netdev - Re: [PATCH 1/4] pch_gbe: Fix transmit queue management

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Date:	Fri, 29 Nov 2013 16:42:29 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	puzman@...il.com
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] pch_gbe: Fix transmit queue management

From: Ondrej Puzman <puzman@...il.com>
Date: Thu, 28 Nov 2013 10:44:39 +0100

> According to Documentation/networking/driver.txt the ndo_start_xmit method should not return NETDEV_TX_BUSY under normal circumstances.
> Stop the transmit queue when tx_ring is full.

Please format your commit message text to 80 columns.

> 
> Signed-off-by: Ondrej Puzman <puzman@...il.com>

You should be instead preventing the transmit method from being invoked
when it might be possible that a request cannot be satisfied.

This means that at the end of a transmit request, you must stop the
queue if a packet with the maximum number of possible descriptors
cannot be satisfied.  Then it is impossible for the transmit function
to be invoked in a situation where it would need to fail for lack of
available transmit descriptors.

This is why drivers decided whether to stop their TX queues based upon
calculations involving MAX_SKB_FRAGS.
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