linux-kernel - Re: [PATCH 04/11] ARM: set arch

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Message-ID: <509DCC5E.7020804@gmail.com>
Date:	Sat, 10 Nov 2012 14:39:10 +1100
From:	Ryan Mallon <rmallon@...il.com>
To:	Stephen Warren <swarren@...dotorg.org>
CC:	Russell King <linux@....linux.org.uk>,
	Olof Johansson <olof@...om.net>, Arnd Bergmann <arnd@...db.de>,
	John Stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, Stephen Warren <swarren@...dia.com>,
	Andrew Victor <linux@...im.org.za>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Hartley Sweeten <hsweeten@...ionengravers.com>,
	Ben Dooks <ben-linux@...ff.org>,
	Kukjin Kim <kgene.kim@...sung.com>
Subject: Re: [PATCH 04/11] ARM: set arch_gettimeoffset directly

On 10/11/12 08:07, Stephen Warren wrote:

> On 11/08/2012 04:06 PM, Ryan Mallon wrote:
>> On 09/11/12 08:01, Stephen Warren wrote:
>>> remove ARM's struct sys_timer .offset function pointer, and instead
>>> directly set the arch_gettimeoffset function pointer when the timer
>>> driver is initialized. This requires multiplying all function results
>>> by 1000, since the removed arm_gettimeoffset() did this. Also,
>>> s/unsigned long/u32/ just to make the function prototypes exactly
>>> match that of arch_gettimeoffset.
> 
>>> +static u32 ep93xx_gettimeoffset(void)
>>> +{
>>> +	int offset;
>>> +
>>> +	offset = __raw_readl(EP93XX_TIMER4_VALUE_LOW) - last_jiffy_time;
>>> +
>>> +	/* Calculate (1000000 / 983040) * offset.  */
>>
>> This comment is now incorrect, it should say:
>>
>> 	/* Calculate (1000000000 / 983040) * offset */
>>
>> or perhaps to better explain what is being done:
>>
>> 	/*
>> 	 * Timer 4 is based on a 983.04 kHz reference clock,
>> 	 * so dividing by 983040 gives a milli-second value.
>> 	 * Refactor the calculation to avoid overflow.
>> 	 */
>>
>>> +	return (offset + (53 * offset / 3072)) * 1000;
> 
> Thanks. I expanded on that slightly and went for:
> 
> 	/*
> 	 * Timer 4 is based on a 983.04 kHz reference clock,
> 	 * so dividing by 983040 gives the fraction of a second,
> 	 * so dividing by 0.983040 converts to uS.
> 	 * Refactor the calculation to avoid overflow.
> 	 * Finally, multiply by 1000 to give nS.
> 	 */
> 


Looks good, thanks.

~Ryan

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