linux-kernel - Re: [PATCH 0/6] get rid of on-stack dma buffers

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Message-Id: <20110322.131552.104076400.davem@davemloft.net>
Date:	Tue, 22 Mar 2011 13:15:52 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	James.Bottomley@...senPartnership.com
Cc:	florian@...kler.org, awalls@...metrocast.net,
	mchehab@...radead.org, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org, js@...uxtv.org, tskd2@...oo.co.jp,
	liplianin@...by, g.marco@...enet.de, aet@...terburn.org,
	pb@...uxtv.org, mkrufky@...uxtv.org, nick@...k-andrew.net,
	max@...eto.com, janne-dvb@...nau.be, oliver@...kum.org,
	greg@...ah.com, rjw@...k.pl, joerg.roedel@....com
Subject: Re: [PATCH 0/6] get rid of on-stack dma buffers

From: James Bottomley <James.Bottomley@...senPartnership.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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