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

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Date:	Thu, 26 Jun 2008 15:02:04 +1000
From:	"Dave Airlie" <airlied@...il.com>
To:	"Arjan van de Ven" <arjan@...radead.org>
Cc:	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	"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 2:36 PM, Arjan van de Ven <arjan@...radead.org> wrote:
> On Thu, 26 Jun 2008 11:23:11 +1000
> "Dave Airlie" <airlied@...il.com> wrote:
>
>> 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.
>
> well kmap does a tlb flush as well... you can't get away from doing a
> flush if you change cpu mapping.
> What you CAN do is play tricks and flush only once in a while and make
> sure you don't recycle mappings in the mean time (like kmap does).
>
> I can totally see doing an iounmap_lazy() and then have an
> iounmap_flush_lazy() thing or something like that....
>

Thats why keithp wants something like kmap_atomic but for ioremap
instead of kmaps.

This would be for short temporary ioremaps like kmap_atomic is.

Dave.
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