On 14 April 2010 19:36, David Woodhouse <dwmw2@infradead.org> wrote:
quoted text
> On Wed, 2010-04-14 at 14:15 -0400, Alan Stern wrote:
>>
>> Since using mem=4096M or GFP_DMA stopped the symptoms, it seems very
>> likely that a buffer is getting allocated above the 4 GB line and not
>> bounced or IOMMU-mapped correctly.
>>
>> David, do you have anything to suggest?  Any ways to check for IOMMU
>> or related errors?
>>
>> The problem, in short, is that USB audio doesn't work properly when
>> Pedro boots a 64-bit kernel on his 4-GB machine.  With a 32-bit kernel
>> it works okay, and it also works if we use dma_alloc_coherent().  The
>> host controller is limited to 32-bit DMA, and the DMA addresses
>> generated by dma_map_single() appear to be normal.
>>
>> At the moment we don't even know if this is caused by a bug in the
>> kernel or a bug in Pedro's hardware.  But he has observed the same
>> problem on two different machines, both using the ICH9 chipset.
>
> Pedro's dmesg suggests that his machine has an IOMMU, but his kernel
> isn't built to support it. So he'll be using swiotlb.
>
> Would be interesting to enable CONFIG_DMAR and check whether the problem
> goes away. If so, we can start looking harder at the swiotlb code.
>
> --
> David Woodhouse                            Open Source Technology Centre
> David.Woodhouse@intel.com                              Intel Corporation
>
>

Turns out CONFIG_DMAR was disabled because of PREEMPT_RT. I disabled
the later and enabled _DMAR. It took a long time to boot, something
wrong with the usb ports. You can see it in the appended dmesg from
time 11s to 100s.

 Then after it booted, I could barely move my USB mouse and lots of
errors appeared on dmesg. I tried to connect the DVB card but it
wouldn't even initialize.

Enabling it with iommu=pt seemed to make no difference.

Thanks,
Pedro