netdev - Re: [PATCH] net: usb: ax88179_178a: fix packet alignment padding

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Message-ID: <ac9ac673933f0e8383c6ab538302058ba2469192.camel@ozlabs.org>
Date:   Wed, 17 Jun 2020 08:56:39 +0800
From:   Jeremy Kerr <jk@...abs.org>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, allan@...x.com.tw, freddy@...x.com.tw,
        pfink@...ist-es.de, linux-usb@...r.kernel.org, louis@...x.com.tw
Subject: Re: [PATCH] net: usb: ax88179_178a: fix packet alignment padding

Hi David,

> It seems logical to me that what the chip does is align up the total
> sub-packet length to a multiple of 4 or larger, and then add those two
> prefix padding bytes.  Otherwise the prefix padding won't necessarily
> and reliably align the IP header after the link level header.

Yep, that makes sense, and is what the driver is currently doing;
between clustered packets, the header is aligned (up) to 8 bytes, then
the 2-byte padding is added to that.

For this change, I have assumed that the packet length behaviour (ie,
describing the un-padded length) is consistent between clustered
packets.

[If you have any hints for forcing clustered packets, I'll see if I can
probe the behaviour a little better to confirm]

Cheers,


Jeremy