[alsa-devel] USB transfer_buffer allocations on 64bit systems

[Daniel Mack]

On Tue, Apr 20, 2010 at 01:16:58AM +0100, Pedro Ribeiro wrote:
> On 15 April 2010 16:20, Alan Stern <stern at rowland.harvard.edu> wrote:
> > On Thu, 15 Apr 2010, Pedro Ribeiro wrote:
> >
> >> I enabled  CONFIG_DMAR_BROKEN_GFX_WA=y and the result is the same. A
> >> delay in the boot process and usb devices don't work properly,
> >> including my USB mouse.
> >>
> >> Strange, since you have the same platform as me. The extra usb devices
> >> you were seeing are because of my docking station - but it makes no
> >> difference whether I'm docked or not for the purposes of the original
> >> bug or this situation right now. The dmesg I'm attaching is without
> >> the computer being docked.
> >
> > It's not possible to determine the reason for the timeout errors
> > between timestamps 16 and 53 from the small amount of debugging
> > information in the log.  Clearly something is going wrong with the
> > communication between the computer and the EHCI controller.  And
> > clearly the kernel config changes are responsible.
> >
> > But I don't know what to do to track it down any farther.
> >
> > Alan Stern
> >
> >
> 
> I guess this is pretty much a dead end until anyone else can reproduce it!

Hmm, I think I finally found the reason for all this confusion. No idea
why I didn't come up with the following explanation any earlier.

The problem is again (summarized):

On 64bit machines, with 4GB or more, the allocated buffers for USB
transfers might be beyond the 32bit boundary. In this case, the IOMMU
should take care and install DMA bounce buffer to copy over the buffer
before the transfer actually happens. The problem is, however, that this
copy mechanism takes place when the URB with its associated buffer is
submitted, not when the EHCI will actually do the transfer.

In the particular case of audio drivers, though, the contents of the
buffers are likely to change after the submission. What we do here
is that we map the audio stream buffers which are used by ALSA to
the output URBs, so they're filled asychronously. Once the buffer is
actually sent out on the bus, it is believed to contain proper audio
date. If it doesn't, that's due to too tight audio timing or other
problems. This breaks once buffers are magically bounced in the
background.

So - long story short: these audio buffers need to be DMA coherent.

The patch below does that, and Pedro excessively tested this patch for
weeks now :) It was just the final explanation _why_ it does the right
thing that was yet missing.

If nobody has objections, can we still squeeze it into .34?

Thanks,
Daniel