On Wed, 10 Jul 2013, Geert Uytterhoeven wrote:

If NO_DMA=y:

drivers/built-in.o: In function `usb_hcd_unmap_urb_setup_for_dma':
drivers/usb/core/hcd.c:1361: undefined reference to `dma_unmap_single'

,,,

Commit d9ea21a779278da06d0cbe989594bf542ed213d7 ("usb: host: make
USB_ARCH_HAS_?HCI obsolete") allowed to enable USB on platforms with
NO_DMA=y, and exposed several input and media USB drivers that just select
USB if USB_ARCH_HAS_HCD, without checking HAS_DMA.

Fix the former by making USB depend on HAS_DMA.

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.

To fix the latter, instead of adding lots of "depends on HAS_DMA", make
those drivers depend on USB, instead of depending on USB_ARCH_HAS_HCD and
selecting USB.  Drivers for other busses (e.g. MOUSE_SYNAPTICS_I2C) already
handle this in a similar way.

That seems reasonable.

Alan Stern