netdev - Re: [PATCH v1 1/4] USB: introduce usb_device_no_sg

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Date:	Sat, 3 Aug 2013 11:53:18 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Ming Lei <ming.lei@...onical.com>
cc:	"David S. Miller" <davem@...emloft.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Oliver Neukum <oneukum@...e.de>,
	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	<netdev@...r.kernel.org>, <linux-usb@...r.kernel.org>
Subject: Re: [PATCH v1 1/4] USB: introduce usb_device_no_sg_limit() helper

On Sat, 3 Aug 2013, Ming Lei wrote:

> Some host controllers(such as xHCI) can support building
> packet from discontinuous buffers, so introduce one flag
> and helper for this kind of host controllers, then the
> feature can help some applications(such as, usbnet) by
> supporting arbitrary length of sg buffers.

> diff --git a/include/linux/usb.h b/include/linux/usb.h
> index 84f14e2..5d03074 100644
> --- a/include/linux/usb.h
> +++ b/include/linux/usb.h
> @@ -337,6 +337,7 @@ struct usb_bus {
>  					 * the ep queue on a short transfer
>  					 * with the URB_SHORT_NOT_OK flag set.
>  					 */
> +	unsigned no_sg_limit:1;		/* no sg list limit */

Why do you call this "no_sg_limit"?  It isn't a limit on the SG list; 
the list can be arbitrarily long, provided all the entries except the 
last are divisible by the maxpacket size.

You could call it "no_sg_constraint" if you want.  Or 
"allow_arbitrary_sg", to put a more positive spin on it.

Alan Stern

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