Re: [PATCH v2 4/8] drm/crtc: Add a generic infrastructure to fake VBLANK events

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Jun 29, 2018 at 01:17:17PM +0200, Boris Brezillon wrote:
> In some cases CRTCs are active but are not able to generating events, at
> least not at every frame at it's expected to.
> This is typically the case when the CRTC is feeding a writeback connector
> that has no job queued. In this situation the CRTC is usually stopped
> until a new job is queued, and this can lead to timeouts when part of
> the pipeline is updated but no new jobs are queued to the active
> writeback connector.
> 
> In order to solve that, we add a ->no_vblank flag to drm_crtc_state
> and ask the CRTC drivers to set it to true when they know they're not
> able to generate VBLANK events. The core drm_atomic_helper_fake_vblank()
> helper can then be used to fake VBLANKs at commit time.
> 
> Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
> ---
>  drivers/gpu/drm/drm_atomic_helper.c | 40 +++++++++++++++++++++++++++++++++++++
>  include/drm/drm_atomic_helper.h     |  1 +
>  include/drm/drm_crtc.h              | 15 ++++++++++++++
>  3 files changed, 56 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index 69063bcf2334..ca586993c2a2 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -2051,6 +2051,46 @@ void drm_atomic_helper_wait_for_dependencies(struct drm_atomic_state *old_state)
>  }
>  EXPORT_SYMBOL(drm_atomic_helper_wait_for_dependencies);
>  
> +/**
> + * drm_atomic_helper_fake_vblank - fake VBLANK events if needed
> + * @old_state: atomic state object with old state structures
> + *
> + * This function walks all CRTCs and fake VBLANK events on those with
> + * &drm_crtc_state.no_vblank set to true and &drm_crtc_state.event != NULL.
> + * The primary use of this function is writeback connectors working in oneshot
> + * mode and faking VBLANK events. In this case they only fake the VBLANK event
> + * when a job is queued, and any change to the pipeline that does not touch the
> + * connector is leading to timeouts when calling
> + * drm_atomic_helper_wait_for_vblanks() or
> + * drm_atomic_helper_wait_for_flip_done().
> + *
> + * This is part of the atomic helper support for nonblocking commits, see
> + * drm_atomic_helper_setup_commit() for an overview.
> + */
> +void drm_atomic_helper_fake_vblank(struct drm_atomic_state *old_state)
> +{
> +	struct drm_crtc_state *old_crtc_state, *new_crtc_state;
> +	struct drm_crtc *crtc;
> +	int i;
> +
> +	for_each_oldnew_crtc_in_state(old_state, crtc, old_crtc_state,
> +				      new_crtc_state, i) {
> +		unsigned long flags;
> +
> +		if (!new_crtc_state->no_vblank && !old_crtc_state->no_vblank)

Uh, this essentially makes it impossible to reset no_vblank. For control
flow state bits we only check the new state for it (see e.g. the various
*_changed or plane_bits or whatever).

> +			continue;
> +
> +		spin_lock_irqsave(&old_state->dev->event_lock, flags);
> +		if (new_crtc_state->event) {
> +			drm_crtc_send_vblank_event(crtc,
> +						   new_crtc_state->event);
> +			new_crtc_state->event = NULL;
> +		}
> +		spin_unlock_irqrestore(&old_state->dev->event_lock, flags);
> +	}
> +}
> +EXPORT_SYMBOL(drm_atomic_helper_fake_vblank);
> +
>  /**
>   * drm_atomic_helper_commit_hw_done - setup possible nonblocking commit
>   * @old_state: atomic state object with old state structures
> diff --git a/include/drm/drm_atomic_helper.h b/include/drm/drm_atomic_helper.h
> index 26aaba58d6ce..99e2a5297c69 100644
> --- a/include/drm/drm_atomic_helper.h
> +++ b/include/drm/drm_atomic_helper.h
> @@ -100,6 +100,7 @@ int __must_check drm_atomic_helper_swap_state(struct drm_atomic_state *state,
>  int drm_atomic_helper_setup_commit(struct drm_atomic_state *state,
>  				   bool nonblock);
>  void drm_atomic_helper_wait_for_dependencies(struct drm_atomic_state *state);
> +void drm_atomic_helper_fake_vblank(struct drm_atomic_state *state);
>  void drm_atomic_helper_commit_hw_done(struct drm_atomic_state *state);
>  void drm_atomic_helper_commit_cleanup_done(struct drm_atomic_state *state);
>  
> diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
> index 23eddbccab10..7435dc66c08a 100644
> --- a/include/drm/drm_crtc.h
> +++ b/include/drm/drm_crtc.h
> @@ -87,6 +87,20 @@ struct drm_plane_helper_funcs;
>   * @zpos_changed: zpos values of planes on this crtc have been updated
>   * @color_mgmt_changed: color management properties have changed (degamma or
>   *	gamma LUT or CSC matrix)
> + * @no_vblank: reflects the ability of a CRTC to send VBLANK events. This state
> + *	usually depends on the pipeline configuration, and the main usuage is
> + *	CRTCs feeding a writeback connector operating in oneshot mode. In this
> + *	case the VBLANK event is only generated when a job is queued to the
> + *	writeback connector, and we want the core to fake VBLANK events when
> + *	this part of the pipeline hasn't changed but others had or when the
> + *	CRTC and connectors are disabled.

s/are disabled/are getting disabled/

You'll never get a request for an event when it's already off.

> + *	__drm_atomic_helper_crtc_duplicate_state() will the value from the
> + *	current state, the CRTC driver is then responsible for updating this
> + *	field when needed.

Not parsing the above ... probably missing a "... will not reset ..."

> + *	Note that, even when no_blank is set to true, the CRTC driver can still
> + *	steal the &drm_crtc_state.event object and send the event on its own.
> + *	That's usually what happens when a job is queued to the writeback
> + *	connector.

The last sentence is confusing imo. Just drop it?

Please use the inline comment style for struct members, and then also
polish the formatting a bit (e.g. paragraph breaks, which are only
possible with the inline style).

With the nits addressed:

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

>   * @plane_mask: bitmask of (1 << drm_plane_index(plane)) of attached planes
>   * @connector_mask: bitmask of (1 << drm_connector_index(connector)) of attached connectors
>   * @encoder_mask: bitmask of (1 << drm_encoder_index(encoder)) of attached encoders
> @@ -118,6 +132,7 @@ struct drm_crtc_state {
>  	bool connectors_changed : 1;
>  	bool zpos_changed : 1;
>  	bool color_mgmt_changed : 1;
> +	bool no_vblank : 1;
>  
>  	/* attached planes bitmask:
>  	 * WARNING: transitional helpers do not maintain plane_mask so
> -- 
> 2.14.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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