linux-kernel - Re: [3/4] dma: document dma_flags_set

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Date:	Thu, 27 Sep 2007 21:33:26 -0600
From:	Grant Grundler <grundler@...isc-linux.org>
To:	akepner@....com
Cc:	Grant Grundler <grundler@...isc-linux.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Jes Sorensen <jes@....com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	RolandDreier <rdreier@...co.com>,
	James Bottomley <James.Bottomley@...eleye.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [3/4] dma: document dma_flags_set_dmabarrier()

On Thu, Sep 27, 2007 at 06:13:02PM -0700, akepner@....com wrote:
> 
> Document dma_flags_set_dmabarrier().
> 
> Signed-off-by: Arthur Kepner <akepner@....com>

This looks really good!

thanks,
grant

Acked-by: Grant Grundler <grundler@...isc-linux.org>

> 
> ---
>  DMA-API.txt |   26 ++++++++++++++++++++++++++
>  1 files changed, 26 insertions(+)
> 
> diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt
> index cc7a8c3..5fc0bba 100644
> --- a/Documentation/DMA-API.txt
> +++ b/Documentation/DMA-API.txt
> @@ -544,3 +544,29 @@ size is the size (and should be a page-sized multiple).
>  The return value will be either a pointer to the processor virtual
>  address of the memory, or an error (via PTR_ERR()) if any part of the
>  region is occupied.
> +
> +int 
> +dma_flags_set_dmabarrier(int dir)
> +
> +Amend dir (one of the enum dma_data_direction values), with a 
> +platform-specific "dmabarrier" attribute.  The dmabarrier attribute 
> +forces a flush of all in-flight DMA when the associated memory 
> +region is written to (see example below.)
> +
> +This provides a mechanism to enforce ordering of DMA on platforms that 
> +permit DMA to be reordered between device and host memory (within a 
> +NUMA interconnect).  On other platforms this is a nop.
> +
> +The dmabarrier would be set when the memory region is mapped for DMA, 
> +e.g.:
> +
> +	int count, flags = dma_flags_set_dmabarrier(DMA_BIDIRECTIONAL);
> +	....
> +	count = dma_map_sg(dev, sglist, nents, flags);
> +
> +As an example of a situation where this would be useful, suppose that 
> +the device does a DMA write to indicate that data is ready and 
> +available in memory.  The DMA of the "completion indication" could 
> +race with data DMA.  Using a dmabarrier on the memory used for 
> +completion indications would prevent the race.
> +
-
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