linux-kernel - re: net: phy: continue searching for C45 MMDs even if first returned ffff:ffff

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Date:   Mon, 20 Jul 2020 17:21:13 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     Vladimir Oltean <vladimir.oltean@....com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: re: net: phy: continue searching for C45 MMDs even if first returned
 ffff:ffff

Hi,

Static analysis by Coverity has found a potential issue with the
following commit in /drivers/net/phy/phy_device.c:

commit bba238ed037c60242332dd1e4c5778af9eba4d9b
Author: Vladimir Oltean <vladimir.oltean@....com>
Date:   Sun Jul 12 19:48:15 2020 +0300

    net: phy: continue searching for C45 MMDs even if first returned
ffff:ffff

The analysis is as follows:

735         * for 802.3 c45 complied PHYs, so don't probe it at first.
736         */

dead_error_condition: The condition (devs_in_pkg & 0x1fffffffU) ==
0x1fffffffU cannot be true.

737        for (i = 1; i < MDIO_MMD_NUM && devs_in_pkg == 0 &&

const: At condition (devs_in_pkg & 0x1fffffffU) == 0x1fffffffU, the
value of devs_in_pkg must be equal to 0.

738             (devs_in_pkg & 0x1fffffff) == 0x1fffffff; i++) {

Logically dead code (DEADCODE)dead_error_line: Execution cannot reach
this statement: if (i == 30 || i == 31) {

To summarize, if devs_in_pkg is zero, then (devs_in_pkg & 0x1fffffffU)
== 0x1fffffffU can never be true, so the loop is never iterated over.

Colin