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

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Message-ID: <20100817083216.GA3330@riccoc20.at.omicron.at>
Date:	Tue, 17 Aug 2010 10:32:16 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linuxppc-dev@...ts.ozlabs.org,
	Rodolfo Giometti <giometti@...ux.it>, netdev@...r.kernel.org,
	devicetree-discuss@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Krzysztof Halasa <khc@...waw.pl>
Subject: Re: [PATCH 1/5] ptp: Added a brand new class driver for ptp clocks.

On Mon, Aug 16, 2010 at 09:59:39PM +0200, Arnd Bergmann wrote:
> Why does it matter how long it takes to read the clock? I wasn't thinking
> of replacing the system clock with this, just exposing the additional
> clock as a new clockid_t value that can be accessed using the existing
> syscalls.

Okay, now I see. You are suggesting this:

      clock_gettime(CLOCK_PTP, &ts);
      clock_settime(CLOCK_PTP, &ts);

I like this. If there is agreement about it, I am happy to implement
the PTP stuff that way.

> Why did you not want to add syscalls? Adding ioctls instead of syscalls
> does not make the interface better, just less visible.

I bet that, had I posted patch set with new syscalls, someone would
have said, "You are adding new syscalls. Can't you just use a char
device instead!"

If you add syscalls and introduce CLOCK_PTP, then you add it to
everyone's kernel, even those people who never heard of PTP. A char
device has the advantage that can it be simply ignored. Also, a
syscall has got to have the right form from the very beginning. If the
next generation of PTP hardware looks very different, then it is not
that much of a crime to change an ioctl interface, provided it has
versioning.

Richard

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