On 4/5/2019 1:29 PM, Maciej W. Rozycki wrote:
Obviously you do need that `mb' before `__ioread' in the second case, just like in the first one, because otherwise the read bus access issued by `__ioread' can be reordered ahead of the write bus access issued by the preceding `__iowrite'.
Please also not that you also need a mb() after read to prevent stale from being read from memory. write_to_memory_for_dma() mb(); <---- DMA ordering requirement.Prefer wmb() if possible. __iowrite(123, INDEX); mb(); <---- alpha arch requirement due to instruction reordering. x = __ioread(DATA); mb(); <---- DMA ordering requirement. Prefer rmb() if possible. read_from_dma_memory() You basically can't remove the mb() after __ioread() and before __iowrite() unless your architecture guarantees IO vs. memory ordering.
