(unknown charset) bus dma: a flag/quirk for page zero

Some hardware interfaces may reserve a special meaning for a (physical) memory
address value of zero.  One example is the OHCI specification where a zero value
in CurrentBufferPointer doesn't mean a physical address, but has a reserved
meaning.  To be honest I don't have another example :) but don't preclude its
existence.

To deal with this peculiarity we could use a special flag/quirk that would
instruct the bus dma code to never use the page zero for communication with the
hardware.
Here's a proof of concept patch that implements the idea:
http://people.freebsd.org/~avg/usb-dma-pagezero.diff

Some concerns:
- not sure if BUS_DMA_NO_PAGEZERO is the best name for the flag
- the patch implements the flag only for x86 at the moment
- usb code uses the flag regardless of the actual controller type

What do you think?

-- 
Andriy Gapon