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

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D0F6D78EA@AcuExch.aculab.com>
Date:	Fri, 7 Mar 2014 15:56:49 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Alan Stern' <stern@...land.harvard.edu>
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"

From: Alan Stern 
> 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.

I was reading patch 2 first.
It would seem to be better to modify patch 1 - so it doesn't set the flag.
Then the changes are limited to the usb tree, and the change to net wouldn't
need to be reapplied in order to test scatter-gather when it is properly fixed.

	David



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