AMD IOMMU: set cmd buffer pointers to zero manually

Impact: set cmd buffer head and tail pointers to zero in case nobody else did
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

arch/x86/kernel/amd_iommu_init.c

@@ -427,6 +427,10 @@ static u8 * __init alloc_command_buffer(struct amd_iommu *iommu)
 	memcpy_toio(iommu->mmio_base + MMIO_CMD_BUF_OFFSET,
 			&entry, sizeof(entry));
 
+	/* set head and tail to zero manually */
+	writel(0x00, iommu->mmio_base + MMIO_CMD_HEAD_OFFSET);
+	writel(0x00, iommu->mmio_base + MMIO_CMD_TAIL_OFFSET);
+
 	iommu_feature_enable(iommu, CONTROL_CMDBUF_EN);
 
 	return cmd_buf;