Re: [PATCH 2/4] drm: Only update final vblank count when precise ts is available

[Matt Roper <matthew.d.roper@intel.com>]

On Wed, Sep 10, 2014 at 05:36:09PM +0200, Daniel Vetter wrote:
> Drivers without a hardware vblank counter simply can't account for the
> vblanks that happened while the vblank interrupt was off. To check
> this grab a vblank timestamp and if the result is dubious follow the
> normal save-and-disable logic.
> 
> Drivers should prevent this by setting vblank_disable_allowed = false,
> but since running vblank interrupts constantly is not good for power
> consumption most drivers lie. Testing for precise vblank timestamps is
> the next best thing we can check for.
> 
> Suggested-by: Mario Kleiner <mario.kleiner.de@gmail.com>
> Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
> Cc: Matt Roper <matthew.d.roper@intel.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>

> ---
>  drivers/gpu/drm/drm_irq.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> index 6eb015020af2..922721ead29a 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -163,8 +163,15 @@ static void vblank_disable_and_save(struct drm_device *dev, int crtc)
>  	 * has been ticking all along until this time. This makes the
>  	 * count account for the entire time between drm_vblank_on() and
>  	 * drm_vblank_off().
> +	 *
> +	 * But only do this if precise vblank timestamps are available.
> +	 * Otherwise we might read a totally bogus timestamp since drivers
> +	 * lacking precise timestamp support rely upon sampling the system clock
> +	 * at vblank interrupt time. Which obviously won't work out well if the
> +	 * vblank interrupt is disabled.
>  	 */
> -	if (!vblank->enabled) {
> +	if (!vblank->enabled &&
> +	    drm_get_last_vbltimestamp(dev, crtc, &tvblank, 0) > 0) {
>  		drm_update_vblank_count(dev, crtc);
>  		spin_unlock_irqrestore(&dev->vblank_time_lock, irqflags);
>  		return;
> -- 
> 1.9.3
> 

-- 
Matt Roper
Graphics Software Engineer
IoTG Platform Enabling & Development
Intel Corporation
(916) 356-2795