linux-kernel - Re: [PATCH 1/5] ptp: Added a brand new class driver for ptp clocks.

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Date:	Mon, 16 Aug 2010 12:24:48 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Richard Cochran <richardcochran@...il.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, devicetree-discuss@...ts.ozlabs.org,
	linux-arm-kernel@...ts.infradead.org,
	Krzysztof Halasa <khc@...waw.pl>,
	Rodolfo Giometti <giometti@...ux.it>
Subject: Re: [PATCH 1/5] ptp: Added a brand new class driver for ptp clocks.

On Mon, Aug 16, 2010 at 4:17 AM, Richard Cochran
<richardcochran@...il.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@...cron.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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