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SubjectRe: [PATCH 1/5] ptp: Added a brand new class driver for ptp clocks.
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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
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