[Openpxa-users] Linux udelay() is way off

[Saravana Kannan]

On 01/20/2011 11:18 AM, Bjørn Forsman wrote:
> On 20 January 2011 18:55, Russell King - ARM Linux
> <linux at arm.linux.org.uk>  wrote:
>> On Thu, Jan 20, 2011 at 06:00:04PM +0100, Marek Vasut wrote:
>>> On Thursday 20 January 2011 17:09:00 Bjørn Forsman wrote:
>>>> Hi,
>>>>
>>>> On my Colibri PXA310, I see that Linux udelay() is only 30 % of the
>>>> length it's supposed to. This is way off and cause problems (e.g.
>>>> 1-Wire device driver breaks). I'm running OpenPXA bootloaders (OBM2 +
>>>> U-Boot) and mainline Linux 2.6.36.
>>>>
>>>> Any ideas how to fix this?
>>>>
>>>> Could it be that U-Boot sets up the system clock to something
>>>> different than what Linux "expects" so that loops_per_jiffy becomes
>>>> wrong? I see some clock setup in<u-boot-pxa>/arch/arm/cpu/pxa/start.S
>>>> but I'm not sure what to do with it (yet). I'd like to try out
>>>> different clock speeds to see if it helps. Any pointers?
>>>>
>>>> I also have a workaround in mind: Try out different lpj= settings on
>>>> the kernel command line until a udelay(1000) actually is 1000 us. Is
>>>> this a sane thing to do?
>>>
>>> CCing relevant people.
>>>
>>> And no, linux kernel should derive it's timing properly so it's a kernel bug.
>>
>> I recently looked into the udelay() timing accuracy, and I saw it only
>> being reduced by .7% at boot time - due to the cycles stolen by the timer
>> IRQ handler.
>>
>> The delay calibration algorithm shouldn't produce big errors due to the
>> way it works.  It first tries to find a delay longer than the timer tick.
>> It then uses a successive approximation technique over about 9 bits of
>> loops_per_jiffy value.
>>
>> So, I wouldn't expect the boot time lpj value to be significantly out.
>>
>> However, what might be happening is you boot at X MHz.  You then use cpufreq
>> and drop the frequency to Y MHz.  You then suspend and resume.  On resume,
>> the boot loader starts the processor again at X MHz, but the kernel still
>> assumes that you're running at Y MHz and has not adjusted the delay loop
>> accordingly.
>>
>> This will result in a much shorter than expected delay.
>>
>> So, the first thing that needs doing is to ascertain when the delay
>> becomes wrong.
>
> I'm not doing any suspend/resume stuff.
>
> But I did have cpufreq enabled (using default 'performance' governor).
> Disabling cpufreq makes udelay() correct. Yay!
>
> In order to try diagnosing the problem I re-enabled cpufreq and booted
> with cpufreq.debug=7.
>

Bjorn,

Assuming your hardware is not weird at scaling and needs something like 
Russell said:
	new_lpj = ref_lpj * (new_freq / ref_freq) + some_offset

The most likely scenario seems to be that the CPU freq driver that's 
specific to your CPU is not setting the CPU to the frequency it thinks 
it's setting it to. It might THINK it's running at 624 MHz, but it might 
really be at 310 MHz or 100 MHz or something. Since, if the frequency at 
which the calibration was done or if the new frequencies are not what 
really what they are reported to be, then the math will go wrong and you 
would see these kind of issues.

If you can somehow measure your CPU freq with an oscilloscope, measure 
these two:
- Just before calibrate_delay is run for the first time.
- When you are seeing the udelay to be wrong.

One of these two won't match what you think the CPU is running at.

-Saravana
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