linux-kernel - Re: [PATCH RFC 2/8] clock device: convert clock

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Message-ID: <20101115094156.GB15936@riccoc20.at.omicron.at>
Date:	Mon, 15 Nov 2010 10:41:56 +0100
From:	Richard Cochran <richardcochran@...il.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	Christoph Lameter <cl@...ux.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Roland McGrath <roland@...hat.com>
Subject: Re: [PATCH RFC 2/8] clock device: convert clock_gettime

On Tue, Nov 09, 2010 at 01:10:58PM -0800, john stultz wrote:
> On Tue, 2010-11-09 at 09:23 +0100, Richard Cochran wrote:
> > On Mon, Nov 08, 2010 at 03:37:03PM -0800, john stultz wrote:
> > > So sort of minor nit here, but is there a reason the clockfd
> > > implementation is primary here and the standard posix implementation
> > > gets pushed off into its own function rather then doing something like:
> > > 
> > > clk = clockid_to_clock_device(id)
> > > if(clk)
> > > 	return clockdev_clock_gettime(clk, user_ts);
> > > [existing sys_clock_gettime()] 
> > > 
> > > As you implemented it, it seems to expect the clockdevice method to be
> > > the most frequent use case, where as its likely to be the inverse. So
> > > folks looking into the more common CLOCK_REALTIME calls have to traverse
> > > more code then the likely more rare clockfd cases.
> > 
> > Actually, what I would like to do is refactor the exisiting posix
> > clock code to use the new framework. The idea is to have a set of
> > static global clock_device*, one per fixed clock. The function
> > clockid_to_clock_device() will include a lookup table, like this:
> > 
> > static clock_device *realtime_clock, *monotinic_clock;
> > 
> > switch (id) {
> > case CLOCK_REALTIME:
> > 	return realtime_clock;
> > case CLOCK_MONOTONIC:
> > 	return monotinic_clock;
> > /* and so on ... */
> > }
> 
> This seems a little over-reaching. I'm not sure I see what benefit would
> come from having clock_devices for the static clock_ids? The extra mutex
> locking and status/null checking for the clock_device would would just
> add unnecessary overhead to the performance critical clock_gettime call.

Okay, next time I'll leave the syscall in posix-timers.c and call out
to the clock_device in the unlikely dynamic case.

Thanks,
Richard
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