[RFC] Fixing CPU Hotplug for RealView Platforms

[will.deacon@arm.com (Will Deacon)]

Hi Russell,

> 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?

This will certainly solve our problem, but people might complain that it's
too heavyweight :) That said, for v7 this may be the only solution as the
platform can require an IMPLEMENTATION DEFINED initialisation routine to be
executed before enabling the D-cache out of reset. This should be something
that the bootloader does for us.

> 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.

As long as the bootloader doesn't mind, then this should work.

Will