linux-kernel - Re: [PATCH] drm/vc4: Return -EBUSY if there's already a pending flip event.

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Date:	Mon, 02 May 2016 17:57:37 -0700
From:	Eric Anholt <eric@...olt.net>
To:	robert.foss@...labora.com, daniel.vetter@...ll.ch,
	airlied@...ux.ie, aniel.vetter@...ll.ch, fengguang.wu@...el.com,
	maarten.lankhorst@...ux.intel.com, julia.lawall@...6.fr,
	alexander.deucher@....com, daniels@...labora.com,
	derekf@....samsung.com, varadgautam@...il.com
Cc:	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	Robert Foss <robert.foss@...labora.com>
Subject: Re: [PATCH] drm/vc4: Return -EBUSY if there's already a pending flip event.

robert.foss@...labora.com writes:

> From: Robert Foss <robert.foss@...labora.com>
>
> As per the docs, atomic_commit should return -EBUSY "if an asycnhronous
> update is requested and there is an earlier update pending".

Note: docs cited here are drm_crtc.h, and the whole quote is:

	 *  - -EBUSY, if an asynchronous updated is requested and there is
	 *    an earlier updated pending. Drivers are allowed to support a queue
	 *    of outstanding updates, but currently no driver supports that.
	 *    Note that drivers must wait for preceding updates to complete if a
	 *    synchronous update is requested, they are not allowed to fail the
	 *    commit in that case.

> Signed-off-by: Robert Foss <robert.foss@...labora.com>
> ---
>  drivers/gpu/drm/vc4/vc4_crtc.c |  6 ++++++
>  drivers/gpu/drm/vc4/vc4_drv.h  |  1 +
>  drivers/gpu/drm/vc4/vc4_kms.c  | 20 ++++++++++++++++++--
>  3 files changed, 25 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/vc4/vc4_crtc.c b/drivers/gpu/drm/vc4/vc4_crtc.c
> index 355ee4b..43193a3 100644
> --- a/drivers/gpu/drm/vc4/vc4_crtc.c
> +++ b/drivers/gpu/drm/vc4/vc4_crtc.c
> @@ -802,3 +802,9 @@ struct platform_driver vc4_crtc_driver = {
>  		.of_match_table = vc4_crtc_dt_match,
>  	},
>  };
> +
> +bool vc4_crtc_has_pending_event(struct drm_crtc *crtc)
> +{
> +	assert_spin_locked(&crtc->dev->event_lock);
> +	return to_vc4_crtc(crtc)->event;
> +}
> diff --git a/drivers/gpu/drm/vc4/vc4_drv.h b/drivers/gpu/drm/vc4/vc4_drv.h
> index fa2ad15..54c1fb5 100644
> --- a/drivers/gpu/drm/vc4/vc4_drv.h
> +++ b/drivers/gpu/drm/vc4/vc4_drv.h
> @@ -414,6 +414,7 @@ extern struct platform_driver vc4_crtc_driver;
>  int vc4_enable_vblank(struct drm_device *dev, unsigned int crtc_id);
>  void vc4_disable_vblank(struct drm_device *dev, unsigned int crtc_id);
>  int vc4_crtc_debugfs_regs(struct seq_file *m, void *arg);
> +bool vc4_crtc_has_pending_event(struct drm_crtc *crtc);
>  
>  /* vc4_debugfs.c */
>  int vc4_debugfs_init(struct drm_minor *minor);
> diff --git a/drivers/gpu/drm/vc4/vc4_kms.c b/drivers/gpu/drm/vc4/vc4_kms.c
> index 4718ae5..dc157a1e 100644
> --- a/drivers/gpu/drm/vc4/vc4_kms.c
> +++ b/drivers/gpu/drm/vc4/vc4_kms.c
> @@ -107,10 +107,26 @@ static int vc4_atomic_commit(struct drm_device *dev,
>  			     bool async)
>  {
>  	struct vc4_dev *vc4 = to_vc4_dev(dev);
> -	int ret;
> -	int i;
>  	uint64_t wait_seqno = 0;
>  	struct vc4_commit *c;
> +	struct drm_crtc_state *crtc_state;
> +	struct drm_crtc *crtc;
> +	unsigned long flags;
> +	int i, ret;
> +
> +	if (async) {
> +		for_each_crtc_in_state(state, crtc, crtc_state, i) {
> +
> +			spin_lock_irqsave(&dev->event_lock, flags);
> +
> +			if (crtc->state->event || 
> +				vc4_crtc_has_pending_event(crtc)) {
> +				spin_unlock_irqrestore(&dev->event_lock, flags);
> +				return -EBUSY;
> +			}
> +			spin_unlock_irqrestore(&dev->event_lock, flags);
> +		}
> +	}

By my reading, the point of returning -EBUSY here is just to avoid
blocking when the compositor asked for nonblocking.  You're checking
that each CRTC involved in the commit doesn't have a queued pageflip
event, except that then we immediately proceed to:

	/* Make sure that any outstanding modesets have finished. */
	ret = down_interruptible(&vc4->async_modeset);
	if (ret) {
		kfree(c);
		return ret;
	}

so this per-CRTC check doesn't prevent blocking if the set of CRTCs for
this commit was disjoint from the last one, right?  To implement the
minimal behavior, I think we'd want to just down_trylock instead in the
async case, I think.  And to implement the disjoint CRTC thing you were
trying for, I think we'd need to track a mask kind of like msm's
pending_crtcs.

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