Problem with non aligned DMA in usbnet on ARM

On Wed, Aug 11, 2010 at 06:08:43PM +0200, Martin Fuzzey wrote:

On Wed, Aug 11, 2010 at 5:04 PM, Greg KH [off-list ref] wrote:

quoted

quoted

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