linux-kernel - Re: [PATCH v8 2/6] bits: introduce fixed-type GENMASK

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Message-ID: <Z-LYs9D8_Q76bDQk@smile.fi.intel.com>
Date: Tue, 25 Mar 2025 18:24:19 +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>,
	Catalin Marinas <catalin.marinas@....com>,
	Anshuman Khandual <anshuman.khandual@....com>,
	linux-arm-kernel@...ts.infradead.org,
	Jani Nikula <jani.nikula@...el.com>
Subject: Re: [PATCH v8 2/6] bits: introduce fixed-type GENMASK_U*()

On Wed, Mar 26, 2025 at 12:59:57AM +0900, Vincent Mailhol via B4 Relay wrote:
> From: Vincent Mailhol <mailhol.vincent@...adoo.fr>
> 
> 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

-Wshift-count-overflow

but it's not so important here. It's good that it's fixed in the comment.
In any case if Yury wants/can do the change, it might be done when applying.
(I.o.w. no need to resend)

> 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)))))
>         |                           ^~
> 
> The result is casted to the corresponding fixed width type. For
> example, GENMASK_U8() returns an u8. Note that because of the C
> promotion rules, GENMASK_U8() and GENMASK_U16() will immediately be
> promoted to int if used in an expression. Regardless, the main goal is
> not to get the correct type, but rather to enforce more checks at
> compile time.
> 
> While GENMASK_TYPE() is crafted to cover all variants, including the
> already existing GENMASK(), GENMASK_ULL() and GENMASK_U128(), for the
> moment, only use it for the newly introduced GENMASK_U*(). The
> consolidation will be done in a separate change.

The change LGTM, FWIW,
Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

-- 
With Best Regards,
Andy Shevchenko