On Thu, 11 Jul 2013, Geert Uytterhoeven wrote:

On Thu, Jul 11, 2013 at 3:01 AM, Alan Stern [off-list ref] wrote:

quoted

On Thu, 11 Jul 2013, Arnd Bergmann wrote:

quoted

On Wednesday 10 July 2013, Alan Stern wrote:

quoted

This isn't right.  There are USB host controllers that use PIO, not
DMA.  The HAS_DMA dependency should go with the controller driver, not
the USB core.

On the other hand, the USB core does call various routines like
dma_unmap_single.  It ought to be possible to compile these calls even
when DMA isn't enabled.  That is, they should be defined as do-nothing
stubs.

The asm-generic/dma-mapping-broken.h file intentionally causes link
errors, but that could be changed.

The better approach in my mind would be to replace code like


      if (hcd->self.uses_dma)

with

      if (IS_ENABLED(CONFIG_HAS_DMA) && hcd->self.uses_dma) {

which will reliably cause that reference to be omitted from object code,
but not stop giving link errors for drivers that actually require
DMA.

How will it give link errors for drivers that require DMA?

It won't. Unless the host driver itself calls into the DMA API, too
(are there any that don't?).

To my knowledge, all the host controller drivers which use DMA _do_
call functions in the DMA API.  So they would still get link errors,
even though the USB core wouldn't.

Therefore adding the appropriate HAS_DMA dependencies should be 
straightforward: Try to build all the drivers and see which ones fail 
to link.

Alan Stern