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

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Date:	Tue, 27 Sep 2011 11:29:07 -0700
From:	Simon Glass <sjg@...omium.org>
To:	Matthieu CASTET <matthieu.castet@...rot.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	"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 Matthieu,

On Mon, Sep 26, 2011 at 12:53 AM, Matthieu CASTET
<matthieu.castet@...rot.com> wrote:
> 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 ?

Yes we could, but we need to be careful that we use a 'time of day'
counter, not dependent on clock speed, CPU load and the like. See my
other email for what I see as the ideal timer. Obviously this will
have to be architecture-specific, but I hope for a simple fallback to
something available on all architectures, with perhaps the ability to
use better timers available on specific architectures which implement
them.

Regards,
Simon

>
>
> Matthieu
>
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