preliminary 750GX PLL control patch
Benjamin Herrenschmidt
benh at kernel.crashing.org
Sun Jul 31 02:18:03 EST 2005
On Tue, 2005-07-26 at 10:28 -0700, Kevin Diggs wrote:
> Hi,
>
> I've attached a patch that adds preliminary access to the HID1
> register in the 750GX (aka the PLL config register). The patch is
> against 2.4.30. It has been lightly tested on a PowerMac 8600 with a
> PowerLogix PowerForce 750GX cpu card.
>
> Depending on the values for the pllx_cfg field, this should
> work on the 750FX as well ... except for the stuff in idle_6xx.S. Since
> there are plenty of people on this list way more smarterer than me I was
> hoping someone could answer a question about the powersave modes that
> some of the PowerPC processors have:
>
> Since, as I understand it, the clock is shut off to much of the
> chip when a powersave mode is engaged does it really buy you that much
> to switch to a lower clock speed? Wouldn't you be better off shutting
> off the unused PLL?
>
> Also, please note that I am not very experienced creating
> patches so please use a backup of your source tree.
Well... I replicated what darwin does here. I don't remember completely
but it's possible that there is some kind of CPU errata related to nap
and clock speed... or maybe it still makes a difference :)
You want to wakeup fast from nap too, so switching off/on the unused PLL
might not be that a good idea. But heh, you are welcome to try &
measure.
Ben.
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