vfio-ccw: document possible errors

Interacting with the I/O and the async regions can yield a number
of errors, which had been undocumented so far. These are part of
the api, so remedy that.
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Message-Id: <20200407111605.1795-1-cohuck@redhat.com>

Documentation/s390/vfio-ccw.rst

@@ -204,15 +204,44 @@ definition of the region is::
 	  __u32   ret_code;
   } __packed;
 
+This region is always available.
+
 While starting an I/O request, orb_area should be filled with the
 guest ORB, and scsw_area should be filled with the SCSW of the Virtual
 Subchannel.
 
 irb_area stores the I/O result.
 
-ret_code stores a return code for each access of the region.
+ret_code stores a return code for each access of the region. The following
+values may occur:
+
+``0``
+  The operation was successful.
+
+``-EOPNOTSUPP``
+  The orb specified transport mode or an unidentified IDAW format, or the
+  scsw specified a function other than the start function.
+
+``-EIO``
+  A request was issued while the device was not in a state ready to accept
+  requests, or an internal error occurred.
+
+``-EBUSY``
+  The subchannel was status pending or busy, or a request is already active.
+
+``-EAGAIN``
+  A request was being processed, and the caller should retry.
+
+``-EACCES``
+  The channel path(s) used for the I/O were found to be not operational.
+
+``-ENODEV``
+  The device was found to be not operational.
+
+``-EINVAL``
+  The orb specified a chain longer than 255 ccws, or an internal error
+  occurred.
 
-This region is always available.
 
 vfio-ccw cmd region
 -------------------
@@ -231,6 +260,29 @@ This region is exposed via region type VFIO_REGION_SUBTYPE_CCW_ASYNC_CMD.
 
 Currently, CLEAR SUBCHANNEL and HALT SUBCHANNEL use this region.
 
+command specifies the command to be issued; ret_code stores a return code
+for each access of the region. The following values may occur:
+
+``0``
+  The operation was successful.
+
+``-ENODEV``
+  The device was found to be not operational.
+
+``-EINVAL``
+  A command other than halt or clear was specified.
+
+``-EIO``
+  A request was issued while the device was not in a state ready to accept
+  requests.
+
+``-EAGAIN``
+  A request was being processed, and the caller should retry.
+
+``-EBUSY``
+  The subchannel was status pending or busy while processing a halt request.
+
+
 vfio-ccw operation details
 --------------------------