On Wed, Aug 11, 2010 at 06:08:43PM +0200, Martin Fuzzey wrote: > On Wed, Aug 11, 2010 at 5:04 PM, Greg KH <greg@xxxxxxxxx> wrote: > >> Here is a pointer to the thread where it was stated that HCD's don't > >> have to handle this. > >> > >> http://kerneltrap.org/mailarchive/linux-usb/2009/4/20/5528164 > > > > No, that thread is about stack vs. heap allocations, not about alignment > > issues. > > > > Well although the issue discussed in that thread was caused by a stack > allocation isn't the issue here the same? > > My understanding is that a heap allocation as returned by kmalloc() will be: > 1) correctly aligned for DMA > and > 2) in a memory zone accessible to DMA > > whereas a stack allocation is not guaranteed to have either of these properties. > > The problem I described in that thread was due to case 1 > (misalignment) rather than the stack memory zone not being accessible > at all to DMA. > To which was the reply was basically "use a heap allocation". > > So the question is are hcds expected to accept arbitarilly aligned but > heap allocated pointers (such as the result of kmalloc() + 1)? It sounds like your HCD doesn't like this, so perhaps we should make that rule :) If you allocate the urb with a kmalloc() call with no offset, does it all work properly? The driver should be calling usb_alloc_urb() which does this automatically for them, right? Or is it trying to allocate things on its own somehow? thanks, greg k-h -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
