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

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Message-Id: <20100511233852B.fujita.tomonori@lab.ntt.co.jp>
Date:	Tue, 11 May 2010 23:38:35 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	konrad.wilk@...cle.com
Cc:	stern@...land.harvard.edu, 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

On Tue, 11 May 2010 10:24:40 -0400
Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> 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)'
> (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?

Or calling dma_unmap_single.

Can you tell me all the exact process of DMA that the usb core and the
driver do?
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