netdev - [PATCH net-next] netdevice: document NETDEV_TX

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Message-ID: <87pp7l5t8a.fsf@rustcorp.com.au>
Date:	Fri, 03 Apr 2015 22:17:17 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	David Miller <davem@...emloft.net>,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Cc:	Stephen Hemminger <stephen@...workplumber.org>
Subject: [PATCH net-next] netdevice: document NETDEV_TX_BUSY deprecation.

This paraphrases DaveM (and steals some of his words) explaining why
a device shouldn't return NETDEV_TX_BUSY, even though it looks so inviting
to driver authors.

See http://www.spinics.net/lists/netdev/msg322350.html

Inspired-by: David Miller <davem@...emloft.net>
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index dcf6ec27739b..a2cad44b8630 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -795,7 +795,10 @@ typedef u16 (*select_queue_fallback_t)(struct net_device *dev,
  * netdev_tx_t (*ndo_start_xmit)(struct sk_buff *skb,
  *                               struct net_device *dev);
  *	Called when a packet needs to be transmitted.
- *	Must return NETDEV_TX_OK , NETDEV_TX_BUSY.
+ *	Returns NETDEV_TX_OK.  Can return NETDEV_TX_BUSY, but you should stop
+ *	the queue before that can happen; it's for obsolete devices and weird
+ *	corner cases, but the stack really does a non-trivial amount
+ *	of useless work if you return NETDEV_TX_BUSY.
  *        (can also return NETDEV_TX_LOCKED iff NETIF_F_LLTX)
  *	Required can not be NULL.
  *
--
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