netdev - RE: [PATCH] USBNET: fix handling padding packet

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B734A@saturn3.aculab.com>
Date:	Thu, 19 Sep 2013 09:29:07 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	Bjørn Mork <bjorn@...k.no>,
	"Oliver Neukum" <oneukum@...e.de>
Cc:	"Ming Lei" <ming.lei@...onical.com>,
	"David S. Miller" <davem@...emloft.net>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	"Network Development" <netdev@...r.kernel.org>,
	"linux-usb" <linux-usb@...r.kernel.org>
Subject: RE: [PATCH] USBNET: fix handling padding packet

> I wonder what the gain of that really is?  Yes, I can see the advantage
> of making the class drivers more effective.  But padding is only
> relevant for the ECM class, isn't it? And are there any ECM class
> devices where SG support matters?

AFAICT the requirement for avoiding ZLP is a property of the slave.
Support for scatter-gather is a property of the host.

So connect an old slave to a modern host and you may be able to use
sg to add the header or padding (and possibly have a fragmented skb).

Generating frames that would have a ZLP should just be a matter of
sending UDP frames with ever increasing lengths - although a
printf in the driver would confirm it.

However the is a report (somewhere) that it would work sometimes
and not others with certain targets.

I've got one of the ASIX USB3 devices here, but the system is running
ubuntu 12.04 - which doesn't have the sg changes. I've not looked far
enough to see if the sg changes in usbnet.c can be applied to that
kernel.

	David