netdev - Re: [PATCH net,stable] net: cdc_ncm: avoid changing RX/TX buffers on MTU changes

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Message-Id: <20151223.225240.1999351612611929837.davem@davemloft.net>
Date:	Wed, 23 Dec 2015 22:52:40 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	bjorn@...k.no
Cc:	netdev@...r.kernel.org, linux-usb@...r.kernel.org,
	mrkiko.rs@...il.com
Subject: Re: [PATCH net,stable] net: cdc_ncm: avoid changing RX/TX buffers
 on MTU changes

From: Bjørn Mork <bjorn@...k.no>
Date: Wed, 23 Dec 2015 13:42:43 +0100

> NCM buffer sizes are negotiated with the device independently of
> the network device MTU.  The RX buffers are allocated by the
> usbnet framework based on the rx_urb_size value set by cdc_ncm. A
> single RX buffer can hold a number of MTU sized packets.
> 
> The default usbnet change_mtu ndo only modifies rx_urb_size if it
> is equal to hard_mtu.  And the cdc_ncm driver will set rx_urb_size
> and hard_mtu independently of each other, based on dwNtbInMaxSize
> and dwNtbOutMaxSize respectively. It was therefore assumed that
> usbnet_change_mtu() would never touch rx_urb_size.  This failed to
> consider the case where dwNtbInMaxSize and dwNtbOutMaxSize happens
> to be equal.
> 
> Fix by implementing an NCM specific change_mtu ndo, modifying the
> netdev MTU without touching the buffer size settings.
> 
> Signed-off-by: Bjørn Mork <bjorn@...k.no>

Applied and queued up for -stable, thanks!
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