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. 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. > 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. 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
