iommu/arm-smmu-v3: Use CMD_SYNC completion MSI
As an IRQ, the CMD_SYNC interrupt is not particularly useful, not least because we often need to wait for sync completion within someone else's IRQ handler anyway. However, when the SMMU is both coherent and supports MSIs, we can have a lot more fun by not using it as an interrupt at all. Following the example suggested in the architecture and using a write targeting normal memory, we can let callers wait on a status variable outside the lock instead of having to stall the entire queue or even touch MMIO registers. Since multiple sync commands are guaranteed to complete in order, a simple incrementing sequence count is all we need to unambiguously support any realistic number of overlapping waiters. Signed-off-by: NRobin Murphy <robin.murphy@arm.com> Signed-off-by: NWill Deacon <will.deacon@arm.com>
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