bus dma: a flag/quirk for page zero

Matthew Jacob mj at feral.com
Tue Jan 10 20:54:37 UTC 2012

I think it would be just simpler to disallow page zero usage period. Can 
you think of any case where physical page 0 is ever a valid DMA address?
At the very least, require bounce buffers.

On Tue, 10 Jan 2012, Andriy Gapon wrote:

> 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
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