Re: [PATCH 1/2] drm: use monotonic time in drm_calc_vbltimestamp_from_scanoutpos

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On 23.10.12 20:53, Imre Deak wrote:
For measuring duration we want to avoid that our start/end timestamps
jump, so use monotonic instead of real time for that.

Signed-off-by: Imre Deak <imre.deak@xxxxxxxxx>
---
  drivers/gpu/drm/drm_irq.c |   18 ++++++++++++------
  1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 89b830d..7dc203d 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -576,7 +576,8 @@ int drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev, int crtc,
  					  unsigned flags,
  					  struct drm_crtc *refcrtc)
  {
-	struct timeval stime, raw_time;
+	ktime_t stime, etime, mono_time_offset;
+	struct timeval tv_etime;
  	struct drm_display_mode *mode;
  	int vbl_status, vtotal, vdisplay;
  	int vpos, hpos, i;
@@ -625,13 +626,14 @@ int drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev, int crtc,
  		preempt_disable();

  		/* Get system timestamp before query. */
-		do_gettimeofday(&stime);
+		stime = ktime_get();

  		/* Get vertical and horizontal scanout pos. vpos, hpos. */
  		vbl_status = dev->driver->get_scanout_position(dev, crtc, &vpos, &hpos);

  		/* Get system timestamp after query. */
-		do_gettimeofday(&raw_time);
+		etime = ktime_get();

Here is possibly a tiny race: The wall_to_monotonic offset value could change between the ktime_get() - which uses it internally for wallclock -> monotonic clock conversion, and the ktime_get_monotonic_offset() query below, so the later subtraction of mono_time_offset from etime would not cancel out the addition to etime inside ktime_get() and you wouldn't get correct walltime back. There seem to be multiple sources of change to the value, e.g., do_settimeofday(), do_adjtimex() - the admin or ntp changing the system clock. The internal code, e.g., ktime_get() use a seqlock to protect against this race.

There's a function ktime_get_real(void) which directly gives you the wall time you want as ktime_t, but then you'd still need to do the ktime_get() query in the !drm_timestamp_monotonic case to calculate duration_ns below.

Same problem in the 2nd patch for get_drm_timestamp(). Otoh, the time window for the race is small and it can only happen in the non-default case of !drm_timestamp_monotonic, so i don't know if it is worth fixing it?

Other than that:

Reviewed-by: mario.kleiner

+		mono_time_offset = ktime_get_monotonic_offset();

  		preempt_enable();

@@ -642,7 +644,7 @@ int drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev, int crtc,
  			return -EIO;
  		}

-		duration_ns = timeval_to_ns(&raw_time) - timeval_to_ns(&stime);
+		duration_ns = ktime_to_ns(etime) - ktime_to_ns(stime);

  		/* Accept result with <  max_error nsecs timing uncertainty. */
  		if (duration_ns <= (s64) *max_error)
@@ -689,14 +691,18 @@ int drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev, int crtc,
  		vbl_status |= 0x8;
  	}

+	etime = ktime_sub(etime, mono_time_offset);
+	/* save this only for debugging purposes */
+	tv_etime = ktime_to_timeval(etime);
  	/* Subtract time delta from raw timestamp to get final
  	 * vblank_time timestamp for end of vblank.
  	 */
-	*vblank_time = ns_to_timeval(timeval_to_ns(&raw_time) - delta_ns);
+	etime = ktime_sub_ns(etime, delta_ns);
+	*vblank_time = ktime_to_timeval(etime);

  	DRM_DEBUG("crtc %d : v %d p(%d,%d)@ %ld.%ld -> %ld.%ld [e %d us, %d rep]\n",
  		  crtc, (int)vbl_status, hpos, vpos,
-		  (long)raw_time.tv_sec, (long)raw_time.tv_usec,
+		  (long)tv_etime.tv_sec, (long)tv_etime.tv_usec,
  		  (long)vblank_time->tv_sec, (long)vblank_time->tv_usec,
  		  (int)duration_ns/1000, i);


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