linux-kernel - Re: [PATCH] powercap/drivers/dtpm: Fix __udivdi3 and __aeabi

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Date:   Mon, 4 Jan 2021 12:20:34 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux PM list <linux-pm@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        kernel test robot <lkp@...el.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] powercap/drivers/dtpm: Fix __udivdi3 and __aeabi_uldivmod
 unresolved symbols


Hi Geert,


On 04/01/2021 09:18, Geert Uytterhoeven wrote:
> Hi Daniel,
> 
> On Wed, Dec 30, 2020 at 4:39 PM Daniel Lezcano
> <daniel.lezcano@...aro.org> wrote:
>> 32 bits architectures do not support u64 division, so the macro
>> DIV_ROUND_CLOSEST is not adequate as the compiler will replace the
>> call to an unexisting function for the platform, leading to an
>> unresolved symbols.
>>
>> Fix this by using the compatible macros:
>>
>> DIV64_U64_ROUND_CLOSEST and DIV_ROUND_CLOSEST_ULL.
>>
>> Reported-by: kernel test robot <lkp@...el.com>
>> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> 
> Thanks for your patch!
> 
>> --- a/drivers/powercap/dtpm.c
>> +++ b/drivers/powercap/dtpm.c
>> @@ -99,8 +99,8 @@ static void __dtpm_rebalance_weight(struct dtpm *dtpm)
>>                 pr_debug("Setting weight '%d' for '%s'\n",
>>                          child->weight, child->zone.name);
>>
>> -               child->weight = DIV_ROUND_CLOSEST(child->power_max * 1024,
>> -                                                 dtpm->power_max);
>> +               child->weight = DIV64_U64_ROUND_CLOSEST(
>> +                       child->power_max * 1024, dtpm->power_max);
> 
> Note that 64-by-64 divisions are expensive on 32-bit platforms.
> 
> Does dtpm.power_max need to be u64?

The dtpm is based on the powercap framework which deals with microwatts
and the functions are expecting u64 values.

The division here happens when there is an update of the dtpm tree which
occurs rarely (at boot time or hotplug).

As the power model is in the vast majority on 64b platforms, the effort
to optimize to u32 sounds not worth, especially that the 32b platforms
supporting the energy model are now obsolete.

> The (lack of) documentation for the dtpm structure does not say what is
> being stored there.
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 


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