[RFC] Fixing CPU Hotplug for RealView Platforms

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Tue, Dec 07, 2010 at 04:43:10PM -0000, Will Deacon wrote:
> Hello,
> 
> Currently, CPU hotplug is broken for RealView platforms. I posted some
> patches previously to try and address this, but they didn't solve the
> problems fully:
> 
> http://lists.infradead.org/pipermail/linux-arm-kernel/2010-September/026157.html
> 
> I'm now revisiting the code and it looks like the main problem is when
> we wish to *leave* the lowpower state. The enter/leave routines look
> like this:
> 
...
> 
> The problem is that by turning off coherency, the contents of the D-cache
> becomes stale. If data is prefetched into L1 between the flush_cache_all
> invocation and disabling the D-cache then this data will still be present
> when we come out of lowpower. Without coherency, we *must not* use this
> data and so a D-cache invalidation to the PoC is required in
> cpu_leave_lowpower().

What if we fixed the cpu_reset functions for v6 and v7, and when a CPU
is taken offline, we actually go through a proper shutdown of that CPU
and call the reset vector, re-entering the boot loader?

We can only do this for CPUs other than the original boot CPU, because
the boot loader should be checking which are the secondary CPUs and
putting those into this simple WFI loop with the GIC appropriately
programmed.

This means when we re-activate the CPU, we'll be waking it up in
exactly the same way as we do when the kernel boots - and we have all
that code around just waiting to be used.