linux-kernel - Re: LinuxPPS low-level IRQs timestamps & ldisc

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Message-ID: <20080602172203.GI26854@enneenne.com>
Date:	Mon, 2 Jun 2008 19:22:03 +0200
From:	Rodolfo Giometti <giometti@...eenne.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: LinuxPPS low-level IRQs timestamps & ldisc

On Mon, Jun 02, 2008 at 04:48:40PM +0100, Alan Cox wrote:
> > I prefere avoid the if clause for getnstimeofday() since each
> > instruction delay may decrease time precision, so:
> 
> Why should everyone else pay the cost of the getnstimeofday they don't
> need ?
> 
> The single conditional check at the start won't make the slightest
> material difference to your accuracy.

In the past we noticed that just moving up the getnstimeofday() of
only one instruction, from:

	port->icount.dcd++;
	getnstimeofday(&ts);

to:

	getnstimeofday(&ts);
	port->icount.dcd++;

the time precision improved from 5ms to 2ms...

However, if you prefere that I add the if clause, I'll do it.

Thanks,

Rodolfo 

-- 

GNU/Linux Solutions                  e-mail:    giometti@...eenne.com
Linux Device Driver                             giometti@...ux.it
Embedded Systems                     phone:	+39 349 2432127
UNIX programming                     skype:     rodolfo.giometti
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