On Tue, 2012-01-10 at 23:15 +0200, Andriy Gapon wrote: > on 10/01/2012 22:53 Ian Lepore said the following: > > 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 still some problems: > - filter func is called for the range (lowaddr, hiaddr], that is lowadr is not > inclusive, as such there is no way to filter page zero > - a bounce page could still be at the physical address zero > - and overriding the above, even worse, bounce pages are allocated in the > range > below lowaddr, so with lowaddr of zero it's impossible to have any bounce > pages
Wow, I didn't realize. That almost reads like a list of bugs in the current busdma design. It seems especially wrong to assume that no hardware in the world now or ever would have its range of DMA-able addresses in the middle of its physical address space. I'll throw one more idea out, (because it just popped into my head, not because I think it's the best possible idea)... Could the problems you list be circumvented (for this situation and maybe others) with a new flag BUS_DMA_ALWAYS_FILTER that makes the filter function run regardless of the low/high addr values? That would add the flexibility to handle any arbitary kinds of ranges no matter what hardware or strange requirements come along. -- 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"
