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

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Date:	Tue, 11 May 2010 10:24:40 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	alsa-devel@...a-project.org, linux-usb@...r.kernel.org,
	tiwai@...e.de, gregkh@...e.de, clemens@...isch.de,
	linux-kernel@...r.kernel.org, chrisw@...s-sol.org,
	iommu@...ts.linux-foundation.org, andi@...stfloor.org,
	daniel@...aq.de, pedrib@...il.com, akpm@...ux-foundation.org,
	dwmw2@...radead.org
Subject: Re: [alsa-devel] USB transfer_buffer allocations on 64bit systems

> > > 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)'
(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?
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