From 8f6fce03ddaf10bfa40c7d6cd59b778d35d41800 Mon Sep 17 00:00:00 2001 From: Mario Kleiner Date: Wed, 30 Oct 2013 05:13:06 +0100 Subject: drm: Push latency sensitive bits of vblank scanoutpos timestamping into kms drivers. MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit A change in locking of some kms drivers (currently intel-kms) make the old approach too inaccurate and also incompatible with the PREEMPT_RT realtime kernel patchset. The driver->get_scanout_position() method of intel-kms now needs to aquire a spinlock, which clashes badly with the former preempt_disable() calls in the drm, and it also introduces larger delays and timing uncertainty on a contended lock than acceptable. This patch changes the prototype of driver->get_scanout_position() to require/allow kms drivers to perform the ktime_get() system time queries which go along with actual scanout position readout in a way that provides maximum precision and to return those timestamps to the drm. kms drivers implementations of get_scanout_position() are asked to implement timestamping and scanoutpos readout in a way that is as precise as possible and compatible with preempt_disable() on a PREMPT_RT kernel. A driver should follow this pattern in get_scanout_position() for precision and compatibility: spin_lock...(...); preempt_disable_rt(); // On a PREEMPT_RT kernel, otherwise omit. if (stime) *stime = ktime_get(); ... Minimum amount of MMIO register reads to get scanout position ... ... no taking of locks allowed here! ... if (etime) *etime = ktime_get(); preempt_enable_rt(); // On PREEMPT_RT kernel, otherwise omit. spin_unlock...(...); v2: Fix formatting of new multi-line code comments. Signed-off-by: Mario Kleiner Reviewed-by: Ville Syrjälä Reviewed-by: Alex Deucher Signed-off-by: Dave Airlie --- include/drm/drmP.h | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) (limited to 'include/drm') diff --git a/include/drm/drmP.h b/include/drm/drmP.h index e6d0cd9f518e..748c72907829 100644 --- a/include/drm/drmP.h +++ b/include/drm/drmP.h @@ -836,12 +836,17 @@ struct drm_driver { /** * Called by vblank timestamping code. * - * Return the current display scanout position from a crtc. + * Return the current display scanout position from a crtc, and an + * optional accurate ktime_get timestamp of when position was measured. * * \param dev DRM device. * \param crtc Id of the crtc to query. * \param *vpos Target location for current vertical scanout position. * \param *hpos Target location for current horizontal scanout position. + * \param *stime Target location for timestamp taken immediately before + * scanout position query. Can be NULL to skip timestamp. + * \param *etime Target location for timestamp taken immediately after + * scanout position query. Can be NULL to skip timestamp. * * Returns vpos as a positive number while in active scanout area. * Returns vpos as a negative number inside vblank, counting the number @@ -858,7 +863,8 @@ struct drm_driver { * */ int (*get_scanout_position) (struct drm_device *dev, int crtc, - int *vpos, int *hpos); + int *vpos, int *hpos, ktime_t *stime, + ktime_t *etime); /** * Called by \c drm_get_last_vbltimestamp. Should return a precise -- cgit v1.2.1