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...(...);
Signed-off-by: Mario Kleiner <mario.kleiner.de@xxxxxxxxx>
---
drivers/gpu/drm/drm_irq.c | 18 ++++++++++--------
include/drm/drmP.h | 10 ++++++++--
2 files changed, 18 insertions(+), 10 deletions(-)
diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 33ee515..2250724 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 +586,15 @@ 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 +605,7 @@ 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
--
1.7.10.4
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