linux-kernel - Re: [PATCH v3] ad7877: keep dma rx buffers in seperate cache lines

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Message-ID: <20100511165228.GC7396@core.coreip.homeip.net>
Date:	Tue, 11 May 2010 09:52:28 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Mike Frysinger <vapier.adi@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oskar Schirmer <os@...ix.com>,
	Michael Hennerich <Michael.Hennerich@...log.com>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	Daniel Glöckner <dg@...ix.com>,
	Oliver Schneidewind <osw@...ix.com>,
	Johannes Weiner <jw@...ix.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Matt Mackall <mpm@...enic.com>
Subject: Re: [PATCH v3] ad7877: keep dma rx buffers in seperate cache lines

On Tue, May 11, 2010 at 09:42:03AM +0300, Pekka Enberg wrote:
> Hi Dmitry,
> 
> On Tue, May 11, 2010 at 9:33 AM, Dmitry Torokhov
> <dmitry.torokhov@...il.com> wrote:
> >> what guarantee exactly do you have for that statement ?
> >
> > The data is kmalloced, kmalloc aligns on cacheline boundary AFAIK which
> > means that next kmalloc data chunk will not share "our" cacheline.
> 
> No, there are no such guarantees. kmalloc() aligns on
> ARCH_KMALLOC_MINALIGN or ARCH_SLAB_MINALIGN depending on which is
> bigger but beyond that, there are no guarantees. You can, of course,
> use kmem_cache_create() with SLAB_HWCACHE_ALIGN to align on cacheline
> boundary.
> 

The architectures that we are trying to deal with here should be forcing
kmalloc to the cache boundary already though - otherwise they would not
be able to used kmalloced memory for DMA buffers at all. Or am I utterly
lost here?

-- 
Dmitry
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