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

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Date:	Thu, 26 Jun 2008 11:23:11 +1000
From:	"Dave Airlie" <airlied@...il.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"Keith Packard" <keithp@...thp.com>,
	"Dave Airlie" <airlied@...ux.ie>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: kmap_atomic_pfn for PCI BAR access?

On Thu, Jun 26, 2008 at 11:18 AM, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> 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?
>

"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."

>From the original mail.

doing tlb flush for iounmap is slow as all hell if you do it a lot,
and we can't afford to mmap the whole aperture it can 1GB.

Dave.
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