On Thu, 16 Sep 1999, Deborah Wallach wrote:
>
> On Thu, 16 Sep 1999, Mark Huang wrote:
>
> > Hi,
> >
> > Is there a way to update the SA-1100's PPCR register and change the core
> > clock frequency on the fly after booting Linux? I've tried running the
> > assembly code that I use to set the core clock before booting, but that
> > just hangs the system usually.
>
> Why would you do that? The problem is that most configurable timings like
> dram, IO bus, etc. are all configured relative to the core clock. By
> changing the clock you may lose your dram refresh rate and then anything
> funny can happen.
>
> One reason to do it would potentially be for power savings. We do it, so
> it definitely is possible. The strategy that we use is to basically shut
> down all the devices, jump to a very tiny piece of code in flash that
> changes the dram configuration parameters, then change the ppcr and then
> start up all the devices again. There's some funkiness about the order you
> do things in depending on whether you're decreasing or increasing the clock
> rate, too.
Is it really worth the trouble since the wait-on-irq mode can be used when
there's nothing left to do? Having a hier clock rate will just lead the
CPU faster in the idle mode and stay longer idle. Power saving should be
about the same in both cases.
It may not be exactly the same for all cases. We're in the process of
investigating that right now.
-Debby
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