> There is no other scalable time source available for fast timer access > than the time stamp counter in the cpu. Other time source require > memory accesses which is inherently slower.
On what hardware ? > An accurate other time source is used to adjust this clock. NTP does that > via the clock interfaces from user space which has its problems with > accuracy. PTP can provide the network synced time access > that would a more accurate calibration of the time. Accuracy does not require speed of access. Accuracy requires predictible latency of access. > Userland code would introduce latencies that would make sub microsecond > time sync very difficult. You can take a multiple micro-second I/O stall or SMI trap on a PC so you already lost the battle on the platform you seem to be discussing. > You dont need the user APIs if you directly use the PTP time source to > steer the system clock. In fact I think you have to do it in kernel space > since user space latencies will degrade accuracy otherwise. PTP is not a 'time source' it is one or more source of time. The distinction is rather important. > It implies clock tuning in userspace for a potential sub microsecond > accurate clock. The clock accuracy will be limited by user space > latencies and noise. You wont be able to discipline the system clock > accurately. Noise matters, latency doesn't. And the kernel is getting more and more real time support all the time. _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
