On 14 April 2010 19:36, David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote: > 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@xxxxxxxxx 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
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