linux-kernel - Computation of return value being discarded in get_cpu_power() in drivers/platform/x86/intel

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Message-ID: <548dd463-3942-00a1-85c3-232897dea1a3@canonical.com>
Date:   Wed, 9 Jun 2021 12:50:07 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     "Hans de Goede <hdegoede"@redhat.com,
        Mark Gross <mgross@...ux.intel.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc:     platform-driver-x86@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Computation of return value being discarded in get_cpu_power() in
 drivers/platform/x86/intel_ips.c

Hi,

I was reviewing some old unassigned variable warnings from static
analysis by Coverity and found an issue introduced with the following
commit:

commit aa7ffc01d254c91a36bf854d57a14049c6134c72
Author: Jesse Barnes <jbarnes@...tuousgeek.org>
Date:   Fri May 14 15:41:14 2010 -0700

    x86 platform driver: intelligent power sharing driver

The analysis is as follows:

drivers/platform/x86/intel_ips.c

 871 static u32 get_cpu_power(struct ips_driver *ips, u32 *last, int period)
 872 {
 873        u32 val;
 874        u32 ret;
 875
 876        /*
 877         * CEC is in joules/65535.  Take difference over time to
 878         * get watts.
 879         */
 880        val = thm_readl(THM_CEC);
 881
 882        /* period is in ms and we want mW */
 883        ret = (((val - *last) * 1000) / period);

Unused value (UNUSED_VALUE)
assigned_value:  Assigning value from ret * 1000U / 65535U to ret here,
but that stored value is not used.

 884        ret = (ret * 1000) / 65535;
 885        *last = val;
 886
 887        return 0;
 888 }

I'm really not sure why ret is being calculated on lines 883,884 and not
being used. Should that be *last = ret on line 885? Looks suspect anyhow.

Colin