[PATCH] ARM: setup_mm_for_reboot(): use flush_cache_louis()

[Nicolas Pitre]

On Tue, 6 Nov 2012, Russell King - ARM Linux wrote:

> On Tue, Nov 06, 2012 at 04:12:27PM -0500, Nicolas Pitre wrote:
> > 
> > ... instead of flush_cache_all().  The later unconditionally flushes
> > the L2 cache on ARMv7 architectures such as Cortex A15 and A7 which
> > is costly and unnecessary in some scenarios where setup_mm_for_reboot()
> > is used.  If L2 has to be flushed as well, it should already be done
> > separately on other architectures anyway.
> 
> Why does the cost at reboot count?  It's a relatively slow operation as
> it is anyway, because you have to wait for the system to shut down, call
> the boot loader, etc.

Because I have a use case with the big.LITTLE switcher where the full 
boot is bypassed but the kernel must be reintered as if the CPU was 
powered up.  This is of course something that _could_ happen multiple 
times in a second, and therefore minimizing its unneeded costs is a good 
thing(tm).

> However, the opposite argument is that the state of the L2 _shouldn't_
> matter - except for one small little detail.  Dirty data, which could
> get evicted and overwrite something that matters.  Generally there won't
> be any dirty data in the L2 cache on normal boot, so this is a situation
> which boot loaders probably don't expect.

In the use case that concerns me, L2 is retained and I'd well prefer if 
it didn't get flushed at all.

> So all in all, I'm not sure of the wiseness of your change.  It's likely
> to cause regressions.

Could you please tell me if you have such a regression in mind?  Of 
course I could carry the few operations performed by 
setup_mm_for_reboot() locally, but that looks like useless code 
duplication.


Nicolas