linux-kernel - Re: [RFC PATCH 06/12] PAT 64b: Add ioremap

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Date:	Thu, 13 Dec 2007 21:28:23 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Roland Dreier <rdreier@...co.com>
Cc:	venkatesh.pallipadi@...el.com, ak@....de, ebiederm@...ssion.com,
	torvalds@...ux-foundation.org, gregkh@...e.de, airlied@...net.ie,
	davej@...hat.com, mingo@...e.hu, tglx@...utronix.de, hpa@...or.com,
	akpm@...ux-foundation.org, arjan@...radead.org,
	jesse.barnes@...el.com, linux-kernel@...r.kernel.org,
	Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: [RFC PATCH 06/12] PAT 64b: Add ioremap_wc support

Roland Dreier <rdreier@...co.com> writes:

>  > --- linux-2.6.24-rc4.orig/include/asm-x86/io_64.h 2007-12-11
> 14:24:56.000000000 -0800
>  > +++ linux-2.6.24-rc4/include/asm-x86/io_64.h 2007-12-11 15:49:52.000000000
> -0800
>  > @@ -142,7 +142,8 @@
>  > * it's useful if some control registers are in such an area and write
> combining
>  >   * or read caching is not desirable:
>  >   */
>  > -extern void __iomem * ioremap_nocache (unsigned long offset, unsigned long
> size);
>  > +extern void __iomem * ioremap_nocache(unsigned long offset, unsigned long
> size);
>  > +extern void __iomem * ioremap_wc(unsigned long offset, unsigned long size);
>
> I think ioremap_wc() needs to be available on all archs for this to be
> really useful to drivers.  It can be a fallback to ioremap_nocache()
> everywhere except 64-bit x86, but it's not nice for every driver that
> wants to use this to need an "#ifdef X86" or whatever.
>
> Also I didn't see anything like pgprot_wc() in the patchset (although
pgprot_writcombined.

> I just skimmed quickly for now).  The use case I actually have would
> be in a a driver's .mmap method, where I want to map device registers
> into userspace with write-combining turned on:
>
> 	vma->vm_page_prot = pgprot_wc(vma->vm_page_prot);
> 	io_remap_pfn_range(vma, vma->vm_start, pfn, PAGE_SIZE,
> vma->vm_page_prot);
>
> where the pfn points into a PCI BAR (which will only be mapped once,
> so no issues with conflicting PATs or anything like that).

Eric

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