On Mon, Aug 23, 2010 at 01:21:39PM -0700, john stultz wrote: > On Thu, 2010-08-19 at 17:38 +0200, Richard Cochran wrote: > > On Thu, Aug 19, 2010 at 02:28:04PM +0200, Arnd Bergmann wrote: > > > My point was that a syscall is better than an ioctl based interface here, > > > which I definitely still think. Given that John knows much more about > > > clocks than I do, we still need to get agreement on the question that > > > he raised, which is whether we actually need to expose this clock to the > > > user or not. > > > > > > If we can find a way to sync system time accurate enough with PTP and > > > PPS, user applications may not need to see two separate clocks at all. > > > > At the very least, one user application (the PTPd) needs to see the > > PTP clock. > > > > > > SYSCALL_DEFINE3(clock_adjtime, const clockid_t, clkid, > > > > int, ppb, struct timespec __user *, ts) > > > > > > > > ppb - desired frequency adjustment in parts per billion > > > > ts - desired time step (or jump) in <sec,nsec> to correct > > > > a measured offset > > > > > > > > Arguably, this syscall might be useful for other clocks, too. > > > > > > This is a mix of adjtime and adjtimex with the addition of > > > the clkid parameter, right? > > > > Sort of, but not really. ADJTIME(3) takes an offset and slowly > > corrects the clock using a servo in the kernel, over hours. > > > > For this function, the offset passed in the 'ts' parameter will be > > immediately corrected, by jumping to the new time. This reflects the > > way that PTP works. After the first few samples, the PTPd has an > > estimate of the offset to the master and the rate difference. The PTPd > > can proceed in one of two ways. > > > > 1. If resetting the clock is not desired, then the clock is set to the > > maximum adjustment (in the right direction) until the clock time is > > close to the master's time. > > > > 2. The estimated offset is added to the current time, resulting in a > > jump in time. > > > > We need clock_adjtime(id, 0, ts) for the second case. > > > > > Have you considered passing a struct timex instead of ppb and ts? > > > > Yes, but the timex is not suitable, IMHO. > > Could you expand on this?
We need to able to specify that the call is for a PTP clock. We could add that to the modes flag, like this: /*timex.h*/ #define ADJ_PTP_0 0x10000 #define ADJ_PTP_1 0x20000 #define ADJ_PTP_2 0x30000 #define ADJ_PTP_3 0x40000 I can live with this, if everyone else can, too. > Could we not add a adjustment mode ADJ_SETOFFSET or something that would > provide the instantaneous offset correction? Yes, but we would also need to add a struct timespec to the struct timex, in order to get nanosecond resolution. I think it would be possible to do in the padding at the end? > You're right that the timex is a little crufty. But its legacy that we > will support indefinitely. So following the established interface helps > maintainability. We can use it for PTP, with the modifications suggested above. Or we can just introduce the clock_adjtime method, instead. > So if the clock_adjtime interface is needed, it would seem best for it > to be generic enough to support not only PTP, but also the NTP kernel > PLL. For the proposed clock_adjime, what else is needed to support clock adjustment in general? I don't mind making the interface generic enough to support any (realistic) conceivable clock adjustment scheme, but beyond the present PTP hardware clocks, I don't know what else might be needed. Richard _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
