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

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Message-ID: <1443125663.32298.32.camel@freescale.com>
Date:	Thu, 24 Sep 2015 15:14:23 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	David Laight <David.Laight@...LAB.COM>
CC:	'Christophe Leroy' <christophe.leroy@....fr>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	"Michael Ellerman" <mpe@...erman.id.au>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 08/25] powerpc/8xx: Map IMMR area with 512k page at a
 fixed address

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?

-Scott


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