IDBL
writel(MU_INBOUND_DOORBELL_HANDSHAKE, base + IDBL);
readl(base + IDBL);
writel(MU_INBOUND_DOORBELL_HANDSHAKE, base + IDBL);
readl(base + IDBL); /* flush */
writel(MU_INBOUND_DOORBELL_RESET, base + IDBL);
readl(base + IDBL); /* flush */
writel(MU_INBOUND_DOORBELL_REQHEADCHANGED, hba->mmio_base + IDBL);
readl(hba->mmio_base + IDBL); /* flush */