Re: [OPW kernel] dma_set_coherent_mask

[Sarah Sharp <sarah.a.sharp@linux.intel.com>]

On Thu, May 16, 2013 at 10:11:00AM -0400, Alan Stern wrote:
> On Wed, 15 May 2013, Sarah Sharp wrote:
> 
> > > If you're using 64-bit DMA then you almost certainly do want to call
> > > dma_set_coherent_mask().  On the plus side, it is guaranteed that if
> > > dma_set_mask() succeeds with a particular mask value then
> > > dma_set_coherent_mask() for the same mask value will also succeed.
> > 
> > So we need to call both dma_set_coherent_mask() and dma_set_mask()?  Or
> > just dma_set_coherent_mask()?
> 
> It depends on what kind of DMA transfers you're going to do.  For
> streaming transfers (the ones that use dma_map_single() or
> dma_map_sg(), for example), you need to call dma_set_mask().  For
> coherent transfers (the ones that use dma_alloc_coherent() or
> dma_pool_create()), you need to call dma_set_coherent_mask().
> 
> If you want to do both kinds of transfers then you need to call both 
> routines.

I think we need the host to be able to do DMA to URB buffers that are
mapped with dma_map_single() or dma_map_sg(), since that's what
usb_hcd_submit_urb() uses.  So the driver needs to call dma_set_mask(),
which it does.

The xHCI endpoint rings are allocated from DMA pools, so we need to
call dma_set_coherent_mask() as well.  Missing that call explains why
I've never seen 64-bit endpoint rings, but I have seen 64-bit URB buffer
pointers.

Ksenia, do you want to add code to add the call to
dma_set_coherent_mask() in those two places in xhci_gen_setup()?  As I
mentioned, let's add this function call first, and then have a separate
commit refactor the copy-paste code into a new function.

Again, thanks for catching this and asking questions!

Sarah Sharp