Re: unreliable time function inside IRQ?

[Lars-Peter Clausen <lars@metafoo.de>]

On 03/31/2014 07:44 PM, Ge Gao wrote:
> Dear Jonathan,
> 	Thanks for answering my question. I am using Panda board(OMAP4430). It is using a 32K counter clock as the high resolution clock(arch/arm/plat-omap/couter_32k.c). But I also observe the same thing in Nexus 7 first generation(announced in 2012), which uses Nvidia Tegra 3 platform. I suspect the data that represents the time is cached, or it is using some older value in the interrupt case.
> It could be the Linux's time function problem. Is that possible? I actually tried using Jiffy, which is the built-in software clock. The time between each interrupt also varies big. My HZ setting is 1000. I also tried a later version of Linux, which is 3.7 or later. The result is the same.
> 	The implementation of the time function is very simple. Below is the function from 32K clock counter. It is reading the hardware clock, compute the difference between a static variable and update the time. For a different clock, only the "clocksource_32k.mult", "clocksource_32k.shift", would be different.
> 	Thanks.

You have to consider that there will be other things running with IRQs off, 
which will cause some jitter for your IRQ handler. This is nothing IIO specific.

- Lars