s390/cio: add dev_busid sysfs entry for each subchannel

Introduce dev_busid, which exports the device-id associated with the
io-subchannel (and message-subchannel). The dev_busid indicates that of
the device which may be physically installed on the corrosponding
subchannel. The dev_busid value "none" indicates that the subchannel
is not valid, there is no I/O device currently associated with the
subchannel.

The dev_busid information would be helpful to write device-specific
udev-rules associated with the subchannel. The dev_busid interface would
be available even when the sch is not bound to any driver or if there is
no operational device connected on it. Hence this attribute can be used to
write udev-rules which are specific to the device associated with the
subchannel.
Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

drivers/s390/cio/css.c

@@ -430,9 +430,26 @@ static ssize_t pimpampom_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(pimpampom);
 
+static ssize_t dev_busid_show(struct device *dev,
+			      struct device_attribute *attr,
+			      char *buf)
+{
+	struct subchannel *sch = to_subchannel(dev);
+	struct pmcw *pmcw = &sch->schib.pmcw;
+
+	if ((pmcw->st == SUBCHANNEL_TYPE_IO ||
+	     pmcw->st == SUBCHANNEL_TYPE_MSG) && pmcw->dnv)
+		return sysfs_emit(buf, "0.%x.%04x\n", sch->schid.ssid,
+				  pmcw->dev);
+	else
+		return sysfs_emit(buf, "none\n");
+}
+static DEVICE_ATTR_RO(dev_busid);
+
 static struct attribute *io_subchannel_type_attrs[] = {
 	&dev_attr_chpids.attr,
 	&dev_attr_pimpampom.attr,
+	&dev_attr_dev_busid.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(io_subchannel_type);