On Wed, May 19, 2010 at 02:17:47PM +0100, David Woodhouse wrote: > On Wed, 2010-05-19 at 23:07 +1000, Nick Piggin wrote: > > On Wed, May 19, 2010 at 01:48:45PM +0100, David Woodhouse wrote: > > > However, we _do_ allow drivers to assume that kmalloc is DMA-safe. That > > > happens to mean "cacheline-aligned" for cache-incoherent architectures, > > > but drivers should never really have to think about that. > > > > DMA-safe for GFP_DMA, or all kmalloc? > > Try not to think about that. Seriously, look over there! A kitten! > > (You have to use the DMA API anyway, so swiotlb will handle things if > you're trying to use pages which are above the range that certain > devices can reach. But we're only talking about the problems of sharing > cache lines here; don't worry about that bit.) Yeah, well, GFP_DMA is pretty crappy anyway. I was just curious, but it's no big deal to support dma with regular kmalloc. > > > > So whenever strengthening API guarantees like this, it is better to be > > > > very careful and conservative. Probably even introducing a new API with > > > > the stronger semantics (even if it is just a wrapper in the case where > > > > KMALLOC_MINALIGNED *is* cacheline sized). > > > > > > We're not talking about strengthening API guarantees. It's _always_ been > > > this way; it's just that some architectures are buggy. > > > > It just appeared, in the post I replied to, that there was a suggestion > > of making it explicitly cacheline aligned. If I misread that, ignore > > me. > > Actually I think you read it just fine -- but it was misguided. They > meant to say "cacheline aligned on architectures with cache-incoherent > DMA which need that". But that's what ARCH_KMALLOC_MINALIGN is _already_ > supposed to do. If any architecture needs to set ARCH_KMALLOC_MINALIGN > but doesn't, that's broken. > > The 'cacheline aligned' misconception did manage to get into the ad7877 > driver in commit 3843384a though -- it now uses ____cacheline_aligned > instead of __attribute__((__aligned__(ARCH_KMALLOC_MINALIGN))) as it > should. OK so long as there is not a "must be cacheline aligned" requirement. Your proposal for a __dma_aligned attribute in an arch header looks like a good idea there. -- To unsubscribe from this list: send the line "unsubscribe linux-input" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
