netdev - RE: PROBLEM: usbnet / ax88179_178a: Panic in usb_hcd_map_urb_for

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Date:	Mon, 13 Jan 2014 13:26:17 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Bjørn Mork' <bjorn@...k.no>
CC:	Thomas Kear <thomas@...r.co.nz>,
	Ben Hutchings <bhutchings@...arflare.com>,
	netdev <netdev@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
Subject: RE: PROBLEM: usbnet / ax88179_178a: Panic in usb_hcd_map_urb_for_dma

From: Bjørn Mork [mailto:bjorn@...k.no]
> David Laight <David.Laight@...LAB.COM> writes:
> > From: Bjørn Mork
> >> David Laight <David.Laight@...LAB.COM> writes:
> >>
> >> > The patch you submitted is wrong.
> >> > Whoever wrote the sg interface was on crack.
> >> > The 'last' marker needs moving as well.
> >>
> >> I'm afraid I don't understand what you meant by this.
> >>
> >> sg_init_table() set the 'last' marker.  AFAICS, you don't need to change
> >> it unless you want to chain lists.
> >>
> >> Care to explain with some code?
> >
> > Just assuming that there will be some code, somewhere, that will try
> > to process the entire sg list - so won't like the entry with a
> > NULL pointer and zero length at the end.
> >
> > If all the places that process the list are given an explicit
> > number of entries, or don't care about the NULL it doesn't matter.
> 
> I believe all processing use the urb->num_sgs field to limit the number
> of entries.  Common interfaces like dma_map_sg() and for_each_sg() limit
> their processing to "nents" entries, and the USB code use the value of
> urb->num_sgs for this parameter.

Which mostly means that the sg_xxx functions are doing a whole load
of unnecessary instructions and memory accesses...

This probably has a lot to do with the significant difference in the
cpu use for the usb3 and 'normal' ethernet interfaces.

While each bit doesn't seem significant, they soon add up.

	David