linux-kernel - Re: Scatter-gather list constraints

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Message-ID: <485FF44C.4010600@csr.com>
Date:	Mon, 23 Jun 2008 20:06:52 +0100
From:	David Vrabel <david.vrabel@....com>
To:	Alan Stern <stern@...land.harvard.edu>
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

Alan Stern wrote:
> 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.

This is what I was thinking.

Can the number of entries in a sg list be limited?  e.g., if the
hardware only had support for say, 64 entries?

David
-- 
David Vrabel, Senior Software Engineer, Drivers
CSR, Churchill House, Cambridge Business Park,  Tel: +44 (0)1223 692562
Cowley Road, Cambridge, CB4 0WZ                 http://www.csr.com/
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