From mboxrd@z Thu Jan  1 00:00:00 1970
From: bjorn.forsman@gmail.com (=?UTF-8?Q?Bj=C3=B8rn_Forsman?=)
Date: Fri, 21 Jan 2011 01:02:34 +0100
Subject: [Openpxa-users] Linux udelay() is way off
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To: linux-arm-kernel@lists.infradead.org
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2011/1/20 Russell King - ARM Linux <linux@arm.linux.org.uk>:
> 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.

Ok, thanks!

>> 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.

Thanks for the pointers!

Best regards,
Bj?rn Forsman