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authorMario Kleiner <mario.kleiner.de@gmail.com>2013-10-30 05:13:06 +0100
committerDave Airlie <airlied@redhat.com>2013-11-06 02:53:41 +0100
commit8f6fce03ddaf10bfa40c7d6cd59b778d35d41800 (patch)
treecada34e1587b003af267a75877e30caf60292779 /include/drm
parentdrm: Remove preempt_disable() from vblank timestamping code. (diff)
downloadlinux-8f6fce03ddaf10bfa40c7d6cd59b778d35d41800.tar.xz
linux-8f6fce03ddaf10bfa40c7d6cd59b778d35d41800.zip
drm: Push latency sensitive bits of vblank scanoutpos timestamping into kms drivers.
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 <mario.kleiner.de@gmail.com> Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> Reviewed-by: Alex Deucher <alexander.deucher@amd.com> Signed-off-by: Dave Airlie <airlied@redhat.com>
Diffstat (limited to 'include/drm')
-rw-r--r--include/drm/drmP.h10
1 files changed, 8 insertions, 2 deletions
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