netdev - Re: [PATCH net V4 2/2] igb: offer a PTP Hardware Clock instead of the timecompare method

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Message-ID: <20120322070900.GD2182@netboy.at.omicron.at>
Date:	Thu, 22 Mar 2012 08:09:01 +0100
From:	Richard Cochran <richardcochran@...il.com>
To:	"Keller, Jacob E" <jacob.e.keller@...el.com>
Cc:	chetan loke <loke.chetan@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"e1000-devel@...ts.sourceforge.net" 
	<e1000-devel@...ts.sourceforge.net>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"Ronciak, John" <john.ronciak@...el.com>,
	"john.stultz@...aro.org" <john.stultz@...aro.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [PATCH net V4 2/2] igb: offer a PTP Hardware Clock instead of
 the timecompare method

On Wed, Mar 21, 2012 at 05:06:09PM +0000, Keller, Jacob E wrote:
> 
> I agree with Chetan. I think it would be best to make sure the
> correct form of locking is done, as we are providing an interface to
> the user. Using a seqlock would allow for preventing the ioctls from
> blocking the hardware timestamp code.

Okay, you can improve the time stamping path in the driver to avoid
contending with callers to clock_gettime. But that will not help the
hundreds of clock_gettime callers from contending with each other.

> It's a fairly simple change for the gettime function (the most
> likely culprit to be hammered) by changing it to use
> timecounter_cyc2time function instead of timecounter_read. (as long
> as timecounter_read is called at least every 1/2 the system time
> overflow, which it should be due to the work task.)
>
> With that change, then the section use a seqlock (along with the
> section for checking hardware timestamps). Other places would do the
> full write lock.

So, you think that clock_gettime should not read the card's time
registers?

Richard


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