On 04/07/2010 08:59 AM, Alan Stern wrote: > On Wed, 7 Apr 2010, Daniel Mack wrote: > >> Hi, >> >> I was pointed to https://bugzilla.kernel.org/show_bug.cgi?id=15580 by >> Pedro Ribeiro, and unfortunately I'm pretty late in the game. I wasn't >> aware of that thread until yesterday. >> >> While the report is quite confusing, the reason seams pretty clear to me >> as I've been thru quite some time-demanding debugging of a very similar >> issue on Mac OS X. But I'm not totally sure whether we really hit the >> same issue here, so I'd like to have your opinions first. >> >> The problem is appearantly the way the transfer buffer is allocated in >> the drivers. In the snd-usb-caiaq driver, I used kzalloc() to get memory >> which works fine on 32bit systems. On x86_64, however, it seems that >> kzalloc() hands out memory beyond the 32bit addressable boundary, which >> the DMA controller of the 32bit PCI-connected EHCI controller is unable >> to write to or read from. Am I correct on this conclusion? > > That seems like the right answer. You are correct that an EHCI > controller capable only of 32-bit memory accesses would not be able to > use a buffer above the 4 GB line. > >> Depending on the condition of the memory management, things might work >> or not, and especially right after a reboot, there's a better chance to >> get lower memory. >> >> The fix is to use usb_buffer_alloc() for that purpose which ensures >> memory that is suitable for DMA. And on x86_64, this also means that the >> upper 32 bits of the address returned are all 0's. > > That is not a good fix. usb_buffer_alloc() provides coherent memory, > which is not what we want. I believe the correct fix is to specify the > GFP_DMA32 flag in the kzalloc() call. > > Of course, some EHCI hardware _is_ capable of using 64-bit addresses. > But not all, and other controller types aren't. In principle we could > create a new allocation routine, which would take a pointer to the USB > bus as an additional argument and use it to decide whether the memory > needs to lie below 4 GB. I'm not sure adding this extra complexity > would be worthwhile. 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. Not to say that something might not be sabotaging this somehow, but this complexity really shouldn't be needed. > >> If what I've stated is true, there are quite some more drivers affected >> by this issue. > > Practically every USB driver, I should think. And maybe a lot of > non-USB drivers too. > >> I collected a list of places where similar fixes are >> needed, and I can send patches if I get a thumbs-up. >> >> Pedro is currently testing a patch I sent out yesterday. > > Good work -- it certainly would have taken me a long time to figure > this out. > > Alan Stern > _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
