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 _______________________________________________ freebsd-current@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"
