[PATCH RFC 12/19] drm/bridge: Add an ->atomic_check() hook

[Laurent Pinchart]

Hi Boris,

Thank you for the patch.

On Thu, Aug 08, 2019 at 05:11:43PM +0200, Boris Brezillon wrote:
> So that bridge drivers have a way to check/reject an atomic operation.
> The drm_atomic_bridge_chain_check() (which is just a wrapper around
> the ->atomic_check() hook) is called in place of
> drm_bridge_chain_mode_fixup() (when ->atomic_check() is not implemented,
> the core falls back to ->mode_fixup(), so the behavior should stay
> the same for existing bridge drivers).
> 
> Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>
> ---
>  drivers/gpu/drm/drm_atomic_helper.c | 11 +++---
>  drivers/gpu/drm/drm_bridge.c        | 59 +++++++++++++++++++++++++++++
>  include/drm/drm_bridge.h            | 23 +++++++++++
>  3 files changed, 87 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index e76ec9648b6f..3fadde38ead7 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -445,12 +445,11 @@ mode_fixup(struct drm_atomic_state *state)
>  		encoder = new_conn_state->best_encoder;
>  		funcs = encoder->helper_private;
>  
> -		ret = drm_bridge_chain_mode_fixup(encoder,
> -					&new_crtc_state->mode,
> -					&new_crtc_state->adjusted_mode);
> -		if (!ret) {
> -			DRM_DEBUG_ATOMIC("Bridge fixup failed\n");
> -			return -EINVAL;
> +		ret = drm_atomic_bridge_chain_check(encoder, new_crtc_state,
> +						    new_conn_state);
> +		if (ret) {
> +			DRM_DEBUG_ATOMIC("Bridge atomic check failed\n");
> +			return ret;
>  		}
>  
>  		if (funcs && funcs->atomic_check) {
> diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
> index 0528ca941855..dcad661daa74 100644
> --- a/drivers/gpu/drm/drm_bridge.c
> +++ b/drivers/gpu/drm/drm_bridge.c
> @@ -583,6 +583,65 @@ void drm_atomic_bridge_chain_enable(struct drm_encoder *encoder,
>  }
>  EXPORT_SYMBOL(drm_atomic_bridge_chain_enable);
>  
> +static int drm_atomic_bridge_check(struct drm_bridge *bridge,
> +				   struct drm_crtc_state *crtc_state,
> +				   struct drm_connector_state *conn_state)
> +{
> +	if (bridge->funcs->atomic_check) {
> +		struct drm_bridge_state *bridge_state;
> +		int ret;
> +
> +		bridge_state = drm_atomic_get_new_bridge_state(crtc_state->state,
> +							       bridge);
> +		if (WARN_ON(!bridge_state))
> +			return -EINVAL;
> +
> +		ret = bridge->funcs->atomic_check(bridge, bridge_state,
> +						  crtc_state, conn_state);
> +		if (ret)
> +			return ret;
> +	} else if (bridge->funcs->mode_fixup) {
> +		if (!bridge->funcs->mode_fixup(bridge, &crtc_state->mode,
> +					       &crtc_state->adjusted_mode))
> +			return -EINVAL;
> +	}
> +
> +	return 0;
> +}
> +
> +/**
> + * drm_atomic_bridge_chain_check() - Do an atomic check on the bridge chain
> + * @encoder: encoder object
> + * @crtc_state: new CRTC state
> + * @conn_state: new connector state
> + *
> + * Calls &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()
> + *
> + * RETURNS:
> + * 0 on success, a negative error code on failure
> + */
> +int drm_atomic_bridge_chain_check(struct drm_encoder *encoder,
> +				  struct drm_crtc_state *crtc_state,
> +				  struct drm_connector_state *conn_state)
> +{
> +	struct drm_bridge *bridge;
> +
> +	list_for_each_entry_reverse(bridge, &encoder->bridge_chain,
> +				    chain_node) {
> +		int ret;
> +
> +		ret = drm_atomic_bridge_check(bridge, crtc_state, conn_state);
> +		if (ret)
> +			return ret;
> +	}
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(drm_atomic_bridge_chain_check);
> +
>  /**
>   * drm_atomic_helper_init_bridge_state() - Initializes a bridge state
>   * @bridge this state is referring to
> diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
> index 9383b4e4b853..5d8fe3709bde 100644
> --- a/include/drm/drm_bridge.h
> +++ b/include/drm/drm_bridge.h
> @@ -379,6 +379,26 @@ struct drm_bridge_funcs {
>  	 */
>  	void (*atomic_destroy_state)(struct drm_bridge *bridge,
>  				     struct drm_bridge_state *state);
> +
> +	/**
> +	 * @atomic_check:
> +	 *
> +	 * This method is responsible for checking bridge state correctness.
> +	 * It can also check the state of the surrounding components in chain
> +	 * to make sure the whole pipeline can work properly.

As this is meant to replace .mode_fixup() you should document that here
and in the .mode_fixup() operation.

There's also the additional question of what parameters can be modified.
Is a bridge .atomic_check() allowed to modify the CRTC or connector
state ? Of so, what are the restrictions on how those can be modified ?
For instance .mode_fixup() can change the pixel clock but isn't allowed
to modify the h/v active values. Those restrictions were not clearly
documented for .mode_fixup() and have led to grey areas. I don't want to
repeat the same mistake for bridges, we need to be very explicit here.

In the list of open questions, what if a bridge changes the pixel clock
to a value it supports (as .mode_fixup() does), and then the previous
bridge in the chain (the next one in traversal order for the
drm_atomic_bridge_chain_check() function) also changes the pixel clock,
to a value that the current bridge doesn't support ? I think we really
need to think about the semantic of this operation.

As the goal of your series is to implement configuration of bus formats
between bridges, shouldn't a bridge .atomic_check() also receive the
state of the next bridge in the chain ?

> +	 *
> +	 * &drm_bridge_funcs.atomic_check() hooks are called in reverse
> +	 * order (from the last to the first bridge).
> +	 *
> +	 * This method is optional.
> +	 *
> +	 * RETURNS:
> +	 * zero if the check passed, a negative error code otherwise.
> +	 */
> +	int (*atomic_check)(struct drm_bridge *bridge,
> +			    struct drm_bridge_state *bridge_state,
> +			    struct drm_crtc_state *crtc_state,
> +			    struct drm_connector_state *conn_state);
>  };
>  
>  /**
> @@ -479,6 +499,9 @@ void drm_bridge_chain_mode_set(struct drm_encoder *encoder,
>  void drm_bridge_chain_pre_enable(struct drm_encoder *encoder);
>  void drm_bridge_chain_enable(struct drm_encoder *encoder);
>  
> +int drm_atomic_bridge_chain_check(struct drm_encoder *encoder,
> +				  struct drm_crtc_state *crtc_state,
> +				  struct drm_connector_state *conn_state);
>  void drm_atomic_bridge_chain_disable(struct drm_encoder *encoder,
>  				     struct drm_atomic_state *state);
>  void drm_atomic_bridge_chain_post_disable(struct drm_encoder *encoder,

-- 
Regards,

Laurent Pinchart