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

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Date:	Mon, 11 Apr 2016 05:00:36 -0700 (PDT)
From:	"Robert Dobrowolski" <robert.dobrowolski@...ux.intel.com>
To:	BjÃ¸rn Mork <bjorn@...k.no>
Cc:	"Robert Dobrowolski" <robert.dobrowolski@...ux.intel.com>,
	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] ...
>
>
>
>
> BjÃ¸rn
>

ok, thanks for feedback I will send v2 patch