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To: David Brownell <david-b@pacbell.net>
cc: James Bottomley <James.Bottomley@SteelEye.com>,
       linux-kernel@vger.kernel.org
Subject: Re: [RFT][PATCH] generic device DMA implementation 
In-Reply-To: Message from David Brownell <david-b@pacbell.net> 
   of "Fri, 27 Dec 2002 17:29:54 PST." <3E0CFE92.7060902@pacbell.net> 
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From: James Bottomley <James.Bottomley@steeleye.com>
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david-b@pacbell.net said:
> The indirection is getting from the USB device (or interface) to the
> object representing the USB controller.  All USB calls need that, at
> least for host-side APIs, since the controller driver is multiplexing
> up to almost 4000 I/O channels.  (127 devices * 31 endpoints, max; and
> of course typical usage is more like dozens of channels.) 

This sounds like a mirror of the problem of finding the IOMMU on parisc (there 
can be more than one).

The way parisc solves this is to look in dev->platform_data and if that's null 
walk up the dev->parent until the IOMMU is found and then cache the IOMMU ops 
in the current dev->platform_data.  Obviously, you can't use platform_data, 
but you could use driver_data for this.  The IOMMU's actually lie on a parisc 
specific bus, so the ability to walk up the device tree without having to know 
the device types was crucial to implementing this.

James


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