linux-kernel - Re: [PATCH v3 2/2] ARM : change fixmap mapping region to support 32 CPUs

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Date:	Fri, 30 May 2014 19:02:12 +0100
From:	Will Deacon <will.deacon@....com>
To:	Rob Herring <robherring2@...il.com>
Cc:	Liu Hua <sdu.liu@...wei.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"wangnan0@...wei.com" <wangnan0@...wei.com>,
	Laura Abbott <lauraa@...eaurora.org>,
	"peifeiyue@...wei.com" <peifeiyue@...wei.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"vitalya@...com" <vitalya@...com>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	"liusdu@....co" <liusdu@....co>, "cyril@...com" <cyril@...com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Rabin Vincent <rabin@....in>
Subject: Re: [PATCH v3 2/2] ARM : change fixmap mapping region to support 32
 CPUs

On Fri, May 30, 2014 at 04:33:46PM +0100, Rob Herring wrote:
> On Tue, Apr 15, 2014 at 6:06 AM, Liu Hua <sdu.liu@...wei.com> wrote:
> > In 32-bit ARM systems, the fixmap mapping region can support
> > no more than 14 CPUs(total: 896k; one CPU: 64K). And we can
> > configure NR_CPUS up to 32. So there is a mismatch.
> >
> > This patch moves fixmapping region downwards to region
> > 0xffc00000-0xffe00000. Then the fixmap mapping region can
> > support up to 32 CPUs
> 
> Ugg, this series breaks using fixmap for anything other than kmap
> since 32 cpus requires all 2MB of the region and nothing is left.
> There's work in flight to support early_ioremap, early console, and RO
> text patching which all use the fixmap region.
> 
> There's a couple of options to solve this:
> 
> - Only support up to 16 cpus. It could be anywhere between 17-31, but
> that seems somewhat unlikely. Are we really ever going to see 32-bit
> 32 core systems?

I'd be fine with restricting the limit to 16 CPUs.

Will
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