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>

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