[PATCH v2] drm/core: Add drm_accurate_vblank_count_and_time, v2.

[Ville Syrjälä]

On Mon, Apr 11, 2016 at 11:42:57AM +0200, Maarten Lankhorst wrote:
> This function is useful for gen2 intel devices which have no frame
> counter, but need a way to determine the current vblank count without
> racing with the vblank interrupt handler.
> 
> intel_pipe_update_start checks if no vblank interrupt will occur
> during vblank evasion, but cannot check whether the vblank handler has
> run to completion. This function uses the timestamps to determine
> when the last vblank has happened, and interpolates from there.
> 
> Changes since v1:
> - Take vblank_time_lock and don't use drm_vblank_count_and_time.
> 
> Cc: Mario Kleiner <mario.kleiner.de at gmail.com>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> ---
>  drivers/gpu/drm/drm_irq.c | 31 +++++++++++++++++++++++++++++++
>  include/drm/drmP.h        |  2 ++
>  2 files changed, 33 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> index 3c1a6f18e71c..d3cecc1932e1 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -303,6 +303,37 @@ static void drm_update_vblank_count(struct drm_device *dev, unsigned int pipe,
>  	store_vblank(dev, pipe, diff, &t_vblank, cur_vblank);
>  }
>  
> +/**
> + * drm_accurate_vblank_count_and_time - retrieve the master vblank counter
> + * @crtc: which counter to retrieve
> + * @tv_ret: last time counter was updated
> + *
> + * This function is similar to @drm_update_vblank_count_and_time but
> + * this function interpolates to handle a race with vblank irq's, and
> + * is only useful for crtc's that have no hw vblank counter.
> + */
> +
> +u32 drm_accurate_vblank_count_and_time(struct drm_crtc *crtc,
> +				       struct timeval *tv_ret)
> +{
> +	struct drm_device *dev = crtc->dev;
> +	u32 vblank, pipe = drm_crtc_index(crtc);
> +	unsigned long flags;
> +
> +	WARN(dev->max_vblank_count, "This function is only useful when a hw counter is unavailable.");

It's useful on everything if you actually want to trust the
software counter to not be stale.

> +
> +	spin_lock_irqsave(&dev->vblank_time_lock, flags);
> +	drm_update_vblank_count(dev, pipe, 0);
> +
> +	vblank = dev->vblank[pipe].count;
> +	*tv_ret = vblanktimestamp(dev, pipe, vblank);

Do we have a user in mind that would care about the timestamp?

> +
> +	spin_unlock_irqrestore(&dev->vblank_time_lock, flags);
> +
> +	return vblank;
> +}
> +EXPORT_SYMBOL(drm_accurate_vblank_count_and_time);
> +
>  /*
>   * Disable vblank irq's on crtc, make sure that last vblank count
>   * of hardware and corresponding consistent software vblank counter
> diff --git a/include/drm/drmP.h b/include/drm/drmP.h
> index 31483c2fef51..1df65922c7c6 100644
> --- a/include/drm/drmP.h
> +++ b/include/drm/drmP.h
> @@ -995,6 +995,8 @@ extern void drm_crtc_vblank_off(struct drm_crtc *crtc);
>  extern void drm_crtc_vblank_reset(struct drm_crtc *crtc);
>  extern void drm_crtc_vblank_on(struct drm_crtc *crtc);
>  extern void drm_vblank_cleanup(struct drm_device *dev);
> +extern u32 drm_accurate_vblank_count_and_time(struct drm_crtc *crtc,
> +					      struct timeval *tv_ret);
>  extern u32 drm_vblank_no_hw_counter(struct drm_device *dev, unsigned int pipe);
>  
>  extern int drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev,
> -- 
> 2.1.0
> 

-- 
Ville Syrjälä
Intel OTC