From: linux@arm.linux.org.uk (Russell King - ARM Linux)
To: linux-arm-kernel@lists.infradead.org
Subject: [Openpxa-users] Linux udelay() is way off
Date: Thu, 20 Jan 2011 22:10:25 +0000 [thread overview]
Message-ID: <20110120221025.GM6335@n2100.arm.linux.org.uk> (raw)
In-Reply-To: <AANLkTikawLY2QcoUdcrF-SmmAbGM1MhzWLZogKb=5VAV@mail.gmail.com>
On Thu, Jan 20, 2011 at 09:25:36PM +0100, Bj?rn Forsman wrote:
> 2011/1/20 Russell King - ARM Linux <linux@arm.linux.org.uk>:
> > On Thu, Jan 20, 2011 at 08:18:10PM +0100, Bj?rn Forsman wrote:
> >> [ ? 11.425363] cpufreq-core: saving 518144 as reference value for
> >> loops_per_jiffy; freq is 312000 kHz
> >> [ ? 11.425523] cpufreq-core: scaling loops_per_jiffy to 1036288 for
> >> frequency 624000 kHz
> >
> > So despite scaling lpj by a factor of two for the doubling in clock
> > frequency, we see a 30% error in it?
>
> No. udelay() is ~30 % of what it should be. My gpio + oscilloscope
> test showed that udelay(50) => 16 us in real time with cpufreq
> enabled. This gives 16/50 = 0.32.
>
> > Is it possible to boot at 624MHz and report the resulting lpj?
>
> I think I need some help doing that. I found some clock setup in
> <u-boot-pxa>/arch/arm/cpu/pxa/start.S (OpenPXA tree) but I'm not
> familiar with it.
>
> > Alternatively, call calibrate_delay() after the cpufreq switch to
> > 624MHz, disabling the 'printed' stuff and see what it reports.
>
> First attempts at moving calibrate_delay() after freq switch failed. I
> will try more. Just had to send this so stop the misunderstanding
> about how much udelay() is off...
Not a good idea to remove it from its current location. I was suggesting
adding another call to calibrate_delay() after the cpufreq stuff has
initialized.
> What do you mean by disabling the 'printed' stuff? The cpufreq debug stuff?
See calibrate_delay() in init/calibrate.c. 'static bool printed;' It's
set true after the first run, and there after it won't print the
information again. So if you want to see it from subsequent runs, you
need to defeat it.
next prev parent reply other threads:[~2011-01-20 22:10 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <AANLkTin_cCzNWhJnD=A2h1qiKpeUrnPrSZPs_Kmfjjd1@mail.gmail.com>
2011-01-20 17:00 ` [Openpxa-users] Linux udelay() is way off Marek Vasut
2011-01-20 17:55 ` Russell King - ARM Linux
2011-01-20 19:18 ` Bjørn Forsman
2011-01-20 19:30 ` Russell King - ARM Linux
2011-01-20 20:25 ` Bjørn Forsman
2011-01-20 22:10 ` Russell King - ARM Linux [this message]
2011-01-21 0:02 ` Bjørn Forsman
2011-01-21 17:46 ` Bjørn Forsman
2011-01-21 18:11 ` Marek Vasut
2011-01-21 18:55 ` Bjørn Forsman
2011-01-23 10:35 ` Marek Vasut
2011-01-25 17:45 ` Bjørn Forsman
2011-01-25 20:18 ` Marek Vasut
2011-01-21 7:29 ` Saravana Kannan
2011-01-21 17:56 ` Bjørn Forsman
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