_FMT_FIXED;
> +			cur_state->output_bus_cfg.format = out_bus_fmt;
> +		}
> +
> +		return 0;
> +	}
> +
> +	/*
> +	 * If the driver implements ->atomic_get_input_bus_fmts() it
> +	 * should also implement the atomic state hooks.
> +	 */
> +	if (WARN_ON(!cur_state))
> +		return -EINVAL;
> +
> +	in_bus_fmts = cur_bridge->funcs->atomic_get_input_bus_fmts(cur_bridge,
> +							cur_state,
> +							crtc_state,
> +							conn_state,
> +							out_bus_fmt,
> +							&num_in_bus_fmts);
> +	if (!num_in_bus_fmts)
> +		return -ENOTSUPP;
> +	else if (!in_bus_fmts)
> +		return -ENOMEM;
> +
> +	if (first_bridge == cur_bridge) {
> +		cur_state->input_bus_cfg.format = in_bus_fmts[0];
> +		cur_state->output_bus_cfg.format = out_bus_fmt;
> +		kfree(in_bus_fmts);
> +		return 0;
> +	}
> +
> +	for (i = 0; i < num_in_bus_fmts; i++) {
> +		ret = select_bus_fmt_recursive(first_bridge, prev_bridge,
> +					       crtc_state, conn_state,
> +					       in_bus_fmts[i]);
> +		if (ret != -ENOTSUPP)
> +			break;
> +	}
> +
> +	if (!ret) {
> +		cur_state->input_bus_cfg.format = in_bus_fmts[i];
> +		cur_state->output_bus_cfg.format = out_bus_fmt;
> +	}
> +
> +	kfree(in_bus_fmts);
> +	return ret;
> +}
> +
> +/*
> + * This function is called by &drm_atomic_bridge_chain_check() just before
> + * calling &drm_bridge_funcs.atomic_check() on all elements of the chain.
> + * It performs bus format negotiation between bridge elements. The negotiation
> + * happens in reverse order, starting from the last element in the chain up to
> + * @bridge.
> + *
> + * Negotiation starts by retrieving supported output bus formats on the last
> + * bridge element and testing them one by one. The test is recursive, meaning
> + * that for each tested output format, the whole chain will be walked backward,
> + * and each element will have to choose an input bus format that can be
> + * transcoded to the requested output format. When a bridge element does not
> + * support transcoding into a specific output format -ENOTSUPP is returned and
> + * the next bridge element will have to try a different format. If none of the
> + * combinations worked, -ENOTSUPP is returned and the atomic modeset will fail.
> + *
> + * This implementation is relying on
> + * &drm_bridge_funcs.atomic_get_output_bus_fmts() and
> + * &drm_bridge_funcs.atomic_get_input_bus_fmts() to gather supported
> + * input/output formats.
> + *
> + * When &drm_bridge_funcs.atomic_get_output_bus_fmts() is not implemented by
> + * the last element of the chain, &drm_atomic_bridge_chain_select_bus_fmts()
> + * tries a single format: &drm_connector.display_info.bus_formats[0] if
> + * available, MEDIA_BUS_FMT_FIXED otherwise.
> + *
> + * When &drm_bridge_funcs.atomic_get_input_bus_fmts() is not implemented,
> + * &drm_atomic_bridge_chain_select_bus_fmts() skips the negotiation on the
> + * bridge element that lacks this hook and asks the previous element in the
> + * chain to try MEDIA_BUS_FMT_FIXED. It's up to bridge drivers to decide what
> + * to do in that case (fail if they want to enforce bus format negotiation, or
> + * provide a reasonable default if they need to support pipelines where not
> + * all elements support bus format negotiation).
> + */
> +static int
> +drm_atomic_bridge_chain_select_bus_fmts(struct drm_bridge *bridge,
> +					struct drm_crtc_state *crtc_state,
> +					struct drm_connector_state *conn_state)
> +{
> +	struct drm_connector *conn = conn_state->connector;
> +	struct drm_encoder *encoder = bridge->encoder;
> +	struct drm_bridge_state *last_bridge_state;
> +	unsigned int i, num_out_bus_fmts;
> +	struct drm_bridge *last_bridge;
> +	u32 *out_bus_fmts;
> +	int ret = 0;
> +
> +	last_bridge = list_last_entry(&encoder->bridge_chain,
> +				      struct drm_bridge, chain_node);
> +	last_bridge_state = drm_atomic_get_new_bridge_state(crtc_state->state,
> +							    last_bridge);
> +
> +	if (last_bridge->funcs->atomic_get_output_bus_fmts) {
> +		const struct drm_bridge_funcs *funcs = last_bridge->funcs;
> +
> +		/*
> +		 * If the driver implements ->atomic_get_output_bus_fmts() it
> +		 * should also implement the atomic state hooks.
> +		 */
> +		if (WARN_ON(last_bridge_state))

