Re: bus dma: a flag/quirk for page zero

On Tue, 2012-01-10 at 22:18 +0200, 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?

I think another way to handle this, one that doesn't require modifying
the busdma_machdep implementation for every architecture, would be for
usb_dma_tag_create() to set lowaddr to zero and provide a filter func
that filters based on both the value zero and the expression currently
being passed as lowaddr.  At least, I think that's how the filterfunc
stuff is supposed to work, I've never actually coded a busdma filter.

This has the advantage I call "locality of strangeness."  If only the
OHCI hardware needs this strange processing, and it seems like in the
future this strangeness will still be more the exception than the rule,
then the strangeness is best kept close to the place where it's needed,
rather than being spread out all over the place (lots of machdep
places).

-- Ian