Date: Fri, 11 Mar 2016 17:15:39 -0600
From: Scott Wood <oss@buserror.net>
To: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>, linuxppc-dev@lists.ozlabs.org,
        linux-kernel@vger.kernel.org
Message-ID: <20160311231539.GB30872@home.buserror.net>
References: <469ed145e21be6720c186686039b569e30ca1ed4.1455032446.git.christophe.leroy@c-s.fr>
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Subject: Re: [v8, 08/23] powerpc/8xx: Map IMMR area with 512k page at a fixed
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On Tue, Feb 09, 2016 at 05:08:02PM +0100, Christophe Leroy wrote:
> Once the linear memory space has been mapped with 8Mb pages, as
> seen in the related commit, we get 11 millions DTLB missed during
> the reference 600s period. 77% of the misses are on user addresses
> and 23% are on kernel addresses (1 fourth for linear address space
> and 3 fourth for virtual address space)
> 
> 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.
> 
> Even if each part of IMMR was mapped only once with 4k pages, it would
> still be several small mappings towards linear area.
> 
> With the patch, on the same principle as what was done for the RAM,
> the IMMR gets mapped by a 512k page.

"the patch" -- this one, that below says it maps IMMR with other sizes?

> 
> In 4k pages mode, we reserve a 4Mb area for mapping IMMR. The TLB
> miss handler checks that we are within the first 512k and bail out
> with page not marked valid if we are outside
> 
> In 16k pages mode, it is not realistic to reserve a 64Mb area, so
> we do a standard mapping of the 512k area using 32 pages of 16k.
> The CPM will be mapped via the first two pages, and the SEC engine
> will be mapped via the 16th and 17th pages. As the pages are marked
> guarded, there will be no speculative accesses.

If IMMR is 512k, why do you need to reserve 4M/64M for it?

-Scott