This looks wrong, with this changed to WARN_ON(!last_bridge_state)
my RK3328 HDMI2.0/YUV444/YUV420/10-bit branch at [1] starts working.

With WARN_ON(last_bridge_state) I get:

[    6.606658] WARNING: CPU: 0 PID: 167 at drivers/gpu/drm/drm_bridge.c:746 drm_atomic_bridge_chain_check+0x2b8/0x308
[    6.606673] Hardware name: Pine64 Rock64 (DT)

[    6.606754] Call trace:
[    6.606759]  drm_atomic_bridge_chain_check+0x2b8/0x308
[    6.606764]  drm_atomic_helper_check_modeset+0x89c/0xab8
[    6.606768]  drm_atomic_helper_check+0x1c/0xa0
[    6.606772]  drm_atomic_check_only+0x464/0x708
[    6.606777]  drm_atomic_commit+0x18/0x58

[1] https://github.com/Kwiboo/linux-rockchip/commits/next-20200122-bus-format

Regards,
Jonas

> +			return -EINVAL;
> +
> +		out_bus_fmts = funcs->atomic_get_output_bus_fmts(last_bridge,
> +							last_bridge_state,
> +							crtc_state,
> +							conn_state,
> +							&num_out_bus_fmts);
> +		if (!num_out_bus_fmts)
> +			return -ENOTSUPP;
> +		else if (!out_bus_fmts)
> +			return -ENOMEM;
> +	} else {
> +		num_out_bus_fmts = 1;
> +		out_bus_fmts = kmalloc(sizeof(*out_bus_fmts), GFP_KERNEL);
> +		if (!out_bus_fmts)
> +			return -ENOMEM;
> +
> +		if (conn->display_info.num_bus_formats &&
> +		    conn->display_info.bus_formats)
> +			out_bus_fmts[0] = conn->display_info.bus_formats[0];
> +		else
> +			out_bus_fmts[0] = MEDIA_BUS_FMT_FIXED;
> +	}
> +
> +	for (i = 0; i < num_out_bus_fmts; i++) {
> +		ret = select_bus_fmt_recursive(bridge, last_bridge, crtc_state,
> +					       conn_state, out_bus_fmts[i]);
> +		if (ret != -ENOTSUPP)
> +			break;
> +	}
> +
> +	kfree(out_bus_fmts);
> +
> +	return ret;
> +}
> +
> +static void
> +drm_atomic_bridge_propagate_bus_flags(struct drm_bridge *bridge,
> +				      struct drm_connector *conn,
> +				      struct drm_atomic_state *state)
> +{
> +	struct drm_bridge_state *bridge_state, *next_bridge_state;
> +	struct drm_bridge *next_bridge;
> +	u32 output_flags = 0;
> +
> +	bridge_state = drm_atomic_get_new_bridge_state(state, bridge);
> +
> +	/* No bridge state attached to this bridge => nothing to propagate. */
> +	if (!bridge_state)
> +		return;
> +
> +	next_bridge = drm_bridge_get_next_bridge(bridge);
> +
> +	/*
> +	 * Let's try to apply the most common case here, that is, propagate
> +	 * display_info flags for the last bridge, and propagate the input
> +	 * flags of the next bridge element to the output end of the current
> +	 * bridge when the bridge is not the last one.
> +	 * There are exceptions to this rule, like when signal inversion is
> +	 * happening at the board level, but that's something drivers can deal
> +	 * with from their &drm_bridge_funcs.atomic_check() implementation by
> +	 * simply overriding the flags value we've set here.
> +	 */
> +	if (!next_bridge) {
> +		output_flags = conn->display_info.bus_flags;
> +	} else {
> +		next_bridge_state = drm_atomic_get_new_bridge_state(state,
> +								next_bridge);
> +		/*
> +		 * No bridge state attached to the next bridge, just leave the
> +		 * flags to 0.
