On Tue, Aug 20, 2019 at 10:48:31AM +0200, Hubert Feurstein wrote: > + /* PTP offset compensation: > + * After the MDIO access is completed (from the chip perspective), the > + * switch chip will snapshot the PHC timestamp. To make sure our system > + * timestamp corresponds to the PHC timestamp, we have to add the > + * duration of this MDIO access to sts->post_ts. Linuxptp's phc2sys > + * takes the average of pre_ts and post_ts to calculate the final > + * system timestamp. With this in mind, we have to add ptp_sts_offset > + * twice to post_ts, in order to not introduce an constant time offset. > + */ > + if (sts) > + timespec64_add_ns(&sts->post_ts, 2 * bus->ptp_sts_offset);
This correction looks good to me. Is the MDIO write delay constant in reality, or does it at least have an upper bound? That is, is it always true that the post_ts timestamp does not point to a time before the PHC timestamp was actually taken? This is important to not break the estimation of maximum error in the measured offset. Applications using the ioctl may assume that the maximum error is (post_ts-pre_ts)/2 (i.e. half of the delay printed by phc2sys). That would not work if the delay could be occasionally 50 microseconds for instance, i.e. the post_ts timestamp would be earlier than the PHC timestamp. -- Miroslav Lichvar