[alsa-devel] USB transfer_buffer allocations on 64bit systems
stern at rowland.harvard.edu
Fri Apr 9 18:01:27 CEST 2010
On Fri, 9 Apr 2010, Pedro Ribeiro wrote:
> On 8 April 2010 17:57, Alan Stern <stern at rowland.harvard.edu> wrote:
> > On Thu, 8 Apr 2010, Daniel Mack wrote:
> >> > > AFAIK, the driver shouldn't have to worry about this at all. When the
> >> > > buffer gets DMA-mapped for the controller, the DMA mapping code should
> >> > > see that the device has a 32-bit DMA mask and either bounce or IOMMU-map
> >> > > the memory so that it appears below 4GB.
> >> >
> >> > That's true. It would of course be more efficient for the buffer to be
> >> > allocated below 4 GB, but it should work okay either way. Daniel, do
> >> > you have any idea why it fails?
> >> No, and I can't do real tests as I lack a 64bit machine. I'll do some
> >> more investigation later today, but for now the only explanation I have
> >> is that not the remapped DMA buffer is used eventually by the EHCI code
> >> but the physical address of the original buffer.
> >> It would of course be best to fix the whole problem at this level, if
> >> possible.
> > It definitely needs to be fixed at this level. But I still think it's
> > appropriate to have new USB core functions for allocating and
> > deallocating I/O memory. The additional price is small compared to
> > constantly bouncing the buffers.
> > Pedro, in the hope of tracking down the problem, can you apply this
> > patch and see what output it produces in the system log when the
> > "interference" happens? (Warning: It will produce quite a lot of
> > output whenever you send data to the audio device -- between 500 and
> > 1000 lines per second.)
> Hi Alan,
> here is the output of the patch you sent me when the interference is triggered.
> The log is long, 1.3mb in size.
I don't see anything suspicious. The transfer_buffer addresses repeat
every 32 URBs, and the DMA addresses cycle almost entirely uniformly
from 0x20000000 to 0x23ffffff in units of 0x2000 (there are a few gaps
where the interval is a little bigger).
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