MMIO_CMD_TAIL_OFFSET
tail = readl(iommu->mmio_base + MMIO_CMD_TAIL_OFFSET);
writel(0x00, iommu->mmio_base + MMIO_CMD_TAIL_OFFSET);
tail = readl(iommu->mmio_base + MMIO_CMD_TAIL_OFFSET);
writel(tail, iommu->mmio_base + MMIO_CMD_TAIL_OFFSET);