bus dma: a flag/quirk for page zero

Andriy Gapon avg at FreeBSD.org
Tue Jan 10 20:18:33 UTC 2012

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

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
Here's a proof of concept patch that implements the idea:

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

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