s390/pci: clarify comment in s390_mmio_read/write

The existing comment was talking about reading in the write part
and vice versa. While we are here make it more clear why restricting
the syscalls to MIO capable devices is okay.
Signed-off-by: NNiklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: NHeiko Carstens <hca@linux.ibm.com>

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(