From mboxrd@z Thu Jan  1 00:00:00 1970
From: u.kleine-koenig@pengutronix.de (Uwe =?iso-8859-1?Q?Kleine-K=F6nig?=)
Date: Mon, 8 Nov 2010 11:01:19 +0100
Subject: One quick quenstion about PXA sched_clock resolution?
In-Reply-To: <3f0ffa72.f9b4.12c2ab386e4.Coremail.bill_carson@126.com>
References: <3f0ffa72.f9b4.12c2ab386e4.Coremail.bill_carson@126.com>
Message-ID: <20101108100119.GR16938@pengutronix.de>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

[added Nico to Cc:]

On Mon, Nov 08, 2010 at 04:56:37PM +0800, rocky wrote:
> Hi
> 
> I'am working on an ARM linux porting project, which need to implement sched_clock of our own.
> have a glance at PXA sched_clock timplementation, puzzled about the following notes.
> 
> 
> /*
>  * This is PXA's sched_clock implementation. This has a resolution
>  * of at least 308 ns and a maximum value of 208 days.
>  *
>  * The return value is guaranteed to be monotonic in that range as
>  * long as there is always less than 582 seconds between successive
>  * calls to sched_clock() which should always be the case in practice.
>  */
> 
> 
> 
> 
> PXA series chips have system clock in 3250000/3249600/3686400 HZ.
> I do the math like this:
> 
> ####Where does 308 ns come from?
> 
> 
> ns of one clcok cycle:
> 307.69     (10^9/3250000);
> 307.73     (10^9/3249600);
> 271.26     (10^9/3686400 )
> this is the highest resolution mentioned in above notes;
Seems correct.  I think "at least" above means 308 ns or better (i.e.
shorter).
> 
> 
> ####Where does 582 seconds come from?
> 
> 660.76(0x80000000/3250000);
> 660.84(0x80000000/3249600); 
> 582.54(0x80000000/3686400 );
Seems correct, too.

> Question is how does 208 days pop out?
> Can anyone give me some hints?
> thanks
I didn't check it, but I guess after 208 days sched_clock overflows.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-K?nig            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |