Messages in this thread | | | Date | Mon, 16 Aug 2010 12:24:48 -0700 | Subject | Re: [PATCH 1/5] ptp: Added a brand new class driver for ptp clocks. | From | john stultz <> |
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On Mon, Aug 16, 2010 at 4:17 AM, Richard Cochran <richardcochran@gmail.com> wrote: > This patch adds an infrastructure for hardware clocks that implement > IEEE 1588, the Precision Time Protocol (PTP). A class driver offers a > registration method to particular hardware clock drivers. Each clock is > exposed to user space as a character device with ioctls that allow tuning > of the PTP clock. > > Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Hey Richard! Its very cool to see this work on lkml! I'm excited to see more work done on ptp. We had a short private thread discussion earlier (I got busy and never replied to your last message, my apologies!), but I wanted to bring up the concerns I have here as well.
A few comments below....
> +** PTP user space API > + > + The class driver creates a character device for each registered PTP > + clock. User space programs may control the clock using standardized > + ioctls. A program may query, enable, configure, and disable the > + ancillary clock features. User space can receive time stamped > + events via blocking read() and poll(). One shot and periodic > + signals may be configured via an ioctl API with semantics similar > + to the POSIX timer_settime() system call.
As I mentioned earlier, I'm not a huge fan of the char device interface for abstracted PTP clocks. If it was just the direct hardware access, similar to RTC, which user apps then use as a timesource, I'd not have much of a problem. But as I mentioned in an earlier private mail, the abstraction level concerns me.
1) The driver-like model exposes a char dev for each clock, which allows for poorly-written userland applications to hit portability issues (ie: /dev/hpet vs /dev/rtc). Granted this isn't a huge flaw, but good APIs should be hard to get wrong.
2) As Arnd already mentioned, the chardev interface seems to duplicate the clock_gettime/settime() and adjtimex() interfaces.
3) I'm not sure I see the benefit of being able to have multiple frequency corrected time domains. In other words, what benefit would you get from adjusting a PTP clock's frequency instead of just adjusting the system's time freq? Having the PTP time as a reference to correct the system time seems reasonable, but I'm not sure I see why userland would want to adjust the PTP clock's freq.
thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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