netpoll: Introduce netpoll_carrier_timeout kernel option

Some PHYs require longer timeouts for carrier detection, and
auto-negotiation process may take indefinite amount of time.

It may be inconvenient to force longer timeouts for sane PHYs,
so let's introduce a kernel command line option.

Since we're using module_param(), the option also can be
changed in runtime.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index d77fbd8..9347f4ad 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1531,6 +1531,11 @@
 			symbolic names: lapic and ioapic
 			Example: nmi_watchdog=2 or nmi_watchdog=panic,lapic
 
+	netpoll.carrier_timeout=
+			[NET] Specifies amount of time (in seconds) that
+			netpoll should wait for a carrier. By default netpoll
+			waits 4 seconds.
+
 	no387		[BUGS=X86-32] Tells the kernel to use the 387 maths
 			emulation library even if a 387 maths coprocessor
 			is present.