[PATCH] drm: Return current vblank value for drmWaitVBlank queries

[Michel Dänzer]

On 18.03.2015 00:44, Chris Wilson wrote:
> When userspace queries the current vblank for the CRTC, we can reply
> with the cached value (using atomic reads to serialise with the vblank
> interrupt as necessary) without having to touch registers. In the
> instant disable case, this saves us from enabling/disabling the vblank
> around every query, greatly reducing the number of registers read and
> written.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Imre Deak <imre.deak at intel.com>
> Cc: Daniel Vetter <daniel.vetter at intel.com>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> Cc: Dave Airlie <airlied at redhat.com>
> ---
>  drivers/gpu/drm/drm_irq.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> index c8a34476570a..6c4570082b65 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -1585,7 +1585,18 @@ int drm_wait_vblank(struct drm_device *dev, void *data,
>  	if (crtc >= dev->num_crtcs)
>  		return -EINVAL;
>  
> -	vblank = &dev->vblank[crtc];
> +	/* Fast-path the query for the current value (without an event)
> +	 * to avoid having to enable/disable the vblank interrupts.
> +	 */
> +	if ((vblwait->request.type & (_DRM_VBLANK_TYPES_MASK | _DRM_VBLANK_FLAGS_MASK)) == _DRM_VBLANK_RELATIVE &&
> +	    vblwait->request.sequence == 0) {
> +		struct timeval now;
> +
> +		vblwait->reply.sequence = drm_vblank_count_and_time(dev, crtc, &now);
> +		vblwait->reply.tval_sec = now.tv_sec;
> +		vblwait->reply.tval_usec = now.tv_usec;
> +		return 0;
> +	}

drm_vblank_count_and_time() doesn't return the correct sequence number
while the vblank interrupt is disabled, does it? It returns the sequence
number from the last time vblank_disable_and_save() was called (when the
vblank interrupt was disabled). That's why drm_vblank_get() is needed here.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer