netdev - TX watchdog vs link-layer flow control

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Date:	Thu, 02 Jun 2011 21:48:40 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	netdev <netdev@...r.kernel.org>
Cc:	linux-net-drivers <linux-net-drivers@...arflare.com>
Subject: TX watchdog vs link-layer flow control

The TX watchdog will fire if and only if a TX queue remains stopped for
a certain period for no apparent reason.  Specifically, it requires
netif_device_present(dev) && netif_running(dev) &&
netif_carrier_ok(dev).

However, even if the link is up it can still be blocked by link-layer
flow control.  A customer report (which has not yet been reproduced
here) suggests that when Ethernet flow control is enabled a switch may
in some circumstances throttle the TX packet rate to the extent that a
TX queue cannot be unblocked before the watchdog fires.  It is certainly
possible for a misbehaving link partner to do this, and this should
probably not be considered as a bug in the local hardware or driver!

TX may also be blocked by a 'remote fault' indication.  This should
possibly be translated into netif_carrier_off(), but I'm not sure that
all drivers will be able to detect remote fault without polling.

Perhaps dev_watchdog() should support a driver operation to poll for
cases like this before it decides that the local device is actually
misbehaving?

Even then, I can't think of a reliable way to detect a pause frame
flood.  Also, drivers might well require process context for such an
operation.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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