linux-kernel - RE: [PATCH 2/2] Revert "USBNET: ax88179_178a: enable tso if usb host supports sg dma"

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Message-ID: <Pine.LNX.4.44L0.1403071016450.1246-100000@iolanthe.rowland.org>
Date:	Fri, 7 Mar 2014 10:18:15 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	David Laight <David.Laight@...LAB.COM>
cc:	'Mathias Nyman' <mathias.nyman@...ux.intel.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"sarah.a.sharp@...ux.intel.com" <sarah.a.sharp@...ux.intel.com>,
	stable <stable@...r.kernel.org>
Subject: RE: [PATCH 2/2] Revert "USBNET: ax88179_178a: enable tso if usb host
 supports sg dma"

On Fri, 7 Mar 2014, David Laight wrote:

> From: Mathias Nyman
> > This reverts commit 3804fad45411b48233b48003e33a78f290d227c8.
> > 
> > This commit, together with commit 247bf557273dd775505fb9240d2d152f4f20d304
> > "xhci 1.0: Limit arbitrarily-aligned scatter gather." were
> > origially added to get xHCI 1.0 hosts and usb ethernet ax88179_178a devices
> > working together with scatter gather. xHCI 1.0 hosts pose some requirement on how transfer
> > buffers are aligned, setting this requirement for 1.0 hosts caused USB 3.0 mass
> > storage devices to fail more frequently.
> 
> This patch doesn't need to be reverted.

Yes, it does.

> Provided the xhci driver doesn't set the flag to say that arbitrary scatter
> gather is supported (ie usb_device_no_sg_constraint(dev->udev)) is false)
> the ax88179_178a driver won't request transmits that need fragmenting.

True.  But xhci-hcd _will_ set the flag, because of patch 1 in this 
series.  In other words, patch 1 makes patch 2 necessary.

Alan Stern

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