On Dec 19, 2012, at 1:26 AM, Hans Petter Selasky wrote: > 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.
No, it mustn't. It can't. The hardware is such that it is impossible to do this, as discussed before. You can't mix access to memory in the same cache line between host and DMA. Warner _______________________________________________ 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"
