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

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Date:	Mon, 13 Jan 2014 13:25:26 +0100
From:	Bjørn Mork <bjorn@...k.no>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Thomas Kear <thomas@...r.co.nz>,
	Ben Hutchings <bhutchings@...arflare.com>,
	netdev <netdev@...r.kernel.org>,
	"linux-usb\@vger.kernel.org" <linux-usb@...r.kernel.org>
Subject: Re: PROBLEM: usbnet / ax88179_178a: Panic in usb_hcd_map_urb_for_dma

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.


Bjørn
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