linux-kernel - Re: [PATCH net-next] net: core: Expose number of link up/down transitions

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Date:   Wed, 17 Jan 2018 16:23:09 -0800
From:   Andrei Vagin <avagin@...tuozzo.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Florian Fainelli <f.fainelli@...il.com>, netdev@...r.kernel.org,
        edumazet@...gle.com, cphealy@...il.com,
        David Decotigny <decot@...glers.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Daniel Borkmann <daniel@...earbox.net>,
        Nikolay Aleksandrov <nikolay@...ulusnetworks.com>,
        Alexei Starovoitov <ast@...com>,
        Roopa Prabhu <roopa@...ulusnetworks.com>,
        Mahesh Bandewar <maheshb@...gle.com>,
        Vlad Yasevich <vyasevich@...il.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jonas Bonn <jonas@...thpole.se>,
        stephen hemminger <stephen@...workplumber.org>,
        Hans Liljestrand <ishkamiel@...il.com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Andrey Vagin <avagin@...nvz.org>,
        Florian Westphal <fw@...len.de>,
        David Ahern <dsahern@...il.com>,
        Xin Long <lucien.xin@...il.com>,
        Matthias Schiffer <mschiffer@...verse-factory.net>,
        Jiri Benc <jbenc@...hat.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] net: core: Expose number of link up/down
 transitions

On Thu, Jan 18, 2018 at 01:06:52AM +0100, Andrew Lunn wrote:
> > What is the idea to have two separate counters? Can a delta between them
> > be a bigger than 1?
> 
> Yes, it can.
> 
> These counters are incremented in netif_carrier_on() /
> netif_carrier_off(). They are not always called in pairs and they can
> be called multiple times for the same event. The phylib will call them
> when it notices the PHY saying the link is down/up, and the MAC driver
> sometimes also calls them.

We check the __LINK_STATE_NOCARRIER bit before changing these counters,
so if we call netif_carrier_on() twice, the counter will be incrimented
only by one, will it not?

void netif_carrier_on(struct net_device *dev)
        if (test_and_clear_bit(__LINK_STATE_NOCARRIER, &dev->state)) {
                atomic_inc(&dev->carrier_changes);

...

void netif_carrier_off(struct net_device *dev)
        if (!test_and_set_bit(__LINK_STATE_NOCARRIER, &dev->state)) {
                atomic_inc(&dev->carrier_changes);


> 
> 	  Andrew