It appears that this problem began with version 3.0.13 of the kernel.
As it turns out, timekeeping.c added the following code in two places.
nsecs += arch_gettimeoffset();
A deeper look reveals the root to be in core.c of mach-ep93xx.
There is a potentially relevant patch out there but I have no idea if it's the
correct solution.
--- In "MrRealityCheck" <> wrote:
>
> So, I've been using a 3.0.4 kernel from ynezz/mcrapet tweaked for a 7350 for
> a while now. I also set the kernel HZ to 1000 so I could get close to
> millisecond timing. That's been working fine. usleep is pretty consistent.
> Also, ITIMER_REAL has been pretty consistent. I've used this to check the
> kernel frequency with some code I found via google. The Hz value hovers
> around 980 Hz. Plus or minus less than 5%.
>
> Now, I wanted to upgrade the kernel. I started with stock 3.5.7 and
> attempted to apply the 7350 specific and TS board specific stuff e.g. the SD
> card drivers. When I build the kernel however, my kernel frequency becomes
> really unpredictable varying from as little as 450 Hz to as much as 950 Hz.
> Both usleep and ITIMER_REAL are affected. I can't figure out what I'm doing
> wrong or if the kernel code itself is to blame. I've tried a 3.6.6 kernel
> and I've tried ynezz's 3.3.8 kernel as well. It's possible I missed
> something because the ynezz code doesn't have 7350-specific stuff.
>
> Any ideas? I've posted something about this to Linuxquestions but have yet
> to get a response.
>
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