linux-kernel - Re: USB transfer_buffer allocations on 64bit systems

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Message-Id: <201004071854.55530.oliver@neukum.org>
Date:	Wed, 7 Apr 2010 18:54:55 +0200
From:	Oliver Neukum <oliver@...kum.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Daniel Mack <daniel@...aq.de>, linux-kernel@...r.kernel.org,
	Pedro Ribeiro <pedrib@...il.com>, akpm@...ux-foundation.org,
	Greg KH <gregkh@...e.de>, alsa-devel@...a-project.org,
	linux-usb@...r.kernel.org
Subject: Re: USB transfer_buffer allocations on 64bit systems

Am Mittwoch, 7. April 2010 16:59:47 schrieb Alan Stern:
> > The fix is to use usb_buffer_alloc() for that purpose which ensures
> > memory that is suitable for DMA. And on x86_64, this also means that the
> > upper 32 bits of the address returned are all 0's.
> 
> That is not a good fix.  usb_buffer_alloc() provides coherent memory, 
> which is not what we want.  I believe the correct fix is to specify the 
> GFP_DMA32 flag in the kzalloc() call.
> 
> Of course, some EHCI hardware is capable of using 64-bit addresses.  
> But not all, and other controller types aren't.  In principle we could
> create a new allocation routine, which would take a pointer to the USB
> bus as an additional argument and use it to decide whether the memory 
> needs to lie below 4 GB.  I'm not sure adding this extra complexity 
> would be worthwhile.

What about XHCI? Do you really want to limit it to 32bits?

	Regards
		Oliver
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