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

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Message-ID: <20100816190003.GB4166@riccoc20.at.omicron.at>
Date:	Mon, 16 Aug 2010 21:00:03 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
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 04:26:23PM +0200, Arnd Bergmann wrote:
> Have you considered integrating the subsystem into the Posix clock/timer
> framework?

Yes, but see below.

> I can't really tell from reading the source if this is possible or
> not, but my feeling is that if it can be done, that would be a much
> nicer interface. We already have clock_gettime/clock_settime/
> timer_settime/... system calls, and while you'd need to add another
> clockid and some syscalls, my feeling is that it will be more
> usable in the end.

You are not the first person to ask about this. See this link for
longer explanation of why I did not go that way:

  http://marc.info/?l=linux-netdev&m=127669810232201&w=2

You *could* offer the PTP clock as a Linux clock source/event device,
and I agree that it would be nicer. However, the problem is, what do
you do with the PHY based clocks?  Just one 16 bit read from a PHY
clock can take 40 usec, and you need four such read operations just to
get the current time value.

Also, I really did not want to add or change any syscalls. I could not
see a practical way to extend the existing syscalls to accommodate PTP
clocks.

Richard
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