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

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Message-ID: <7e13032d-1472-9c50-1dba-9dcebc76729f@redhat.com>
Date:   Thu, 10 Jun 2021 17:55:19 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Colin Ian King <colin.king@...onical.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Mark Gross <mgross@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Jesse Barnes <jsbarnes@...gle.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: Re: Computation of return value being discarded in get_cpu_power() in
 drivers/platform/x86/intel_ips.c

Hi,

On 6/10/21 1:55 PM, Joonas Lahtinen wrote:
> (Address for Hans was corrupt in previous message, which confused my mail
> client. Sorry for duplicate message, the other is without From: field).
> 
> + Jesse
> 
> Quoting Colin Ian King (2021-06-09 14:50:07)
>> 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.

This has already been fixed (yesterday actually) in linux-next:

https://git.kernel.org/pub/scm/linux/kernel/git/pdx86/platform-drivers-x86.git/commit/?h=for-next&id=13c3b4f76073d73dd81e418295902676153f6cb5

Regards,

Hans