linux-kernel - Re: [PATCH 1/3 v3] dma: document dma_{un}map_{single|sg}

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Message-ID: <20080312011954.GN12370@sgi.com>
Date:	Tue, 11 Mar 2008 18:19:54 -0700
From:	akepner@....com
To:	Michael Ellerman <michael@...erman.id.au>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	Grant Grundler <grundler@...isc-linux.org>,
	Tony Luck <tony.luck@...el.com>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Jes Sorensen <jes@....com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Roland Dreier <rdreier@...co.com>,
	David Miller <davem@...emloft.net>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-kernel@...r.kernel.org, Mark Nelson <mdnelson@....ibm.com>
Subject: Re: [PATCH 1/3 v3] dma: document dma_{un}map_{single|sg}_attrs() interface


I have a new proposal for the documentation portion of this 
patchset. The code changes essentially amount to 
s/DMA_ATTR_SYNC_ON_WRITE/DMA_ATTR_BARRIER/ so thought I'd send 
just the doc change for comments now.

The description of the DMA_ATTR_BARRIER is now very short and 
generic - no particular architecture is even mentioned. I can add 
a sentence or two near the architecture-specific changes in 
arch/ia64/sn/pci/pci_dma.c about why the implementation works 
on ia64/sn, etc.

diff --git a/Documentation/DMA-attributes.txt b/Documentation/DMA-attributes.txt
index e69de29..a4106ec 100644
--- a/Documentation/DMA-attributes.txt
+++ b/Documentation/DMA-attributes.txt
@@ -0,0 +1,24 @@
+			DMA attributes
+			==============
+
+This document describes the semantics of the DMA attributes that are 
+defined in linux/dma-attrs.h. 
+
+DMA_ATTR_BARRIER
+----------------
+
+DMA_ATTR_BARRIER is a barrier attribute for DMA.  DMA to a memory 
+region with the DMA_ATTR_BARRIER attribute forces all pending DMA 
+writes to complete, and thus provides a mechanism to strictly order 
+DMA from a device across all intervening busses and bridges.  This 
+barrier is not specific to a particular type of interconnect, it 
+applies to the system as a whole, and so its implementation must 
+account for the idiosyncracies of the system all the way from the 
+DMA device to memory. 
+
+As an example of a situation where DMA_ATTR_BARRIER would be useful, 
+suppose that a 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.  Mapping the memory used for completion indications 
+with DMA_ATTR_BARRIER would prevent the race.
+

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