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

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Message-ID: <20100511160923.GZ30801@buzzloop.caiaq.de>
Date:	Tue, 11 May 2010 18:09:23 +0200
From:	Daniel Mack <daniel@...aq.de>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>, gregkh@...e.de,
	konrad.wilk@...cle.com, tiwai@...e.de,
	USB list <linux-usb@...r.kernel.org>, clemens@...isch.de,
	Kernel development list <linux-kernel@...r.kernel.org>,
	chrisw@...s-sol.org, iommu@...ts.linux-foundation.org,
	andi@...stfloor.org, 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, May 11, 2010 at 12:06:09PM -0400, Alan Stern wrote:
> On Wed, 12 May 2010, FUJITA Tomonori wrote:
> 
> > > > 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?
> 
> By kmalloc().  Right, Daniel?

Yes, and that's precisely the reason for the whole thread ;)

[...]

> > The driver does only DMA_TO_DEVICE? Or you see DMA problems only with
> > DMA_TO_DEVICE?
> 
> The particular test that Pedro is running uses audio output only -- 
> he's sending sound data to a speaker and it comes out noisy.
> 
> But the audio data has to come from somewhere, and I don't remember 
> where.  Pedro, does the noise occur only when you're playing sound that 
> comes from a different USB device?  What happens if you play sounds 
> that are stored on your hard disk, like an MP3 file?

AFAIK that was playback from a file, yes. It would be interesting to
have some results for the other direction as well, that's true.

Thanks,
Daniel

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