netdev - Many USB ethernet devices are broken over xhci

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D46624F@AcuExch.aculab.com>
Date:	Mon, 27 Jan 2014 16:06:22 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	Sarah Sharp <sarah.a.sharp@...ux.intel.com>
Subject: Many USB ethernet devices are broken over xhci

Many of the net/usb ethernet drivers (including common ones like
the smsc95xx) will fail to transmit all packet length properly
when connected to a USB3 port (ie using the xhci driver).

The underlying problem is that they assume the host controller
will honour the URB_ZERO_PACKET flag - which usbnet sets because
they specify FLAG_SEND_ZLP.

However no one has ever added support for URB_ZERO_PACKET to
the xhci driver - so packets that need padding (probably 512
bytes after any header is added) will not be sent correctly
and may have very adverse effects on the usb target.

The ax179_178a driver avoids this by not setting FLAG_SEND_ZLP,
modifying the packet header, and appending an extra zero byte.
(Which has been responsible for its own set of panics.)

I don't think this can be fixed by just clearing (or ignoring)
FLAG_SEND_ZLP as the extra byte will also confuse things.

It needs to be fixed in the xhci code.

I wrote this patch a while ago - worked for me with the ax179_178a
driver. http://www.spinics.net/lists/linux-usb/msg97370.html

The patch is a bit difficult to read, the v1 version contained a copy of
the new function. http://www.spinics.net/lists/linux-usb/msg97183.html

I don't think anything significant has changed (in the main kernel
sources) since I wrote the patch.

	David

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