Date: Tue, 22 Mar 2011 13:15:52 -0700 (PDT)
Message-Id: <20110322.131552.104076400.davem@davemloft.net>
To: James.Bottomley@HansenPartnership.com
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Subject: Re: [PATCH 0/6] get rid of on-stack dma buffers
From: David Miller <davem@davemloft.net>
In-Reply-To: <1300800904.3290.7.camel@mulgrave.site>
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	<20110321220315.7545a61a@schatten.dmk.lab>
	<1300800904.3290.7.camel@mulgrave.site>
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From: James Bottomley <James.Bottomley@HansenPartnership.com>
Date: Tue, 22 Mar 2011 08:35:04 -0500

> The API will round up so that the correct region covers the API.
> However, if you have other structures packed into the space (as very
> often happens on stack), you get cache line interference in the CPU if
> they get accessed:  The act of accessing an adjacent object pulls in
> cache above your object and destroys DMA coherence.  This is the
> principle reason why DMA to stack is a bad idea.

Another major real reason we can't DMA on-stack stuff is because the
stack is mapped virtually on some platforms.

And that is the original reason the restriction was put in place.
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