RE: + restore-missing-sysfs-max_cstate-attr.patch added to -mm tree

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>On Fri, 30 Nov 2007 14:06:55 -0800
>"Pallipadi, Venkatesh" <[email protected]> wrote:
>Please dont go off-list like this.  I put Mark's original 
>mailing list cc's

Sorry for missing some cc's earlier. I blindly did a reply-all to the
mm-commits mail I got.

>> I will have to Nack this. The reason max_cstate was initentionally
>> removed due to couple of reasons:
>It broke userspace without any warning or migration period, afaict.

Yes. That's true. I will have to take the blame for that. It has been
known for a while during cpuidle development. But, it was never
documented as deprecating.
>> 1) All in kernel users of max_cstate should rather be using
>> pm_qos/latency interfaces. All such max_cstate usages must already be
>> migrated.
>That code isn't merged.

All kernel part is already merged. I mean, there are do drivers that
depend on max_cstate. They use latency_notifier thing today and their
migration to pm_qos part is not merged yet.

>> 2) Supporting max_cstate as a dynamic parameter cleanly is no longer
>> possible in acpi/processor_idle.c as the C-state policy has moved to
>> cpuidle instead. It can be done if it is needed. But, just 
>below patch
>> will not really work with cpuidle.
>> Selecting max_cstate at boot time as a debug option still 
>works without
>> this patch.
>> So, just this patch will not get back the functionality with cpuidle.
>> Infact changing it at run time will have no effect. Question 
>however is:
>> Is there a real need to revive this parameter so that user can change
>> max_cstate at run time?
>It is not known whether Mark is actually writing to this 
>thing.  Perhaps
>read-only permissions would be a suitable fix?

Exporting it as read only should be OK. We also need to know if there
are hard user space dependency on writing to this from userspace.

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