On Mon, Aug 16, 2010 at 4:17 AM, Richard Cochran
[off-list ref] 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 <redacted>

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
+
+ =A0 The class driver creates a character device for each registered PTP
+ =A0 clock. User space programs may control the clock using standardized
+ =A0 ioctls. A program may query, enable, configure, and disable the
+ =A0 ancillary clock features. User space can receive time stamped
+ =A0 events via blocking read() and poll(). One shot and periodic
+ =A0 signals may be configured via an ioctl API with semantics similar
+ =A0 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