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.
Nicolas
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