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

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Message-ID: <9f0215c4-62e5-4dfa-8dac-732682600c8c@wanadoo.fr>
Date: Wed, 5 Mar 2025 23:38:19 +0900
From: Vincent Mailhol <mailhol.vincent@...adoo.fr>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
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 05/03/2025 at 23:30, Andy Shevchenko wrote:
> 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.

This is my mistake. In an earlier draft, it was __GENMASK_t(),
meanwhile, I dropped the __ prefix but forget to update the patch
description.

> _t is used for typedef simple types. It's unfortunate to have it
> in such a macro.

Ack.

> Perhaps T or TYPE will suffice. Or perhaps we want
> __GENMASK_Uxx() here?

If no objection, I have a preference for GENMASK_TYPE().

>> 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.

Same as above. The description is incorrect. I will fix this in v5.

>> + * 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.

Yours sincerely,
Vincent Mailhol