Continual reading from the PowerPc time base register is not stable

[Csdncannon]

I am really sorry that the previously attached code is wrong, this one
"timebase.c" is the right one, and the "log_timebase" file is the right log.

We are using FreeScale PowerPc 8378, kernel 2.6.28 and compiled as 32-bit.


Thanks
Gino

2010/3/25 Arnd Bergmann <arnd at arndb.de>

> On Thursday 25 March 2010, Benjamin Herrenschmidt wrote:
> > On Thu, 2010-03-25 at 10:41 +0800, Csdncannon wrote:
> > >          In my program, the value of the 64-bit time base register is
> > > read out, and you will find the later value is even smaller than the
> > > earlier value from the log “log_timebase”. While the kernel depends on
> > > the accuracy of the timebase for the compensation of the lost PIT
> > > interrupt, the negative value between two continual timebase reading
> > > will bring to the jump of the jiffies. And this timebase problem will
> > > bring to the instability of the gettimeofday system call.
> > >
> > >          Do you have any idea about this problem, thanks for your any
> > > advice. Attached is the code and log.
> >
> > This is a concern, it should definitely not happen. What machine is
> > that ? is the code compiled 32-bit or 64-bit ? What kernel version ?
> >
> > Arnd, any chance that could relate to the bug you've been chasing on
> > Cell ?
>
> We're still busy with the problem analysis on Cell, waiting for a time
> slot to run the next test kernel. So far it seems like the timebase
> is actually synchronized at a significant accuracy on QS22 to never
> cause this problem with correct code, however it is possible to
> observe incorrect timebase values on Cell whenever the mftb instruction
> is not serialized with memory accesses, e.g. by using an isync in front
> of the mftb. On Power6 and other CPUs, that problem will not happen.
>
>        Arnd
>
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