linux-kernel - Re: [PATCH] Add io-mapping functions to dynamically map large device apertures

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Date:	Mon, 3 Nov 2008 09:37:27 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Keith Packard <keithp@...thp.com>
Cc:	Eric Anholt <eric@...olt.net>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Add io-mapping functions to dynamically map large
	device apertures


* Keith Packard <keithp@...thp.com> wrote:

> On Fri, 2008-10-31 at 10:21 +0100, Ingo Molnar wrote:
> 
> > it's ugly and inflexible to put x86 dependencies into generic headers. 
> > (even though with a high likelyhood 32-bit x86 will be the only arch 
> > to ever implement the iomap_atomic() APIs)
> > 
> > Instead please add a HAVE_ATOMIC_IOMAP define to arch/x86/Kconfig:
> > 
> >  config HAVE_ATOMIC_IOMAP
> >          def_bool y
> >          depends on X86_32
> > 
> > ... and use #ifndef HAVE_ATOMIC_IOMAP in include/linux/io-mapping.h 
> > instead of #ifdef CONFIG_X86_64.
> 
> Just to clarify the issue here: there are two separate 
> implementations of the io_mapping API -- one for 'large address 
> space' machines where ioremap_wc can handle the typical graphics 
> aperture within the kernel virtual map, and the other using 
> iomap_atomic_prot_pfn for machines with puny address spaces.
> 
> All large address space machines can provide the io_mapping API 
> without any archtecture-specific support. For efficient 32-bit 
> io_mapping support, we require the new iomap_atomic_prot_pfn 
> function.
> 
> So, it seems like what I want to do is use the large address space 
> code on any machine which supports it, and then use the 
> iomap_atomic_prot_pfn version for small address space machines which 
> have the iomap_atomic_prot_pfn function.

Correct.

> What I think you're suggesting is to just assume that machines 
> without iomap_atomic_prot_pfn have address spaces large enough to 
> support the ioremap_wc path. The alternative is to create a third 
> (slow) path (which I did before the iomap_atomic_prot_pfn API was 
> introduced) that uses ioremap_wc at run time for small address space 
> machines without iomap_atomic_prot_pfn.
> 
> Let me know which you'd prefer and I'll get a patch out ASAP.

Please lets keep it simple: i.e. always use ioremap_wc() when there's 
no iomap_atomic_prot_pfn() 32-bit API provided.

( and by all means ioremap_wc() will just work fine on most 32-bit
  architectures out of box: they dont go about trying to map hundreds 
  of megabytes of graphics aperture. If they nevertheless need it, 
  they can implement iomap_atomic_prot_pfn() to add support. )

	Ingo
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