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. > 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
