RFC: Leave sysfs nodes alone during hotplug

[Todd Poynor]

On Mon, Jul 7, 2014 at 12:30 PM, Saravana Kannan <skannan at codeaurora.org> wrote:
...
>> Though these are the requirements I have from them:
>> - On hotplug files values should get reset ..
>> - On suspend/resume values must be retained.
>
> Hmm... There's actually enough interest in NOT reseting across hotplug
> because it's also used by thermal when a CPU gets too hot and then it's
> plugged in later. So, userspace has no way to cleanly restore the values.
> But that's a separate topic.

For Android's usage we're also interested in both:

1. not removing and recreating the cpufreq sysfs files for a CPU on
hotplug events (we currently use hotplug uevents to reset file
ownership such that power policy can be controlled by non-root).

2. not resetting the contents of policy files such as scaling_max_freq
(also fixed up from uevents) or stats files (we currently keep a
separate persistent time_in_state for battery accounting purposes).

>>> Any objections to leaving them alone during hotplug? If those files are
>>> read/written to when the entire cluster is hotplugged off, we could just
>>> return an error. I'm not saying it would be impossible to fix all these
>>> deadlock and race issues in the current code -- but it seems like a lot
>>> of
>>> pointless effort to remove/add sysfs nodes.

No objections from our standpoint.


Thanks -- Todd