Re: Problem with non aligned DMA in usbnet on ARM

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

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)?

regards,

Martin

thanks,

greg k-h