linux-kernel - Re: [PATCH v5 1/4] uapi: Provide DIV_ROUND

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Message-ID: <aXjNyHaJDHoCPRJO@smile.fi.intel.com>
Date: Tue, 27 Jan 2026 16:38:00 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Jani Nikula <jani.nikula@...ux.intel.com>
Cc: Cristian Ciocaltea <cristian.ciocaltea@...labora.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Sandy Huang <hjc@...k-chips.com>,
	Heiko Stübner <heiko@...ech.de>,
	Andy Yan <andy.yan@...k-chips.com>,
	Louis Chauvet <louis.chauvet@...tlin.com>,
	Haneen Mohammed <hamohammed.sa@...il.com>,
	Melissa Wen <melissa.srw@...il.com>,
	Robert Mader <robert.mader@...labora.com>, kernel@...labora.com,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-rockchip@...ts.infradead.org,
	Nícolas F. R. A. Prado <nfraprado@...labora.com>,
	Diederik de Haas <diederik@...ow-tech.com>
Subject: Re: [PATCH v5 1/4] uapi: Provide DIV_ROUND_CLOSEST()

On Tue, Jan 27, 2026 at 03:58:13PM +0200, Jani Nikula wrote:
> On Tue, 27 Jan 2026, Cristian Ciocaltea <cristian.ciocaltea@...labora.com> wrote:
> > Currently DIV_ROUND_CLOSEST() is only available for the kernel via
> > include/linux/math.h.
> >
> > Expose it to userland as well by adding __KERNEL_DIV_ROUND_CLOSEST() as
> > a common definition in uapi.
> >
> > Additionally, ensure it allows building ISO C applications by switching
> > from the 'typeof' GNU extension to the ISO-friendly __typeof__.
> 
> I am not convinced that it's a good idea to make the implementation of
> kernel DIV_ROUND_CLOSEST() part of the kernel UAPI, which is what this
> change effectively does.
> 
> I'd at least like to get an ack from Andy Shevchenko first (Cc'd).

Thanks for Cc'ing me!

So, the history of the DIV_ROUND_UP() to appear in UAPI is a response to
the ethtool change that missed the fact that this was a kernel internal macro.
Giving a precedent there is no technical issues to add DIV_ROUND_CLOSEST()
to UAPI as proposed. Main question here is: Does DRM headers in question
(that are going to use it) really need this?

Interestingly that DRM also started using __KERNEL_DIV_ROUND_UP() in UAPI
at some point, which kinda makes an argument for allowing the other one.

Also fun fact: this series plead for a new macro for division while ignoring
existing (UAPI) macros for masks and bits.

0xffffU is effectively __GENMASK(15, 0). (And if you change the code, avoid
using variables inside GENMASK() macros, it may generate an awful code, the
GENMASK($HI, $LO) << foo is preferred over GENMASK(foo + $DELTA, foo) case.
GENMASK(foo - 1, 0) OTOH is fine, however be always careful against overflows
with left shifts, as BIT(foo) - 1 may not work for foo == 32, while GENMASK()
may not work for foo == 0).

So, I have no objections for either choice
Acked-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

...

But if you go that direction, please, fix up the style.

> > +/*
> > + * Divide positive or negative dividend by positive or negative divisor
> > + * and round to closest integer. Result is undefined for negative
> > + * divisors if the dividend variable type is unsigned and for negative
> > + * dividends if the divisor variable type is unsigned.
> > + */
> > +#define __KERNEL_DIV_ROUND_CLOSEST(x, divisor)(		\
> > +{							\

Use ({ on this line together...

> > +	__typeof__(x) __x = x;				\
> > +	__typeof__(divisor) __d = divisor;		\

+ blank line here.

> > +	(((__typeof__(x))-1) > 0 ||			\
> > +	 ((__typeof__(divisor))-1) > 0 ||		\
> > +	 (((__x) > 0) == ((__d) > 0))) ?		\
> > +		(((__x) + ((__d) / 2)) / (__d)) :	\
> > +		(((__x) - ((__d) / 2)) / (__d));	\
> > +}							\
> > +)

...as here join }) to be a single line.

+ blank line.

> >  #endif /* _UAPI_LINUX_CONST_H */

-- 
With Best Regards,
Andy Shevchenko