linux-kernel - Re: [alsa-devel] USB transfer_buffer allocations on 64bit systems

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Date:	Tue, 11 May 2010 10:57:07 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	USB list <linux-usb@...r.kernel.org>, <tiwai@...e.de>,
	<gregkh@...e.de>, <clemens@...isch.de>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	<chrisw@...s-sol.org>, <iommu@...ts.linux-foundation.org>,
	<andi@...stfloor.org>, <daniel@...aq.de>, <pedrib@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	<dwmw2@...radead.org>
Subject: Re: [alsa-devel] USB transfer_buffer allocations on 64bit systems

On Tue, 11 May 2010, Konrad Rzeszutek Wilk wrote:

> > > > Either the data isn't getting written to the buffer correctly or else
> > > > the buffer isn't getting sent to the device correctly.  Can anybody
> > > > suggest a means of determining which is the case?
> > > 
> > > I can't say anything about this log that including only DMA addresses.
> > > I'm not familiar with how the USB core does DMA stuff. And the USB
> > > stack design that the USB core does DMA stuff (allocating, mappings,
> > > etc) makes debugging DMA issues really difficult.
> > 
> > The DMA stuff is simple enough in this case.  The urb->transfer_buffer
> > address is passed to dma_map_single(), and the DMA address it returns
> > is stored in urb->transfer_dma.  Those are the two values printed out
> > by the debugging patch.
> 
> Is that address (urb->transfer_dma) the same as 
'virt_to_phys(urb->transfer_buffer)'

I don't know.  We didn't print out the value of
virt_to_phys(urb->transfer_buffer).  All I can say is that
urb->transfer_dma is the value returned by dma_map_single().

> (if not, then SWIOTLB is being utilized) and is the dma_sync_* done on the
> urb->transfer_dma (to properly sync the data from the SWIOTLB to the
> transfer_buffer) before you start using the urb->transfer_buffer?

There are no calls to any dma_sync_* routines.  Daniel will have to
check me on this, but I believe that urb->transfer_buffer is filled
before dma_map_single() is called and it isn't touched again until
after dma_unmap_single() (which occurs before urb's completion handler
is called).

Alan Stern

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