On Tuesday 18 December 2012 20:12:29 Andrew Turner wrote:
> On Tue, 18 Dec 2012 13:06:44 +0100
> Hans Petter Selasky <hsela...@c2i.net> wrote:
> > Hi Andrew,
> > 
> > > The BUS_DMA_COHERENT flag does nothing on armv6 as we need the cache
> > > enabled for atomic operations to work correctly and we would have to
> > > disable the cache on the entire page. This is acceptable behaviour
> > > from the description of the flag in the busdma man page.
> > 
> > Yes, but when I allocate memory from the USB stack, then I want that
> > memory to not be cached. It is simply not that useful to have that
> > memory cached. I didn't check the latest state of busdma, but if I'm
> > not mistaken, if the BUS_DMA_COHERENT flag is set on the DMA tag, the
> > flush/invalidate will simply return and do nothing. Maybe that is the
> > problem ...
> The exact meaning of BUS_DMA_COHERENT depends on the architecture. The
> code is still required to call bus_dmamap_sync with a coherent map but
> when the flag is implemented the cost of the operation will be reduced.
> It doesn't guarantee the memory is uncached, it may be implemented that
> way but the USB code can't rely on it.
> Andrew

The USB code doesn't rely on this flag. I'm wondering if BUSDMA by accident 
sets this flag, so that the cache sync ops are not called.

freebsd-usb@freebsd.org mailing list
To unsubscribe, send any mail to "freebsd-usb-unsubscr...@freebsd.org"

Reply via email to