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 --- drivers/gpu/drm/drm_irq.c | 20 ++++++++++++-------- include/drm/drmP.h | 10 ++++++++-- 2 files changed, 20 insertions(+), 10 deletions(-)
diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c index 33ee515..d80d952 100644 --- a/drivers/gpu/drm/drm_irq.c +++ b/drivers/gpu/drm/drm_irq.c @@ -219,7 +219,7 @@ int drm_vblank_init(struct drm_device *dev, int num_crtcs) for (i = 0; i < num_crtcs; i++) init_waitqueue_head(&dev->vblank[i].queue);
- DRM_INFO("Supports vblank timestamp caching Rev 1 (10.10.2010).\n"); + DRM_INFO("Supports vblank timestamp caching Rev 2 (21.10.2013).\n");
/* Driver specific high-precision vblank timestamping supported? */ if (dev->driver->get_vblank_timestamp) @@ -586,14 +586,17 @@ int drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev, int crtc, * code gets preempted or delayed for some reason. */ for (i = 0; i < DRM_TIMESTAMP_MAXRETRIES; i++) { - /* Get system timestamp before query. */ - stime = ktime_get(); - - /* Get vertical and horizontal scanout pos. vpos, hpos. */ - vbl_status = dev->driver->get_scanout_position(dev, crtc, &vpos, &hpos); + /* + * Get vertical and horizontal scanout position vpos, hpos, + * and bounding timestamps stime, etime, pre/post query. + */ + vbl_status = dev->driver->get_scanout_position(dev, crtc, &vpos, + &hpos, &stime, &etime);
- /* Get system timestamp after query. */ - etime = ktime_get(); + /* + * Get correction for CLOCK_MONOTONIC -> CLOCK_REALTIME if + * CLOCK_REALTIME is requested. + */ if (!drm_timestamp_monotonic) mono_time_offset = ktime_get_monotonic_offset();
@@ -604,6 +607,7 @@ int drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev, int crtc, return -EIO; }
+ /* Compute uncertainty in timestamp of scanout position query. */ duration_ns = ktime_to_ns(etime) - ktime_to_ns(stime);
/* Accept result with < max_error nsecs timing uncertainty. */ diff --git a/include/drm/drmP.h b/include/drm/drmP.h index 2b954ad..48d15f0 100644 --- a/include/drm/drmP.h +++ b/include/drm/drmP.h @@ -835,12 +835,17 @@ 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 @@ -857,7 +862,8 @@ 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