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Date:	Thu, 30 Jan 2014 22:50:21 +0100
From:	Bjørn Mork <>
To:	Sarah Sharp <>
Cc:	David Laight <David.Laight@...LAB.COM>,
	"linux-usb\" <>,
	"netdev\" <>,
	Greg Kroah-Hartman <>,
	David Miller <>,
	Dan Williams <>,
	"Nyman\, Mathias" <>,
	Mark Lord <>,
	Alan Stern <>,
	Freddy Xin <>
Subject: Re: [PATCH RFC 1/1] usb: Tell xhci when usb data might be misaligned

FWIW, the plan looks fine to me.  Just adding a couple of hints to
simplify the implementation.

Sarah Sharp <> writes:

> Let's do this fix the right way, instead of wall papering over the
> issue.  Here's what we should do:
> 1. Disable scatter-gather for the ax88179_178a driver when it's under an
>    xHCI host.

No need to make this conditional.  SG is only enabled in the
ax88179_178a driver if udev->bus->no_sg_constraint is true, so it
applies only to xHCI hosts in the first place.

> 2. Revert the following commits:
>    f2d9b991c549 xhci: Set scatter-gather limit to avoid failed block writes.
>    d6c9ea9069af xhci: Avoid infinite loop when sg urb requires too many trbs
>    35773dac5f86 usb: xhci: Link TRB must not occur within a USB payload burst
> 3. Dan and Mathias can work together to come up with an overall plan to
>    change the xHCI driver architecture to be fully compliant with the TD
>    fragment rules.  That can be done over the next few kernel releases.
> The end result is that we don't destabilize storage or break userspace
> USB drivers, we don't break people's xHCI host controllers,
> the ax88179_178a USB ethernet devices still work under xHCI (a bit with
> worse performance), and other USB ethernet devices still get the
> performance improvement introduced in 3.12.

No other usbnet drivers has enabled SG...  Which is why you have only
seen this problem with the ax88179_178a devices.  So there is no
performance improvement to keep.

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