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

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Message-Id: <201004092251.51739.oliver@neukum.org>
Date:	Fri, 9 Apr 2010 22:51:51 +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 Freitag, 9. April 2010 17:15:43 schrieb Alan Stern:
> > > Then usb_submit_urb(urb[i]) will copy the appropriate four bytes to a
> > > bounce buffer and map the bounce buffer.  Accesses to the other parts
> > > of xbuf won't violate the cacheline rules, because xbuf isn't mapped
> > > for DMA -- only the bounce buffer is.  When urb[i] completes, the
> > > bounce buffer contents will be copied back to the original four bytes
> > > in xbuf.  Again, there is no violation of cacheline rules.
> > 
> > I think you are assuming that either every or no part of the buffer is mapped
> > for DMA in place. I don't think you can assume that.
> 
> Yes I can, because the code that makes this decision is part of 
> usbcore and it is under m

It seems to me that in usbcore you can positively know that a buffer
will be mapped. However if the mapping is not done in usbcore you
cannot know what the HCD driver will do to a buffer, in particular
you don't know whether it will be processed by PIO or mapped for
DMA.

Maybe I understand this wrongly. Which code exactly were you refering to?

	Regards
		Oliver
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