On Thu, 23 Sep 2010, Richard Cochran wrote: > Support for obtaining timestamps from a PHC already exists via the > SO_TIMESTAMPING socket option, integrated in kernel version 2.6.30. > This patch set completes the picture by allow user space programs to > adjust the PHC and to control its ancillary features. Is there a way to use the PHC as a system clock? I think the main benefit of PTP is to have syncronized time on multiple machines in a cluster. That may mean getting rid of ntp and using an in kernel PHC based way to sync time. > So as far as the POSIX standard is concerned, offering a clock id > to represent the PHC would be acceptable. Sure but what would you do with it? HPET timer support has no such need. > 3.2.1 Using the POSIX Clock API > -------------------------------- > > Looking at the mapping from PHC operation to the POSIX clock API, > we see that two of the basic clock operations, marked with *, have > no POSIX equivalent. The items marked NA are peculiar to PHCs and > will be discussed separately, below. > > Clock Operation POSIX function > -----------------------------+----------------------------- > Set time clock_gettime > Get time clock_settime > Shift the clock * > Adjust clock frequency * > -----------------------------+----------------------------- > Time stamp external events NA > Enable PPS events NA > Periodic output signals NA > One shot or periodic alarms timer_create, timer_settime > > In contrast to the standard Linux system clock, a PHC is > adjustable in hardware, for example using frequency compensation > registers or a VCO. The ability to directly tune the PHC is > essential to reap the benefit of hardware timestamping. There is a reason for not being able to shift posix clocks: The system has one time base. The various clocks are contributing to maintaining that sytem wide time. I do not understand why you want to maintain different clocks running at different speeds. Certainly interesting for some uses I guess that I do not have the energy to imagine right now. But can we get the PTP killer feature of synchronized accurate system time first? > 3.3 Synchronizing the Linux System Time > ======================================== > > One could offer a PHC as a combined clock source and clock event > device. The advantage of this approach would be that it obviates > the need for synchronization when the PHC is selected as the system > timer. However, some PHCs, namely the PHY based clocks, cannot be > used in this way. Why not? Do PHY based clock not at least provide a counter that increments in synchronized intervals throughout the network? > Instead, the patch set provides a way to offer a Pulse Per Second > (PPS) event from the PHC to the Linux PPS subsystem. A user space > application can read the PPS events and tune the system clock, just > like when using other external time sources like radio clocks or > GPS. User space is subject to various latencies created by the OS etc. I would that in order to have fine grained (read microsecond) accurary we would have to run the portions that are relevant to obtaining the desired accuracy in the kernel. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
