On Wed, May 19, 2010 at 01:48:45PM +0100, David Woodhouse wrote: > On Wed, 2010-05-12 at 13:03 +1000, Nick Piggin wrote: > > I don't think it's necessarily a good idea. MINALIGN is an enforced > > minimum alignment and the allocator has no leeway in reducing this. > > In a UP system, or in a memory constrained system, it might be a better > > idea to pack objects more tightly, for example. > > > > If we allow drivers to assume kmalloc is cacheline aligned, it will be > > (practically) impossible to revert this because it would require driver > > audits. > > No, we definitely don't, and shouldn't, allow drivers to assume that > kmalloc is cacheline-aligned. Good. > 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? Either way, yes the arch should take care of those details. > > 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. > > But it looks like ARM, PowerPC, SH, MIPS, Microblaze, AVR32 and all > unconditionally cache-coherent architectures _do_ get it right already. > > > I think adding to the DMA API would be a better idea. If the arch knows > > that kmalloc is suitable for the job directly, it can be used. Drivers > > can use the new interface, and kmalloc doesn't get saddled with > > alignment requirements. > > No, that would be a change which would require auditing all drivers. The > _current_ rule is that buffers returned from kmalloc() are OK for DMA. > > -- > David Woodhouse Open Source Technology Centre > David.Woodhouse@xxxxxxxxx Intel Corporation -- 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
