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

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Date:	Wed, 26 Jun 2013 23:34:10 +0200
From:	Leif Lindholm <leif.lindholm@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.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 Wed, Jun 26, 2013 at 11:23:50PM +0200, Arnd Bergmann wrote:
> > Is this an issue here, since (unlike x86) this early_ioremap only works
> > before paging_init()?
> 
> The main problem is that the total fixmap size is only around 900kb,
> and we want to reserve at least 64kb per cpu for kmap_atomic.
> If you want to fit multiple 128kb mappings in there, you run out of
> space really fast.
 
Sorry, I still don't get it. Are you saying that kmap_atomic is
available before kmap_init() (in paging_init())?

If not, all of my mappings are discarded (well, abandoned to be more
correct), so I don't see how it affects kmap.

> > > Extending the iotable mechanism on ARM would be the convenient
> > > solution for us I think.
> > 
> > Could that easily be extended to give similar semantics sufficiently
> > that we can progress with merging more of the UEFI and ACPI support 
> > together as common code with x86/ia64?
> 
> I don't know what the requirements are, but the idea with iotable
> is that the mappings stay around at run time, while it seems you want
> to discard them at some point.
 
Indeed - almost immediately.

x86 early_ioremap can coexist with kmap; the intent of my
implementation is to use the kmap region only before kmap is available.

/
    Leif
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