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

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Message-ID: <Pine.LNX.4.44L0.1005111057540.1834-100000@iolanthe.rowland.org>
Date:	Tue, 11 May 2010 11:04:55 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
cc:	konrad.wilk@...cle.com, <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, FUJITA Tomonori wrote:

> 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?

1. The audio driver stores data in urb->transfer_buffer.

2. The audio driver calls usb_submit_urb(urb).

3. The USB core does
		urb->transfer_dma = dma_map_single(controller, 
			urb->transfer_buffer,
			urb->transfer_buffer_length,
			DMA_TO_DEVICE);

4. The host controller driver tells the hardware to carry out the data 
   transfer.

5. When the hardware says the transfer is finished, the USB core does
		dma_unmap_single(controller,
			urb->transfer_dma,
			urb->transfer_buffer_length,
			DMA_TO_DEVICE);

6. The audio driver's completion handler is called:
		(urb->complete)(urb);

Alan Stern

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