linux-kernel - Re: One of these things (CONFIG

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Message-ID: <CAKGA1bkXx2qn+c6sKqXsty-aFe1r-W79uyem0O8p6JNeeq6zxQ@mail.gmail.com>
Date:	Mon, 21 Jan 2013 19:06:59 -0600
From:	Matt Sealey <matt@...esi-usa.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Linux ARM Kernel ML <linux-arm-kernel@...ts.infradead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>
Subject: Re: One of these things (CONFIG_HZ) is not like the others..

On Mon, Jan 21, 2013 at 6:51 PM, John Stultz <john.stultz@...aro.org> wrote:
> On 01/21/2013 02:54 PM, Matt Sealey wrote:
>>
>> On Mon, Jan 21, 2013 at 4:36 PM, John Stultz <john.stultz@...aro.org>
>> wrote:
>>>
>>> On 01/21/2013 01:14 PM, Matt Sealey wrote:
>
> As far as jiffies rating, from jiffies.c:
>     .rating        = 1, /* lowest valid rating*/
>
> So I'm not sure what you mean by "the debug on the kernel log is telling me
> it has a higher resolution".

Oh, it is just if I actually don't run setup_sched_clock on my
platform, it gives a little message (with #define DEBUG 1 in
sched_clock.c) about who setup the last sched_clock. Since you only
get one chance, and I was fiddling with setup_sched_clock being probed
from multiple possible timers from device tree (i.MX3 has a crapload
of valid timers, which one you use right now is basically forced by
the not-quite-fully-DT-only code and some funky iomap tricks).

And what I got was, if I use the real hardware timer, it runs at 66MHz
and says it has 15ns resolution and wraps every 500 seconds or so. The
jiffies timer says it's 750MHz, with a 2ns resoluton.. you get the
drift. The generic reporting of how "good" the sched_clock source is
kind of glosses over the quality rating of the clock source and at
first glance (if you're not paying that much attention), it is a
little bit misleading..

> Yes, in the case I was remembering, the 60HZ was driven by the electrical
> line.

While I have your attention, what would be the minimum "good" speed to
run the sched_clock or delay timer implementation from? My rudimentary
scribblings in my notebook give me a value of "don't bother" with less
than 10KHz based on HZ=100, so I'm wondering if a direct 32.768KHz
clock would do (i.MX osc clock input if I can supply it to one of the
above myriad timers) since this would be low-power compared to a 66MHz
one (by a couple mA anyway). I also have a bunch of questions about
the delay timer requirements.. I might mail you personally.. or would
you prefer on-list?

-- 
Matt Sealey <matt@...esi-usa.com>
Product Development Analyst, Genesi USA, Inc.
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