netdev - Re: [PATCH v6 0/8] ptp: IEEE 1588 hardware clock support

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Date:	Thu, 23 Sep 2010 22:34:17 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Christoph Lameter <cl@...ux.com>
Cc:	john stultz <johnstul@...ibm.com>,
	Richard Cochran <richardcochran@...il.com>,
	linux-kernel@...r.kernel.org, devicetree-discuss@...ts.ozlabs.org,
	linux-api@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>,
	David Miller <davem@...emloft.net>,
	Krzysztof Halasa <khc@...waw.pl>,
	Peter Zijlstra <peterz@...radead.org>,
	Rodolfo Giometti <giometti@...ux.it>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v6 0/8] ptp: IEEE 1588 hardware clock support

> There is no other scalable time source available for fast timer access
> than the time stamp counter in the cpu. Other time source require
> memory accesses which is inherently slower.

On what hardware ?

> An accurate other time source is used to adjust this clock. NTP does that
> via the clock interfaces from user space which has its problems with
> accuracy. PTP can provide the network synced time access
> that would a more accurate calibration of the time.

Accuracy does not require speed of access. Accuracy requires predictible
latency of access.

> Userland code would introduce latencies that would make sub microsecond
> time sync very difficult.

You can take a multiple micro-second I/O stall or SMI trap on a PC so you
already lost the battle on the platform you seem to be discussing.

> You dont need the user APIs if you directly use the PTP time source to
> steer the system clock. In fact I think you have to do it in kernel space
> since user space latencies will degrade accuracy otherwise.

PTP is not a 'time source' it is one or more source of time. The
distinction is rather important.

> It implies clock tuning in userspace for a potential sub microsecond
> accurate clock. The clock accuracy will be limited by user space
> latencies and noise. You wont be able to discipline the system clock
> accurately.

Noise matters, latency doesn't. And the kernel is getting more and more
real time support all the time.

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