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 15:25:39 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Rob Herring <robherring2@...il.com>
cc:	Liu Hua <sdu.liu@...wei.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	wangnan0@...wei.com, Laura Abbott <lauraa@...eaurora.org>,
	peifeiyue@...wei.com, Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	vitalya@...com, Santosh Shilimkar <santosh.shilimkar@...com>,
	liusdu@....co, 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, 30 May 2014, Rob Herring wrote:

> 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 wouldn't rule that out.  I've seen 16-core ARM chips in 2008 (although 
they didn't go into production).  Silly limitations like that always 
come back to bite you.  And we have better alternatives.

> - Reduce KM_TYPE_NR from 16 to 15. Based on the comment for it, we
> probably don't want to do that. Is increasing it to the default of 20
> worthwhile? Some of the options here would allow doing that.
> - Add 0xffe00000-0xfff00000 to the fixmap region. This would make
> fixmap span 2 PMDs with the top PMD having a mixture of uses like we
> had before.

That would be my preferred approach.  Note here it could be 
0xffe00000-0xfffe0000 to include the whole of the previous fixmap area 
curently unused.

> - push the PCI i/o space down to 0xfec00000 and make fixmap 4MB. This
> is a cleaner solution as the 2 PMDs are only used for fixmap. This may
> require some static mapping adjustments on some platforms.

No need.  With the latest changes, the fixmap area is between 0xffc00000 
and 0xffe00000 (there is apparently a mistake in 
Documentation/arm/memory.txt).  So currently 0xff000000-0xffc00000 is 
free, which makes the fixmap area far away from the PCI i/o area with 
plenti of space in between.

> - Same as previous option, but convert the PCI i/o space to fixmap
> entries. We don't really need all 2MB for PCI.

See above.

> Also, there is an error in the documentation below:
> 
> >
> > Signed-off-by: Liu Hua <sdu.liu@...wei.com>
> > ---
> >  Documentation/arm/memory.txt   |  2 +-

Yep, good that you spotted it as well.  I failed to catch it during my 
review so I'll send a patch.


Nicolas
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