[PATCH 01/13] ARM: suspend: use hash of cpu_logical_map value to index into save array

[Lorenzo Pieralisi]

On Thu, Jul 25, 2013 at 05:06:51PM +0100, Nicolas Pitre wrote:
> On Thu, 25 Jul 2013, Dave Martin wrote:
> 
> > On Wed, Jul 24, 2013 at 02:55:00PM -0400, Nicolas Pitre wrote:
> > > On Wed, 24 Jul 2013, Dave Martin wrote:
> > > 
> > > > But this patch commits us to requiring that on the suspend path 
> > > > specifically -- I think that ought to be mentioned somewhere. A 
> > > > comment in the preamble for __cpu_suspend would be enough, I think.
> > > 
> > > What comment would you suggest?  I want to make sure the possible 
> > > confusion you see is properly addressed.
> > 
> > I think we just need to state that the value of
> > cpu_logical_map(smp_processor_id()) must be the MPIDR of the physical
> > CPU the suspending logical CPU will resume on.  Consequently, if doing a
> > migration, cpu_logical_map() must be updated appropriately somewhere
> > between cpu_pm_enter() and cpu_suspend().
> 
> Excellent.  I've amended the patch with this:
> 
> diff --git a/arch/arm/kernel/suspend.c b/arch/arm/kernel/suspend.c
> index 2835d35234..caf938db62 100644
> --- a/arch/arm/kernel/suspend.c
> +++ b/arch/arm/kernel/suspend.c
> @@ -17,6 +17,11 @@ extern void cpu_resume_mmu(void);
>  /*
>   * Hide the first two arguments to __cpu_suspend - these are an implementation
>   * detail which platform code shouldn't have to know about.
> + *
> + * On SMP systems, the value of cpu_logical_map(smp_processor_id()) must be
> + * the MPIDR of the physical CPU the suspending logical CPU will resume on.
> + * Consequently, if doing a physical CPU migration, cpu_logical_map() must be
> + * updated appropriately somewhere between cpu_pm_enter() and cpu_suspend().
>   */
>  int cpu_suspend(unsigned long arg, int (*fn)(unsigned long))
>  {
> 
> I've put it against cpu_suspend() rather than __cpu_suspend(() as this 
> is what users should care about.
> 
> ACK?

We need this patch to allow IKS to store a cpu context at a specific
index, let's be honest. It is a moot point and I am not very happy
about changing this code for a very specific usage, but the way code is
implemented makes the change acceptable. I really do not think we should
write guidelines on how cpu_suspend users have to change cpu_logical_map
though, that's not needed apart from IKS and that should be limited to IKS
code only.

Again, that's just my opinion, but cpu_suspend API must be kept as it is
and we should not encourage people to use it in creative ways.

Lorenzo