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

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Message-ID: <CACVXFVOGxWe5+o0hLROiPN43OCDekV2Ovz1yXr95H1m+yHGn4w@mail.gmail.com>
Date:	Thu, 16 Jan 2014 09:28:49 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	David Laight <David.Laight@...lab.com>
Cc:	Bjørn Mork <bjorn@...k.no>,
	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

On Mon, Jan 13, 2014 at 9:26 PM, David Laight <David.Laight@...lab.com> wrote:
>>
>> 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.

If you plan to remove the 'nents' parameter, I am wondering if it is
a good idea, because sg_nents() should be more heavy. Not mention
sometimes the callers just want to map/unmap part of entries.


Thanks,
-- 
Ming Lei
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