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

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Message-Id: <20100512003450P.fujita.tomonori@lab.ntt.co.jp>
Date:	Wed, 12 May 2010 00:34:27 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	stern@...land.harvard.edu
Cc:	fujita.tomonori@....ntt.co.jp, alsa-devel@...a-project.org,
	gregkh@...e.de, konrad.wilk@...cle.com, tiwai@...e.de,
	linux-usb@...r.kernel.org, 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 11:04:55 -0400 (EDT)
Alan Stern <stern@...land.harvard.edu> wrote:

> 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.

How urb->transfer_buffer is allocated?


> 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);

The mapping error is checked via dma_mapping_error? Even if an mapping
error happens, no data corruption happens due to that (needs
something like retrying the request)?


> 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);

The driver does only DMA_TO_DEVICE? Or you see DMA problems only with
DMA_TO_DEVICE?
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