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 at intel.com>
> ---
> 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);
>
>
