linux-kernel - Re: [PATCH 2/3] define ARM-specific dma_coherent_write

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Message-ID: <1315319837.2313.1.camel@deneb.redhat.com>
Date:	Tue, 06 Sep 2011 10:37:15 -0400
From:	Mark Salter <msalter@...hat.com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	ming.lei@...onical.com, stern@...land.harvard.edu
Subject: Re: [PATCH 2/3] define ARM-specific dma_coherent_write_sync

On Tue, 2011-09-06 at 15:32 +0100, Catalin Marinas wrote:
> That's what mb() and wmb() do already, at least on ARM. Why do we need
> another API? IIRC from past discussions on linux-arch around barriers,
> the mb() should be sufficient in the case of DMA coherent buffers.
> That's why macros like writel() on ARM have the mb() added by default
> (for cases where you start the DMA transfer by writing to a device
> register). 

For USB EHCI, the driver does not necessarily write to a register after
writing to DMA coherent memory. In some cases, the controller polls for
information written by the driver.

--Mark


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