linux-kernel - Re: Scatter-gather list constraints

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Date:	Mon, 23 Jun 2008 11:12:13 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	David Vrabel <david.vrabel@....com>
cc:	Kernel development list <linux-kernel@...r.kernel.org>,
	AntonioLin <antonio.lin@...ormicro.com>,
	<linux-usb@...r.kernel.org>,
	"Perez-Gonzalez, Inaky" <inaky.perez-gonzalez@...el.com>
Subject: Re: Scatter-gather list constraints

On Mon, 23 Jun 2008, David Vrabel wrote:

> Note that this 1024 byte multiple is for one particular WUSB mass
> storage device.  The WUSB standard permits max packet sizes of up 3584
> (in multiples of 512), but I suspect WUSB mass storage devices will only
> use 512, 1024, or 2048.
> 
> For a solution, we may be able to do something if the HWA host
> controller is passed a single URB with an s-g list (rather than one URB
> per s-g list entry) and was careful about how it segmented the URB into
> transfers to the rpipe.

That would be ideal.  However there is no way to pass an S-G list along 
with an URB; there's no field for it in the data structure.  And none 
of the existing host controller drivers support such a thing.

I suppose we could add a field to struct urb and add a flag indicating 
whether the controller driver supports S-G lists.

Alan Stern

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