linux-kernel - Re: kmap_atomic_pfn for PCI BAR access?

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Message-ID: <4862EE4C.4060009@goop.org>
Date:	Wed, 25 Jun 2008 18:18:04 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Keith Packard <keithp@...thp.com>
CC:	Dave Airlie <airlied@...ux.ie>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: kmap_atomic_pfn for PCI BAR access?

Keith Packard wrote:
> The graphics memory BAR is generally fairly good sized; on Intel chips,
> it's between 256M and 1G (and growing). I want to write data into this
> region from kernel space, but it's really too big to map the whole thing
> into kernel address space, especially on 32-bit systems. ioremap is not
> a good option here -- it's way too slow.
>
> With CONFIG_HIGHMEM enabled, I can use kmap_atomic_pfn (well, actually
> the kmap_atomic_proc_pfn included in the DRM tree) and things work quite
> well -- performance is good, with barely any measurable time spent in
> the PTE whacking (~1%).
>
> However, with CONFIG_HIGHMEM disabled, there aren't any PTEs reserved
> for this kind of mapping fun. This makes me suspect that abusing
> kmap_atomic for this operation would not be appreciated. 
>
> Should I use kmap_atomic_pfn to reach my PCI BAR like this?
>
> Would it be reasonable to supply a patch that made this work even
> without CONFIG_HIGHMEM?
>   

Usually people use ioremap to map device memory.  Wouldn't that work in 
this case?

    J

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