On Sun, Aug 28, 2011 at 01:00:07PM -0400, Alan Stern wrote: > It won't do that. All it will do is guarantee that the CPU writes out > dumy->hw_token before it writes out or reads in any values executed > after the mb. You're right from the perspective of how things are defined today. However, that isn't how things work on ARM. With ARMv6 and ARMv7, we have weak memory ordering. This includes so called "DMA coherent" memory. This means that the architecture does not guarantee the order of writes to DMA coherent memory (which is non- cacheable normal memory) without an intervening 'data synchronization barrier' (dsb). Even that may not be sufficient without also poking at the L2 cache controller. We get around some of that by ensuring that our MMIO read/write macros contain the necessary barriers to ensure that DMA memory is up to date before the DMA agent is programmed. However, this doesn't cater for agents which continue to run in the background. These agents will need some kind of barrier to ensure that the write becomes visible - there's no way to get around that. Maybe we need yet another new barrier macro... -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
