linux-kernel - Re: [PATCH v4 3/8] bits: introduce fixed-type genmasks

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Message-ID: <Z8hf7pN84-64LWPv@smile.fi.intel.com>
Date: Wed, 5 Mar 2025 16:30:06 +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 v4 3/8] bits: introduce fixed-type genmasks

On Wed, Mar 05, 2025 at 10:00:15PM +0900, Vincent Mailhol via B4 Relay wrote:
> From: Yury Norov <yury.norov@...il.com>
> 
> Add __GENMASK_t() which generalizes __GENMASK() to support different

Is it with double underscore? I do not see it.

_t is used for typedef simple types. It's unfortunate to have it
in such a macro. Perhaps T or TYPE will suffice. Or perhaps we want
__GENMASK_Uxx() here?

> 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:41:31: error: left shift count >= width of type [-Werror=shift-count-overflow]
> 	   41 |          (((t)~0ULL - ((t)(1) << (l)) + 1) & \
> 	      |                               ^~

...

> + * __GENMASK_U*() depends on BITS_PER_TYPE() which would not work in the asm

Where are the double underscore variants? I see it only for U128.

> + * code as BITS_PER_TYPE() relies on sizeof(), something not available in
> + * asm. Nethertheless, the concept of fixed width integers is a C thing which
> + * does not apply to assembly code.

-- 
With Best Regards,
Andy Shevchenko