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

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Message-Id: <201004082324.31481.oliver@neukum.org>
Date:	Thu, 8 Apr 2010 23:24:31 +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 Donnerstag, 8. April 2010 18:59:38 schrieb Alan Stern:
> On Thu, 8 Apr 2010, Oliver Neukum wrote:
> 
> > Am Mittwoch, 7. April 2010 17:46:17 schrieb Alan Stern:
> > > Or alternatively, instead of allocating regular memory the routine
> > > could simply fail.  Then the caller would be responsible for checking
> > > and using regular memory instead of dma-consistent memory.  Of course,
> > > that would put an even larger burden on the caller than just forcing it
> > > to keep track of what flag to use.
> > 
> > Then it would be sensible to pass it a filled URB, modify it or return
> > an error code.
> 
> That would work, but it doesn't match the way existing drivers use the
> interface.  For example, the audio driver allocates a 16-byte coherent
> buffer and then uses four bytes from it for each of four different 
> URBs.

That will not work with any fallback that does not yield a coherent buffer.

	Regards
		Oliver
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