netdev - dsa_master_find_slave()'s time complexity and potential performance hit

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Date:   Tue, 2 Mar 2021 13:51:42 +0800
From:   DENG Qingfang <dqfext@...il.com>
To:     netdev <netdev@...r.kernel.org>, Andrew Lunn <andrew@...n.ch>,
        Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...emloft.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        linux-kernel@...r.kernel.org,
        Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Birger Koblitz <git@...ger-koblitz.de>,
        Bjørn Mork <bjorn@...k.no>,
        Stijn Segers <foss@...atilesystems.org>
Subject: dsa_master_find_slave()'s time complexity and potential performance hit

Since commit 7b9a2f4bac68 ("net: dsa: use ports list to find slave"),
dsa_master_find_slave() has been iterating over a linked list instead
of accessing arrays, making its time complexity O(n).
The said function is called frequently in DSA RX path, so it may cause
a performance hit, especially for switches that have many ports (20+)
such as RTL8380/8390/9300 (There is a downstream DSA driver for it,
see https://github.com/openwrt/openwrt/tree/openwrt-21.02/target/linux/realtek/files-5.4/drivers/net/dsa/rtl83xx).
I don't have one of those switches, so I can't test if the performance
impact is huge or not.