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Date:	Sun, 9 May 2010 13:00:49 +0900
From:	FUJITA Tomonori <>
Subject: [PATCH -mm] Documentation: add networking driver's mapping error
 handling to DMA-API-HOWTO

I think that it's a good idea to add what drivers should to do exactly
in the DMA mapping failure case.

Seems some networking drivers call dev_kfree_skb() and return
NETDEV_TX_OK, however, some return NETDEV_TX_BUSY without freeing the
skb. (and many don't even check the failure, which we need to fix).

What networking drivers are supposed to do in the case?

From: FUJITA Tomonori <>
Subject: [PATCH -mm] Documentation: add networking driver's mapping error handling to DMA-API-HOWTO

Adds the concrete DMA mapping error handling for Networking drivers on
the transmit path.

Signed-off-by: FUJITA Tomonori <>
 Documentation/DMA-API-HOWTO.txt |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/Documentation/DMA-API-HOWTO.txt b/Documentation/DMA-API-HOWTO.txt
index 52618ab..8c3235c 100644
--- a/Documentation/DMA-API-HOWTO.txt
+++ b/Documentation/DMA-API-HOWTO.txt
@@ -740,6 +740,11 @@ failure can be determined by:
+Networking drivers must call dev_kfree_skb to free the socket buffer
+and return NETDEV_TX_OK if the DMA mapping fails on the transmit hook
+(ndo_start_xmit). This means that the socket buffer is just dropped in
+the failure case.
 This document, and the API itself, would not be in it's current

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