linux-kernel - Re: [RFC PATCH 0/3] Add accurate boot timing to a Linux system

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Date:	Mon, 26 Sep 2011 09:53:33 +0200
From:	Matthieu CASTET <matthieu.castet@...rot.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Simon Glass <sjg@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 0/3] Add accurate boot timing to a Linux system

Hi,

Russell King - ARM Linux a écrit :
> On Fri, Sep 23, 2011 at 04:03:15PM -0700, Simon Glass wrote:
>> An accurate timer is required to make the numbers meaningful. Many
>> modern platforms have a microsecond timer. This patch set uses a
>> function called timer_get_us() to read the timer.
> 
> Not another 'get a time value' function.  Why do we need soo many?
> We have - at least:
> 
> ktime_get (and various flavours of it)
> do_gettimeofday
> getnstimeofday
> sched_clock
> 
> Do we really need yet another one which will have to be multiplexed
> amongst platforms, requiring scaling and so forth from whatever the
> platform provides?
> 
> Remember that ARM timers are virtually all MMIO mapped, which means
> they don't work during early kernel bringup when the MMU mappings for
> the hardware have not been setup.  (That's the reason stuff like
> sched_clock for printk doesn't work early.)
Doesn't cortexA-8 (and A9 ?) have a cycle counter that can be read by
coprocessor 15 ?

Couldn't we use that counter for early stuff on those architectures ?


Matthieu
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