diff --git a/arch/s390/pci/pci_mmio.c b/arch/s390/pci/pci_mmio.c
index 38efa3e852c4..401cf670a243 100644
--- a/arch/s390/pci/pci_mmio.c
+++ b/arch/s390/pci/pci_mmio.c
@@ -155,10 +155,12 @@ SYSCALL_DEFINE3(s390_pci_mmio_write, unsigned long, mmio_addr,
 		return -EINVAL;
 
 	/*
-	 * Only support read access to MIO capable devices on a MIO enabled
-	 * system. Otherwise we would have to check for every address if it is
-	 * a special ZPCI_ADDR and we would have to do a get_pfn() which we
-	 * don't need for MIO capable devices.
+	 * We only support write access to MIO capable devices if we are on
+	 * a MIO enabled system. Otherwise we would have to check for every
+	 * address if it is a special ZPCI_ADDR and would have to do
+	 * a get_pfn() which we don't need for MIO capable devices.  Currently
+	 * ISM devices are the only devices without MIO support and there is no
+	 * known need for accessing these from userspace.
 	 */
 	if (static_branch_likely(&have_mio)) {
 		ret = __memcpy_toio_inuser((void  __iomem *) mmio_addr,
@@ -282,10 +284,12 @@ SYSCALL_DEFINE3(s390_pci_mmio_read, unsigned long, mmio_addr,
 		return -EINVAL;
 
 	/*
-	 * Only support write access to MIO capable devices on a MIO enabled
-	 * system. Otherwise we would have to check for every address if it is
-	 * a special ZPCI_ADDR and we would have to do a get_pfn() which we
-	 * don't need for MIO capable devices.
+	 * We only support read access to MIO capable devices if we are on
+	 * a MIO enabled system. Otherwise we would have to check for every
+	 * address if it is a special ZPCI_ADDR and would have to do
+	 * a get_pfn() which we don't need for MIO capable devices.  Currently
+	 * ISM devices are the only devices without MIO support and there is no
+	 * known need for accessing these from userspace.
 	 */
 	if (static_branch_likely(&have_mio)) {
 		ret = __memcpy_fromio_inuser(