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

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Date:	Mon, 7 Jul 2008 00:05:28 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Dave Airlie <airlied@...il.com>,
	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 Mon, 7 Jul 2008 16:53:54 +1000
Nick Piggin <nickpiggin@...oo.com.au> wrote:

> On Thursday 26 June 2008 13:11, Jeremy Fitzhardinge wrote:
> > Dave Airlie wrote:
> 
> > > 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.
> >
> > Maybe Nick's vmap reimplementation would help here.  It effectively
> > allows you to map stuff into the vmalloc space, and do lazy tlb
> > flushes to mitigate the cost of map/unmap.  He posted the patches
> > week or so ago.
> 
> Yeah, it can _really_ help. I'd posted some performance numbers with
> the patch which might prompt you to take another look at ioremap.
> 
> One thing I still haven't implemented in that patch are CPU-local
> mappings (which only require a local flush to flush)... I found that
> after the improvements I did implement, they didn't help much for
> my workloads, so I suspect you might find the same thing... 

just to complicate things with ioremap() you have to deal with the
various caching-coherency requirements.. making things lazy and per CPU
complicates that a ton (and I suspect won't actually make things
cheaper)

-- 
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