linux-kernel - Re: [PATCH 2/2] net: cdc_ncm: respect operator preferred MTU reported by MBIM

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Message-ID: <CAC5Y2nMdPM8w3=dgd2as5z8huL1XLPjon8wSmR-2e3=JGWtCpA@mail.gmail.com>
Date:	Mon, 17 Mar 2014 17:46:27 -0700
From:	Ben Chan <benchan@...omium.org>
To:	Bjørn Mork <bjorn@...k.no>
Cc:	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
	netdev@...r.kernel.org, Oliver Neukum <oliver@...kum.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Greg Suarez <gsuarez@...thmicro.com>
Subject: Re: [PATCH 2/2] net: cdc_ncm: respect operator preferred MTU reported
 by MBIM

On Mon, Mar 17, 2014 at 2:27 PM, Bjørn Mork <bjorn@...k.no> wrote:

>
> This sounds all reasonable to me. Thanks for taking the time to explain
> it in such detail. I did know that some vendors set wMaxSegmentSize too
> low, but had no idea that vendors were using the extended descriptor
> instead of MBIM_CID_IP_CONFIGURATION.
>
> If so, then yes, it does make sense for the driver to base the default
> MTU on this descriptor.

Hopefully, subsequent MBIM specifications would further clarify and
simplify things a bit, but it's gonna be a slow process as we all know
:-/

>
>>> wMTU access needs le16_to_cpu.
>>
>> Good catch. I will fix it in patch v2.
>
> Tip: I found this because my test script/makefile includes
> "C=1 CF=-D__CHECK_ENDIAN__"
>
> I find that very useful when dealing with USB on a little endian system,
> like most of us have.  It's all too easy to miss a conversion otherwise.
>

Thanks again for the review and tip. I've submitted patch v2 to
address the le16_to_cpu conversion.


>>> Could we move this final MTU correction from cdc_ncm_setup to
>>> cdc_ncm_bind_common to avoid bloating the device struct with another
>>> descriptor pointer we donæt really need to keep around?
>>>
>>> I know we look into descriptors in cdc_ncm_setup, because we have to,
>>> but ideally I would have loved to see cdc_ncm_setup dealing with *just*
>>> the NCM/MBIM specific control setup messages and cdc_ncm_bind_common
>>> dealing with all the functional descriptors.  That seems most logic to
>>> me (but is of course only my personal opinion and nothing else - I do
>>> not know what the original cdc_ncm author intended)
>>>
>>
>> I understand the argument against the extra descriptor pointer. But I
>> think it's better to keep the mtu related code together so that one
>> can easily see how MTU is determined when trying to change or refactor
>> the code. I haven't looked into what cdc_ncm_setup was originally
>> intended for. If we'd like to avoid adding an extra pointer in
>> cdc_ncm_ctx, we could have cdc_ncm_bind passing a locally scoped
>> context to cdc_ncm_setup.
>
> No, the extra pointer doesn't matter much. Just keep it as it is.
>
>
> Bjørn
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