linux-kernel - Re: [PATCH v6 2/7] bits: introduce fixed-type genmasks

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Message-ID: <Z8sw9P5__EAW8RQh@smile.fi.intel.com>
Date: Fri, 7 Mar 2025 19:46:28 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: mailhol.vincent@...adoo.fr
Cc: Yury Norov <yury.norov@...il.com>,
	Lucas De Marchi <lucas.demarchi@...el.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Jani Nikula <jani.nikula@...ux.intel.com>,
	Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@...el.com>,
	Tvrtko Ursulin <tursulin@...ulin.net>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org,
	Andi Shyti <andi.shyti@...ux.intel.com>,
	David Laight <David.Laight@...lab.com>,
	Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
	Jani Nikula <jani.nikula@...el.com>
Subject: Re: [PATCH v6 2/7] bits: introduce fixed-type genmasks

On Sat, Mar 08, 2025 at 01:48:49AM +0900, Vincent Mailhol via B4 Relay wrote:
> From: Yury Norov <yury.norov@...il.com>
> 
> Add GENMASK_TYPE() which generalizes __GENMASK() to support different
> types, and implement fixed-types versions of GENMASK() based on it.
> The fixed-type version allows more strict checks to the min/max values
> accepted, which is useful for defining registers like implemented by
> i915 and xe drivers with their REG_GENMASK*() macros.
> 
> The strict checks rely on shift-count-overflow compiler check to fail
> the build if a number outside of the range allowed is passed.
> Example:
> 
>   #define FOO_MASK GENMASK_U32(33, 4)
> 
> will generate a warning like:
> 
>   include/linux/bits.h:51:27: error: right shift count >= width of type [-Werror=shift-count-overflow]
>      51 |               type_max(t) >> (BITS_PER_TYPE(t) - 1 - (h)))))
>         |                           ^~

...

>  /*
>   * Missing asm support
>   *

> + * GENMASK_U*() depends on BITS_PER_TYPE() which relies on sizeof(),

s/depends/depend/ (we are already referring to a plural)

> + * something not available in asm. Nethertheless, fixed width integers
> + * is a C concept. Assembly code can rely on the long and long long
> + * versions instead.
>   */

-- 
With Best Regards,
Andy Shevchenko