On Tuesday 18 December 2012 21:32:46 Warner Losh wrote: > On Dec 18, 2012, at 12:44 PM, Hans Petter Selasky wrote: > > 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. > > No. It doesn't. >
Hi, > Does the USB code still touch bytes in the same cache line while the DMA is > still outstanding? Yes, only for data. The busdma API must handle this case properly as discussed before. --HPS _______________________________________________ freebsd-usb@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-usb To unsubscribe, send any mail to "freebsd-usb-unsubscr...@freebsd.org"
