Hello Richard, Thank you for spotting this issue. We haven’t been hit by it (yet!) because we use an old version of linuxptp. I hope to be able to move to a more recent release at some point. The fix you propose seems good to me.
Thanks! -- Delio Brignoli AudioScience Inc On 22 Oct 2015, at 10:45, Richard Cochran <[email protected]> wrote: > > We figure the neighbor rate ratio in port_nrate_calculate(): > > delta t3 / delta t4 = peer_interval / local_interval > > Originally the peer delay was figured like this: > > pd = tmv_sub(tmv_sub(t4, t1), tmv_sub(t3, t2)); > > Since t4-t1 are local, and t3-t2 are from the peer, there will be an > error if the local and peer are not syntonized. > > In commit bd001fde (in v1.5) Delio changed the peer delay calculation: > > adj_t41 = p->nrate.ratio * tmv_dbl(tmv_sub(t4, t1)); > pd = tmv_sub(dbl_tmv(adj_t41), tmv_sub(t3, t2)); > > This converts the local interval t4-t1 into the time base of the peer, > and so the peer delay is then expressed with the peer's frequency. > This follows clause 11.2.15.2.4 from IEEE 802.1-AS. > > Now, with the tsproc in place, the peer delay figured like this: > > /* delay = ((t2 - t3) * rr + (t4 - t1)) / 2 */ > > t23 = tmv_sub(tsp->t2, tsp->t3); > if (tsp->clock_rate_ratio != 1.0) > t23 = dbl_tmv(tmv_dbl(t23) * tsp->clock_rate_ratio); > t41 = tmv_sub(tsp->t4, tsp->t1); > delay = tmv_div(tmv_add(t23, t41), 2); > > The ratio peer/local is applied to t3-t2, but these are peer time > stamps to begin with! That scaling is wrong, and even if the code did > (t2-t3)*(1/rr), it would still change the meaning of the peer delay, > since it would be then expressed in the local time base. > > [ The tsproc code is correct for figuring E2E delay, assuming that > tsp->clock_rate_ratio is GM/local, since t3-t2 is a local time > interval. ] > > We need to fix this and make a new release soon, since the error in > the peer delay will impact some configurations, especially gPTP with > free running local clocks. In order to fix this, I am thinking of > adding a e2e/p2p flag to tsproc_create. When set to p2p, the formula > > delay = ((t2 - t3) + rr * (t4 - t1)) / 2 > > would be used. That would restore the behavior of v1.5. > > Thoughts? > > Thanks, > Richard > > ------------------------------------------------------------------------------ > _______________________________________________ > Linuxptp-devel mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/linuxptp-devel ------------------------------------------------------------------------------ _______________________________________________ Linuxptp-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/linuxptp-devel
