linux-kernel - Re: [PATCH 5/7] [RFC] force 32-byte aligned kmallocs

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Message-ID: <BANLkTimc3jQTOJdqAiqbPxGQNVWe39VqfQ@mail.gmail.com>
Date:	Thu, 19 May 2011 19:47:55 -0500
From:	Eric Van Hensbergen <ericvh@...il.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	bg-linux@...ts.anl-external.org
Subject: Re: [PATCH 5/7] [RFC] force 32-byte aligned kmallocs

On Thu, May 19, 2011 at 7:36 PM, Benjamin Herrenschmidt
<benh@...nel.crashing.org> wrote:
> On Wed, 2011-05-18 at 16:24 -0500, Eric Van Hensbergen wrote:
>>
>> -#ifdef CONFIG_NOT_COHERENT_CACHE
>> +#if defined(CONFIG_NOT_COHERENT_CACHE) || defined(CONFIG_BGP)
>>  #define ARCH_DMA_MINALIGN    L1_CACHE_BYTES
>>  #endif
>
> Is DMA cache coherent on BG/P ? That's odd for a 4xx base :-)
>

My understanding of things (which could be totally wrong) is that the
DMA we care about on BG/P (namely the Torus and Collective networks)
is coherent at the L2.  Of course the change in question is talking
about L1_CACHE_BYTES, so my reading of this is that its a sleazy way
of getting aligned mallocs that make interactions with the tightly
coupled networks easier/more-efficient.  I'm open to alternative
suggestions.

      -eric
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