Hi all! There are read prefetch optimization in several PPC specific functions responsible for copying memory (copy_page, __copy_tofrom_user). Current implementations will try to prefetch up to 4 (MAX_COPY_PREFETCH) cache lines _after_ the end of the source buffer.
Unfortunately, it's not a good idea on non-coherent cache CPUs. This prefetching may establish cache lines for memory ranges that require exactly the opposite (e.g. DMA read buffer). Here is one example of the incorrect behavior (from the real life :() Let's consider two adjacent pages in RAM. First page contains some data and the second one is used as target for DMA read. Please, note that these two pages are completely unrelated to each other. Before initiating "read" we invalidate cache lines for addresses in the second page range (e.g. with consistent_sync_page(...., PCI_DMA_FROMDEVICE)). Then somewhere else in the kernel we initiate copying from the _first_ page, e.g. to the user space with __copy_tofrom_user. During copying, this function calls dcbt to prefetch up to 4 cache lines of the source data. This will also result in prefetching 4 cache lines _after_ the end of the source buffer, e.g. _first_ 4 cache lines of the _second_ page. If DMA read hasn't started filling this memory, we'll may get old data instead of real DMA read. I think we should disable prefetch if CONFIG_NONCOHERENT_CACHE is defined. Other more complex solutions are possible, e.g. we can still prefetch our own buffer but don't touch anything outside (I'll try to do some performance testing to determine whether it's worth the effort :). Patch against linuxppc-2.4: http://kernel.ebshome.net/read_prefetch-2.4.diff Patch against linuxppc-2.5 (not tested, but compiles): http://kernel.ebshome.net/read_prefetch-2.6.diff Comments, suggestions... Eugene. P.S. I suspect that "speculative" dcache fetch past the end of the memory on 440 is really copy_page() prefetching source data. I'm guilty :), it was my interpretation of the strange machine check we saw a year ago ... ** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/
