linux-kernel - Re: [PATCH v1] net: cdc_ncm: update datagram size after changing mtu

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Date:	Wed, 06 Apr 2016 16:36:50 +0200
From:	Bjørn Mork <bjorn@...k.no>
To:	Robert Dobrowolski <robert.dobrowolski@...ux.intel.com>
Cc:	linux-usb@...r.kernel.org, rafal.f.redzimski@...el.com,
	stable@...r.kernel.org, oliver@...kum.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] net: cdc_ncm: update datagram size after changing mtu

Robert Dobrowolski <robert.dobrowolski@...ux.intel.com> writes:

> From: Rafal Redzimski <rafal.f.redzimski@...el.com>
>
> Current implementation updates the mtu size and notify cdc_ncm
> device using USB_CDC_SET_MAX_DATAGRAM_SIZE request about datagram
> size change instead of changing rx_urb_size.
>
> Whenever mtu is being changed, datagram size should also be
> updated.

Definitely!  Thanks for this.  But looking at the code I believe you
need to fix the calculation of maxmtu too.  It is currently:

  int maxmtu = ctx->max_datagram_size - cdc_ncm_eth_hlen(dev);

And cdc_ncm_set_dgram_size() updates ctx->max_datagram_size with the new
mtu, meaning that you can only reduce the mtu.  We should probably use
cdc_ncm_max_dgram_size() instead here.

And cdc_ncm_set_dgram_size() takes the datagram size with header as
input (ref the above maxmtu calucalution), so it probably needs to
called as

  cdc_ncm_set_dgram_size(dev, new_mtu + cdc_ncm_eth_hlen(dev));

to get it right.  I think.  None of this is tested on an actual device
yet...  Care to test if I'm right, and do a v2 if necessry?

> Cc: <stable@...r.kernel.org>

This should be dropped for net.  Ask David to queue it for stable
instead.  I usually do that by using a subject prefix like

 [PATCH net,stable v1] ...

Bjørn