[PATCH][ ARM cpu hotplug 1/2 ] extract common code for arm cpu hotplug

[Russell King - ARM Linux]

On Tue, Nov 30, 2010 at 11:03:31AM +0000, Russell King - ARM Linux wrote:
> I see no point to what is being proposed in this thread.  It's _soo_
> little code that the platforms have to implement that it really is
> not worth the effort.
> 
> How do you know whether separating out the cache flushes from the
> wait-for-interrupt is an acceptable thing to do?  On the Realview
> platforms, I suspect it's not acceptable.  That means your attempts
> to move the cache flusing into a separate function from the wait-for-
> interrupt will cause problems - as entering a function creates a
> stack frame, and therefore writes to memory which can hit the cache.
> 
> Leave it as is.  The generic interface for platforms to implement is:
> 
> platform_do_lowpower() - does whatever's necessary to idle etc the CPU
> platform_cpu_kill() - returns 1 if there's nothing to be done
> platform_cpu_disable() - returns 0 if the CPU can be taken offline
> 
> Trying to get rid of platform_cpu_kill and platform_cpu_disable, and
> then splitting platform_do_lowpower into three new smaller functions
> is NOT an improvement.

Note also that most implementations of this get it wrong.  Realview
assumes ARMv6 MPcore layout of the auxcontrol register, which is
cut'n'pasted into s5pv310 (ARMv7) and Tegra (ARMv7) without thinking.
The only implementation which has put some thought into it is the
UX500 one, which is extremely simple.

So, out of the four existing implementations, at least two need fixing
to be different, and I'll bet that they do have some way to power down
the offlined CPUs for greater power savings.

On that basis, there is absolutely no way to start thinking about
collecting these up into common code, as you don't know what the proper
implementation for these platforms should look like.