linux-kernel - Re: [PATCH v16 1/2] math64.h: provide rounddown

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Message-ID: <685bf6ac.050a0220.357231.66b2@mx.google.com>
Date: Wed, 25 Jun 2025 15:16:25 +0200
From: Christian Marangi <ansuelsmth@...il.com>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
Cc: Uwe Kleine-König <ukleinek@...nel.org>,
	linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org
Subject: Re: [PATCH v16 1/2] math64.h: provide rounddown_u64 variant for
 rounddown macro

On Wed, Jun 25, 2025 at 02:39:13PM +0300, Andy Shevchenko wrote:
> On Wed, Jun 25, 2025 at 02:00:38AM +0200, Christian Marangi wrote:
> > There is currently a problem with the usage of rounddown() macro with
> > u64 dividends. This causes compilation error on specific arch where
> > 64-bit division is done on 32-bit system.
> > 
> > To be more specific GCC try to optimize the function and replace it
> > with __umoddi3() but this is actually not compiled in the kernel.
> > 
> > Example:
> > pwm-airoha.c:(.text+0x8f8): undefined reference to `__umoddi3'
> > 
> > To better handle this, introduce a variant of rounddown() macro,
> > rounddown_u64() that can be used exactly for this scenario.
> > 
> > The new rounddown_u64() in math64.h uses do_div() to do the heavy work
> > of handling internally all the magic for the 64-bit division on 32-bit
> > (and indirectly fix the compilation error).
> 
> ...
> 
> > static inline u64 roundup_u64(u64 x, u32 y)
> >  {
> >  	return DIV_U64_ROUND_UP(x, y) * y;
> >  }
> 
> ...
> 
> > +static inline u64 rounddown_u64(u64 x, u32 y)
> > +{
> > +	u64 tmp = x;
> > +	return x - do_div(tmp, y);
> > +}
> 
> Can it be implemented as above?
> 
> 	return DIV_U64_ROUND_DOWN(x, y) * y;
> 
> (yes, it seems we are missing the DIV_U64_ROUND_DOWN() implementation).
> 

Guess it would be

#define DIV_U64_ROUND_DOWN(ll, d)		\
	({ u32 _tmp = (d); div_u64((ll), _tmp); })


But isn't that just directly div_u64?? (maybe the dividend is enforced
u32 with the cast)

and in math.h I can also notice

#define DIV_ROUND_DOWN_ULL(ll, d) \
	({ unsigned long long _tmp = (ll); do_div(_tmp, d); _tmp; })

tons of macro that do the same thing ahhahah

Really seems I'm opening a can of worm.

Also also division + subtraction isn't less CPU intensive than division
+ multiplication?

I know the compiler does magic on these internally but still...

-- 
	Ansuel