linux-kernel - Re: [PATCH v15 1/2] math.h: provide rounddown

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Message-ID: <CAHp75VcWW=RaHS9Yb8BcK2Jt7qtNOQzA3eDOZQ88RQG63981cQ@mail.gmail.com>
Date: Tue, 24 Jun 2025 09:08:32 +0300
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Christian Marangi <ansuelsmth@...il.com>
Cc: Uwe Kleine-König <ukleinek@...nel.org>, 
	Lukas Wunner <lukas@...ner.de>, 
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>, 
	Herbert Xu <herbert@...dor.apana.org.au>, Andy Shevchenko <andy@...nel.org>, 
	Jonathan Cameron <Jonathan.Cameron@...wei.com>, linux-kernel@...r.kernel.org, 
	linux-pwm@...r.kernel.org
Subject: Re: [PATCH v15 1/2] math.h: provide rounddown_ull variant for
 rounddown MACRO

On Tue, Jun 24, 2025 at 12:11 AM Christian Marangi <ansuelsmth@...il.com> wrote:
>
> There is currently a problem with the usage of rounddown MACRO with

rounddown() with

> u64 dividends. This cause compilation error on specific arch where

causes

> 64bit division is done on 32bit system.

on the 32-bit


> To be more specific GCC try optimize the function and replace it with

to optimize

> __umoddi3 but this is actually not compiled in the kernel.

__umoddi3()

> Example:
> pwm-airoha.c:(.text+0x8f8): undefined reference to `__umoddi3'
>
> To better handle this, introduce a variant of rounddown MACRO,

rounddown(),

> rounddown_ull that can be used exactly for this scenario.

rounddown_ull()

> rounddown_ull new MACRO use the do_div MACRO that do the heavy work of

The rounddown_ull() is a new macro that uses do_div() to do the heavy work of

> handling internally all the magic for 64bit division on 32bit (and

for the 64-bit divisions on the 32-bit platforms (and

> indirectly fix the compilation error).

...

> - Add this patch

Why are math64 APIs not usable here?

-- 
With Best Regards,
Andy Shevchenko