netdev - Re: TX watchdog vs link-layer flow control

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Date:	Thu, 02 Jun 2011 13:55:07 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	bhutchings@...arflare.com
Cc:	netdev@...r.kernel.org, linux-net-drivers@...arflare.com
Subject: Re: TX watchdog vs link-layer flow control

From: Ben Hutchings <bhutchings@...arflare.com>
Date: Thu, 02 Jun 2011 21:48:40 +0100

> 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.

Frankly, if the switch can't take packets for several seconds I want a
notification as that's a serious condition.

In lieu of a reliable way to poll for these kinds of cases in order to
distinguish properly, I think what happens now is the best we can do.
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