net: core: try to runtime-resume detached device in __dev_open

A netdevice may be marked as detached because the parent is
runtime-suspended and not accessible whilst interface or link is down.
An example are PCI network devices that go into PCI D3hot, see e.g.
__igc_shutdown() or rtl8169_net_suspend().
If netdevice is down and marked as detached we can only open it if
we runtime-resume it before __dev_open() calls netif_device_present().

Therefore, if netdevice is detached, try to runtime-resume the parent
and only return with an error if it's still detached.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -143,6 +143,7 @@
 #include <linux/net_namespace.h>
 #include <linux/indirect_call_wrapper.h>
 #include <net/devlink.h>
+#include <linux/pm_runtime.h>
 
 #include "net-sysfs.h"
 
@@ -1492,8 +1493,13 @@ static int __dev_open(struct net_device *dev, struct netlink_ext_ack *extack)
 
 	ASSERT_RTNL();
 
-	if (!netif_device_present(dev))
-		return -ENODEV;
+	if (!netif_device_present(dev)) {
+		/* may be detached because parent is runtime-suspended */
+		if (dev->dev.parent)
+			pm_runtime_resume(dev->dev.parent);
+		if (!netif_device_present(dev))
+			return -ENODEV;
+	}
 
 	/* Block netpoll from trying to do any rx path servicing.
 	 * If we don't do this there is a chance ndo_poll_controller