netdev - Re: [RFC PATCH net-next 2/4] net: dsa: remove the "dsa_to_port in a loop" antipattern from the core

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Date:   Tue, 10 Aug 2021 11:33:39 +0800
From:   DENG Qingfang <dqfext@...il.com>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        Woojung Huh <woojung.huh@...rochip.com>,
        UNGLinuxDriver@...rochip.com, Sean Wang <sean.wang@...iatek.com>,
        Landen Chao <Landen.Chao@...iatek.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        George McCollister <george.mccollister@...il.com>
Subject: Re: [RFC PATCH net-next 2/4] net: dsa: remove the "dsa_to_port in a loop" antipattern from the core

On Mon, Aug 09, 2021 at 10:03:18PM +0300, Vladimir Oltean wrote:
> Ever since Vivien's conversion of the ds->ports array into a dst->ports
> list, and the introduction of dsa_to_port, iterations through the ports
> of a switch became quadratic whenever dsa_to_port was needed.

So, what is the benefit of a linked list here? Do we allow users to
insert/delete a dsa_port at runtime? If not, how about using a
dynamically allocated array instead?

> 
> dsa_to_port can either be called directly, or indirectly through the
> dsa_is_{user,cpu,dsa,unused}_port helpers.
> 
> Introduce a basic switch port iterator macro, dsa_switch_for_each_port,
> that works with the iterator variable being a struct dsa_port *dp
> directly, and not an int i. It is an expensive variable to go from i to
> dp, but cheap to go from dp to i.
> 
> This macro iterates through the entire ds->dst->ports list and filters
> by the ports belonging just to the switch provided as argument.
> 
> While at it, provide some more flavors of that macro which filter for
> specific types of ports: at the moment just user ports and CPU ports.
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>