linux-kernel - Re: Adding kmap_atomic_prot_pfn (was: [git pull] drm patches for 2.6.27-rc1)

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Date:	Thu, 23 Oct 2008 18:50:15 -0700
From:	Keith Packard <keithp@...thp.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	keithp@...thp.com, Andrew Morton <akpm@...ux-foundation.org>,
	mingo@...e.hu, nickpiggin@...oo.com.au, airlied@...ux.ie,
	linux-kernel@...r.kernel.org, jbarnes@...tuousgeek.org,
	dri-devel@...ts.sf.net, yinghai@...nel.org
Subject: Re: Adding kmap_atomic_prot_pfn  (was: [git pull] drm patches for
	2.6.27-rc1)

On Thu, 2008-10-23 at 14:24 -0700, Linus Torvalds wrote:

> The whole point of that function has absolutely nothing to do with 
> highmem, and it *must* be useful on non-highmem configurations to be 
> appropriate.

Right, I'd just like my io_mapping_map_atomic_wc to be able to rapidly
map random pages from my PCI BAR in WC mode. The code in your tree uses
kmap_atomic_pfn, which does the mapping, but use the wrong prot bits. On
a system with MTRR failure, this all ends badly, with factors of 20
performance drops for copying data from the CPU to the aperture.

> So I'd much rather create a new <linux/kmap.h> or something. Or just 
> expose this from to <asm/fixmap.h> or something. Let's not confuse this 
> with highmem, even if the implementation _historically_ was due to that.

Sure, we readily admit that we're abusing the highmem API. So we wrapped
that abusive code in a pretty package that can be re-implemented however
you desire.

How (and when) would you like to see this fixed?

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

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