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

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Message-Id: <201306270013.55333.arnd@arndb.de>
Date:	Thu, 27 Jun 2013 00:13:55 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Leif Lindholm <leif.lindholm@...aro.org>
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 Wednesday 26 June 2013, Leif Lindholm wrote:
> 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.

Sorry, I was under the assumption that the mappings are meant to
stay around.

> > > > 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.

So if you never plan to use fixmap and early_ioremap at the same time,
why even bother using the fixmap code? Wouldn't it be easier to just
use the same memory area and ensure we never use fixmap before 
we're done with early_ioremap?

	Arnd
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