On Thu, Jan 29, 2009 at 09:36:13PM +0900, Atsushi Nemoto wrote: > On Wed, 28 Jan 2009 18:30:47 +0000, Ralf Baechle <ralf@xxxxxxxxxxxxxx> wrote: > > --- a/arch/mips/lib/memcpy.S > > +++ b/arch/mips/lib/memcpy.S > > @@ -21,7 +21,7 @@ > > * end of memory on some systems. It's also a seriously bad idea on non > > * dma-coherent systems. > > */ > > -#if !defined(CONFIG_DMA_COHERENT) || !defined(CONFIG_DMA_IP27) > > +#ifdef CONFIG_DMA_NONCOHERENT > > #undef CONFIG_CPU_HAS_PREFETCH > > #endif > > #ifdef CONFIG_MIPS_MALTA > > This makes IP27 (and all other coherent platforms) use prefetch. Is > prefetch OK for all of them? > > I suppose memcpy_fromio() should not use PREFETCH, at least. The idea here is that we have two issues with prefetching: o Prefetching beyond the end of the source or destination range on a in-coherent range might bring back stale values from a DMA I/O buffer resulting in data corruption. Hardware DMA coherency will avoid this issue. o IP27 has full blown hardware coherency. Historically CONFIG_DMA_COHERENT was not able to cope with something of the complexity of IP27, so there was a separate CONFIG_DMA_IP27 and the broken logic expression was meant to treat CONFIG_DMA_COHERENT and CONFIG_DMA_IP27 the same as for prefetching. o Prefetching beyond the end of physical memory can cause exceptions on some systems. The Malta has this problem. Thus no prefetching on Malta or non-coherent systems. Ralf
