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

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Date:	Fri, 31 Oct 2008 09:59:29 -0700
From:	Keith Packard <keithp@...thp.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	keithp@...thp.com, Eric Anholt <eric@...olt.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Add io-mapping functions to dynamically map large
	device apertures

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.

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.

-- 
keith.packard@...el.com

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