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

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Date:	Thu, 27 Jun 2013 01:25:06 +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 Thu, Jun 27, 2013 at 12:13:55AM +0200, Arnd Bergmann wrote:
> > 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.
 
Ok, just so I'm not completely lost :)

No, the purpose is just like for x86 - do early parsing of things like
the UEFI system and configuration tables, DMI and ACPI, in order to
populate global structs and stuff.

> > 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?

Well, I did have a crazy idea that much/most of the early_ioremap code
could be made generic and shared between x86 and arm (and any other
32-bit architecture). Using the fixmap macros would make that possible
with a minimum of ifdefs.

If we ever wanted early_ioremap() to work like on x86, beyond kmap_init()
(but not beyond the "booting" system state), using the same macros would
help there too. I had no need for that for my EFI patches.

/
    Leif
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