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SubjectRe: [linux-pm] [PATCH] cpuidle: avoid singing capacitors
Date
On Sunday, 9 of March 2008, Pierre Ossman wrote:
> I'm beginning to think this is a lost cause. I've tried several variants, all without satisfactory results.
>
> In case anyone else has any more ideas, I'll detail what I've found influences the noise:
>
> 1. C state
>
> This is the big one. There is no noise as long as C3 is avoided (processor.max_cstate).
>
> 2. uhci_hcd driver
>
> USB is somehow involved in this problem. Unloading the uhci_hcd driver almost
> entirely kills the noise on a 1000 HZ NO_HZ kernel. On a 100 HZ, no NO_HZ
> kernel, the effect is very small, but still there.
>
> 3. Low speed USB devices
>
> Related, the noise goes away if I insert a USB mouse (low speed).
> A high-speed device does not effect the noise, neither does the two built-in
> low speed devices (a fingerprint reader and a bluetooth host).
>
> 4. Battery and AC
>
> The noise increases with the battery present and also when the AC supply is
> removed.
>
> 5. Second core
>
> Disabling the second core makes the noise go away. This might be
> a subset of 1., as I've been told that a stopped core enters C1.
>
>
> Changing HZ and NO_HZ has no noticeable effect on the problem (except
> the odd behaviour in 2.). This is further supported by the fact that Windows
> also has the problem (which should behave close to 100 HZ without NO_HZ).
>
>
> So for now, the only viable workaround is processor.max_cstate....

Well, there may be some users willing to trade some battery life for having a
quiet box. :-)

Perhaps it's worth documenting?

Thanks,
Rafael


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