> +		 */
> +		if (next_bridge_state)
> +			output_flags = next_bridge_state->input_bus_cfg.flags;
> +	}
> +
> +	bridge_state->output_bus_cfg.flags = output_flags;
> +
> +	/*
> +	 * Propage the output flags to the input end of the bridge. Again, it's
> +	 * not necessarily what all bridges want, but that's what most of them
> +	 * do, and by doing that by default we avoid forcing drivers to
> +	 * duplicate the "dummy propagation" logic.
> +	 */
> +	bridge_state->input_bus_cfg.flags = output_flags;
> +}
> +
>  /**
>   * drm_atomic_bridge_chain_check() - Do an atomic check on the bridge chain
>   * @bridge: bridge control structure
>   * @crtc_state: new CRTC state
>   * @conn_state: new connector state
>   *
> - * Calls &drm_bridge_funcs.atomic_check() (falls back on
> + * First trigger a bus format negotiation before calling
> + * &drm_bridge_funcs.atomic_check() (falls back on
>   * &drm_bridge_funcs.mode_fixup()) op for all the bridges in the encoder chain,
>   * starting from the last bridge to the first. These are called before calling
>   * &drm_encoder_helper_funcs.atomic_check()
> @@ -616,16 +850,33 @@ int drm_atomic_bridge_chain_check(struct drm_bridge *bridge,
>  				  struct drm_crtc_state *crtc_state,
>  				  struct drm_connector_state *conn_state)
>  {
> +	struct drm_connector *conn = conn_state->connector;
>  	struct drm_encoder *encoder;
>  	struct drm_bridge *iter;
> +	int ret;
>  
>  	if (!bridge)
>  		return 0;
>  
> +	ret = drm_atomic_bridge_chain_select_bus_fmts(bridge, crtc_state,
> +						      conn_state);
> +	if (ret)
> +		return ret;
> +
>  	encoder = bridge->encoder;
>  	list_for_each_entry_reverse(iter, &encoder->bridge_chain, chain_node) {
>  		int ret;
>  
> +		/*
> +		 * Bus flags are propagated by default. If a bridge needs to
> +		 * tweak the input bus flags for any reason, it should happen
> +		 * in its &drm_bridge_funcs.atomic_check() implementation such
> +		 * that preceding bridges in the chain can propagate the new
> +		 * bus flags.
> +		 */
> +		drm_atomic_bridge_propagate_bus_flags(iter, conn,
> +						      crtc_state->state);
> +
>  		ret = drm_atomic_bridge_check(iter, crtc_state, conn_state);
>  		if (ret)
>  			return ret;
> 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
> @@ -995,6 +995,38 @@ drm_atomic_crtc_effectively_active(const struct drm_crtc_state *state)
>  	return state->active || state->self_refresh_active;
>  }
>  
> +/**
> + * 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
>   */
> @@ -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 04815ab948dd..e9f407993c8b 100644
> --- a/include/drm/drm_bridge.h
> +++ b/include/drm/drm_bridge.h
> @@ -370,6 +370,72 @@ struct drm_bridge_funcs {
>  	void (*atomic_destroy_state)(struct drm_bridge *bridge,
>  				     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:
>  	 *
> @@ -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.
>  	 */
> @@ -571,6 +645,14 @@ 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,
> 
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