linux-kernel - Re: [PATCH v2] usb: gadget: ncm: Avoid dropping datagrams of properly parsed NTBs

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Date: Thu, 1 Feb 2024 11:14:47 +0530
From: Krishna Kurapati PSSNV <quic_kriskura@...cinc.com>
To: Maciej Żenczykowski <maze@...gle.com>
CC: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hardik Gajjar
	<hgajjar@...adit-jv.com>, <linux-usb@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <quic_ppratap@...cinc.com>,
        <quic_wcheng@...cinc.com>, <quic_jackp@...cinc.com>
Subject: Re: [PATCH v2] usb: gadget: ncm: Avoid dropping datagrams of properly
 parsed NTBs



On 2/1/2024 12:24 AM, Maciej Żenczykowski wrote:

> 
> I believe so, yes.
> 
> I believe they are trying to avoid having to send ZLPs.
> That's determined *purely* by the size of things as they show up on
> the usb cable (ie. the size of the usb xfer).
> ie. that's where things that are a multiple of 512 (USB2) or 1024
> (USB3) need an extra 0 byte sized packet to prevent ZLP.
> 
> The actual size of the NTB doesn't matter.
> 
> That said... maybe we're overcomplicating this...
> Maybe it's enough to just remove this modulo check entirely (I know I
> asked for it before).
> 
> Ultimately if we just do:
> 
> // Windows NCM driver avoids USB ZLPs by adding a 1-byte zero pad as needed
> if (to_process == 1 && !*(u8*)(ntb_ptr + block_len)) --to_process;
> 
> it'll fix the problem too, and perhaps be easier to understand?
> 
I agree. This will simplify the check and also cover all cases.
To keep diff minimal from tested version (because issue comes up easily 
with a particular test here), I can remove the second check in the if 
block in the v2 and push it as v3.

Regards,
Krishna,