[PATCH 7/7] ARM: tegra30: cpuidle: add LP2 driver for CPU0

[Peter De Schrijver]

On Tue, Oct 16, 2012 at 12:33:07AM +0200, Colin Cross wrote:
> On Mon, Oct 15, 2012 at 8:59 AM, Stephen Warren <swarren at wwwdotorg.org> wrote:
> > On 10/15/2012 01:56 AM, Joseph Lo wrote:
> >> There is still one thing you should know. Because we are planning to
> >> upstream "CPUquiet" framework that is a CPU auto hotplug mechanism. It
> >> will auto hotplug the CPUs depends on the system is busy or not. So when
> >> system is idle, there will be only one CPU online (i.e, CPU0). The
> >> secondary CPUs will all be hot plugged (i.e, offline and power gate). We
> >> need to think about do we still need coupled cpuidle on Tegra30 if we
> >> are going to use "CPUquiet".
> >
> > CPUquiet isn't relevant at all. First, a user may presumably disable
> > CPUquiet's Kconfig option (it had better have one, and the system had
> > better work with it disabled). Second, even if CPUquiet is enabled, I
> > don't imagine there is a 100% guarantee that hot(un)plug will happen
> > before cpuidle kicks in, is there? Finally, is there any guarantee that
> > CPUquiet will actually be accepted upstream?
> 
> CPUquiet is a glorified hotplug governor, and hotplug governors have
> been widely rejected upstream, so I wouldn't plan on seeing it
> accepted.

Note that nothing in CPUquiet enforces the use of hotplug. It assumes there
is a way to put a CPU in a quiet state which means it doesn't get interrupted,
doesn't do any work and doesn't respond to IPIs. Currently only hotplug
provides this state, but it's possible to provide a driver which uses a
different mechanism to provide the same state. We need this state to be able
to switch to the low power cluster which has only 1 CPU. IPIs to non-existing
cores would hang the system.

Cheers,

Peter.