Re: [PATCH v2 08/25] powerpc/8xx: Map IMMR area with 512k page at a fixed address

From: Scott Wood
Date: Thu Sep 24 2015 - 16:14:38 EST

On Thu, 2015-09-24 at 11:41 +0000, David Laight wrote:
> From: Christophe Leroy
> > Sent: 22 September 2015 17:51
> ...
> > Traditionaly, each driver manages one computer board which has its
> > own components with its own memory maps.
> > But on embedded chips like the MPC8xx, the SOC has all registers
> > located in the same IO area.
> >
> > When looking at ioremaps done during startup, we see that
> > many drivers are re-mapping small parts of the IMMR for their own use
> > and all those small pieces gets their own 4k page, amplifying the
> > number of TLB misses: in our system we get 0xff000000 mapped 31 times
> > and 0xff003000 mapped 9 times.
> Isn't this a more general problem?
> If there are multiple remap requests for the same physical page
> shouldn't the kernel be just increasing a reference count somewhere
> and returning address in the same virtual page?
> This should probably happen regardless of the address.
> I presume it must be done for cacheable mappings.

Why would you assume that?


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