netdev - Re: [PATCH net 2/3] net: cdc_mbim: send ZLP after max sized NTBs

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Message-ID: <CAAzmmD4ebn6p_xojieGkk5dzyVfhDizfcneUAUL=ZUzucH2nww@mail.gmail.com>
Date:	Tue, 22 Jan 2013 19:16:34 +0200
From:	Yauheni Kaliuta <yauheni@...iuta.org>
To:	Bjørn Mork <bjorn@...k.no>
Cc:	netdev@...r.kernel.org, linux-usb@...r.kernel.org,
	Greg Suarez <gsuarez@...thmicro.com>,
	Alexey Orishko <alexey.orishko@...ricsson.com>,
	Oliver Neukum <oneukum@...e.de>
Subject: Re: [PATCH net 2/3] net: cdc_mbim: send ZLP after max sized NTBs

Hi!

On Tue, Jan 22, 2013 at 12:01 AM, Bjørn Mork <bjorn@...k.no> wrote:
> Yauheni Kaliuta <y.kaliuta@...il.com> writes:
>>>>>>> "BM" == Bjørn Mork writes:
>>
>>  > We normally avoid sending ZLPs by padding NTBs with a zero byte
>>  > if the NTB is shorter than dwNtbOutMaxSize, resulting in a short
>>  > USB packet instead of a ZLP.  But in the case where the NTB length
>>  > is exactly dwNtbOutMaxSize and this is an exact multiplum of
>>  > wMaxPacketSize, then we must send a ZLP.
>>
>> The idea of NCM was to avoid extra ZLPs. If your transfer is exactly
>> dwNtbOutMaxSize, it's known, you can submit such request on the receiver
>> side and you do not need any EOT indicatation, so the frametime can be
>> used for useful data.
>
> Yes, that makes sense.  And I understand that both you and Alexey are of
> this opinion.
>
> But this idea is by no means made clear (to me) in the spec.  I do not
> think the current wording is precise enough to expect every implementor
> to understand any such intent.  The only place I find either "short
> packet" or ZLP mentioned in the NCM spec is in tables 3-1 and 3-2,
> describing the 16bit and 32bit NTH formats, in the description of the
> (d)wBlockLength fields.  And even there it is only mentioned in the
> context of the special (d)wBlockLength = 0x0000 handling.

I agree, it could be a bit unclear,

"
If exactly dwNtbInMaxSize or dwNtbOutMaxSize
bytes are sent, and the size is a multiple of wMax-
PacketSize for the given pipe, then no ZLP shall be
sent.
"
is an independent clause.

>
> If the intent was to prevent ZLPs, then it would have been wise to write
> that in the NCM standard. As it stands you have to use a lot of
> imagination to read that intent into the current spec.

Well, I hope the guys took your complains into account and will fix
the wording in the future spec versions.
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