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To: David Gibson <david@gibson.dropbear.id.au>,
       James Bottomley <James.Bottomley@SteelEye.com>,
       "Adam J. Richter" <adam@yggdrasil.com>, linux-kernel@vger.kernel.org
Subject: Re: [RFC] generic device DMA implementation 
In-Reply-To: Message from David Gibson <david@gibson.dropbear.id.au> 
   of "Thu, 05 Dec 2002 16:05:36 +1100." <20021205050536.GE1500@zax.zax> 
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Date: Thu, 05 Dec 2002 09:03:24 -0600
From: James Bottomley <James.Bottomley@steeleye.com>
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david@gibson.dropbear.id.au said:
> But if you have the sync points, you don't need a special allocater
> for the memory at all - any old RAM will do.  So why not just use
> kmalloc() to get it. 

Because with kmalloc, you have to be aware of platform implementations.  Most 
notably that cache flush/invalidate instructions only operate at the level of 
certain block of memory (called the cache line width).  If kmalloc returns 
less than a cache line width you have the potential for severe cockups because 
of the possibility of interfering cache operations on adjacent kmalloc regions 
that share the same cache line.

the dma_alloc... function guarantees to avoid this for you by passing the 
allocation to the platform layer which knows the cache characteristics and 
requirements for the machine (and dma controller) you're using.

James


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