linux-kernel - Re: [PATCH 0/2] arm: add early

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Message-Id: <201306262052.09640.arnd@arndb.de>
Date:	Wed, 26 Jun 2013 20:52:09 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Leif Lindholm <leif.lindholm@...aro.org>, nico@...aro.org,
	patches@...aro.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org,
	"Russell King - ARM Linux" <linux@....linux.org.uk>
Subject: Re: [PATCH 0/2] arm: add early_ioremap() support

On Tuesday 25 June 2013, Leif Lindholm wrote:
> x86 and ia64 have the early_ioremap()/early_iounmap() functions, which are
> useful for supporting things like UEFI, ACPI and SMBIOS, where configuration
> tables need to be parsed before proper memory management is available,
> regardless of highmem status.
> 
> This patchset implements a restricted form of early_ioremap(), available
> before paging_init() only. Like the x86 code on which it is based, it
> (p)re-uses the fixmap regions for its virtual mapping range. Up to 7
> simultaneous mappings of up to 128KB can be accommodated in the available
> fixmap space.

+rmk

I made a similar suggestion to extending the use of fixmap recently, see
"Re: SCU registers mapping for CA9/CA5 cores". Russell pointed out that
fixmap is intentionally limited to just kmap_atomic uses at the moment
and changing that would potentially have a significant impact when we
run out of pages in the fixmap area.

The method we use on ARM normally is the iotable_init() function, which
requires hardcoding a virtual address at the moment.

It might be nicer to change that code than to put early_ioremap into
fixmap. Note that early_ioremap in fixmap is a bit of a kludge on x86
as well because it is very much /not/ a fixed mapping like the rest
of fixmap, they just use it because it's convenient.

Extending the iotable mechanism on ARM would be the convenient
solution for us I think.

	Arnd
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