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

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

On Tue, Nov 06, 2012 at 05:53:20PM -0500, Nicolas Pitre wrote:
> 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.

I think I included all the relevant information in the original email.