[PATCH v4 1/2] drm/vblank: warn on sending stale event

[Robert Beckett]

Nak - I forgot the requested doc changes. Ill re-send

On Fri, 2019-06-28 at 13:05 +0100, Robert Beckett wrote:
> Warn when about to send stale vblank info and add advice to
> documentation on how to avoid.
> 
> Signed-off-by: Robert Beckett <bob.beckett at collabora.com>
> ---
>  drivers/gpu/drm/drm_vblank.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_vblank.c
> b/drivers/gpu/drm/drm_vblank.c
> index 603ab105125d..7dabb2bdb733 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -918,6 +918,19 @@ EXPORT_SYMBOL(drm_crtc_arm_vblank_event);
>   *
>   * See drm_crtc_arm_vblank_event() for a helper which can be used in
> certain
>   * situation, especially to send out events for atomic commit
> operations.
> + *
> + * Care should be taken to avoid stale timestamps. If:
> + *   - your driver has vblank support (i.e. dev->num_crtcs > 0)
> + *   - the vblank irq is off (i.e. no one called
> drm_crtc_vblank_get)
> + *   - from the vblank code's pov the pipe is still running (i.e.
> not
> + *     in-between a drm_crtc_vblank_off()/on() pair)
> + * If all of these conditions hold then drm_crtc_send_vblank_event
> is
> + * going to give you a garbage timestamp and and sequence number
> (the last
> + * recorded before the irq was disabled). If you call
> drm_crtc_vblank_get/put
> + * around it, or after vblank_off, then either of those will have
> rolled things
> + * forward for you.
> + * So, drivers should call drm_crtc_vblank_off() before this
> function in their
> + * crtc atomic_disable handlers.
>   */
>  void drm_crtc_send_vblank_event(struct drm_crtc *crtc,
>  				struct drm_pending_vblank_event *e)
> @@ -925,8 +938,12 @@ void drm_crtc_send_vblank_event(struct drm_crtc
> *crtc,
>  	struct drm_device *dev = crtc->dev;
>  	u64 seq;
>  	unsigned int pipe = drm_crtc_index(crtc);
> +	struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
>  	ktime_t now;
>  
> +	WARN_ONCE(dev->num_crtcs > 0 && !vblank->enabled && !vblank-
> >inmodeset,
> +		  "sending stale vblank info\n");
> +
>  	if (dev->num_crtcs > 0) {
>  		seq = drm_vblank_count_and_time(dev, pipe, &now);
>  	} else {