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: <063D6719AE5E284EB5DD2968C1650D6D1CBA15B8@AcuExch.aculab.com>
Date:	Thu, 24 Sep 2015 11:41:25 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Christophe Leroy' <christophe.leroy@....fr>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	"Michael Ellerman" <mpe@...erman.id.au>,
	"scottwood@...escale.com" <scottwood@...escale.com>
CC:	"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

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.

Whether things like the IMMR should be mapped with a larger TLB
is a separate matter.

	David