netdev - Re: [PATCH net-next 2/2] igb: offer a PTP Hardware Clock instead of the timecompare method

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Date:	Tue, 13 Dec 2011 04:41:33 +0100
From:	Richard Cochran <richardcochran@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, e1000-devel@...ts.sourceforge.net,
	Jacob Keller <jacob.e.keller@...el.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	John Ronciak <john.ronciak@...el.com>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH net-next 2/2] igb: offer a PTP Hardware Clock instead
 of the timecompare method

On Tue, Dec 13, 2011 at 04:28:52AM +0100, Eric Dumazet wrote:
> 
> Adding a (shared) spinlock on a multiqueue device is source of extra
> delay (because of extra cache line trafic), I guess.
> 
> It seems current code doesnt need a spinlock, maybe it was a bug ?

(I didn't think about the old code. I only deleted it. ;)

The spinlock is needed because reading the 64 bit time value involves
reading two 32 registers. The first read latches the value. Ditto for
writing.

In addition, here we have to watch the most significant bit for
one-to-zero transistion, in order to keep count of the overflow.

It is too bad that we have to take the spinlock for every time stamped
packet, but it is the hardware's fault for not providing a 64 bit wide
nanosecond time register.

Richard
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