linux-kernel - Re: [PATCH v4 4/8] bits: introduce fixed-type BIT

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Message-ID: <d7f3150d-0167-44be-90b2-17f8a050687c@wanadoo.fr>
Date: Wed, 5 Mar 2025 23:48:10 +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 4/8] bits: introduce fixed-type BIT

On 05/03/2025 at 23:33, Andy Shevchenko wrote:
> On Wed, Mar 05, 2025 at 10:00:16PM +0900, Vincent Mailhol via B4 Relay wrote:
>> From: Lucas De Marchi <lucas.demarchi@...el.com>
>>
>> Implement fixed-type BIT to help drivers add stricter checks, like was
> 
> Here and in the Subject I would use BIT_Uxx().
> 
>> done for GENMASK().
> 
> ...
> 
>> +/*
>> + * Fixed-type variants of BIT(), with additional checks like GENMASK_t(). The
> 
> GENMASK_t() is not a well named macro.

Ack. I will rename to GENMASK_TYPE().

>> + * following examples generate compiler warnings due to shift-count-overflow:
>> + *
>> + * - BIT_U8(8)
>> + * - BIT_U32(-1)
>> + * - BIT_U32(40)
>> + */
>> +#define BIT_INPUT_CHECK(type, b) \
>> +	BUILD_BUG_ON_ZERO(const_true((b) >= BITS_PER_TYPE(type)))
>> +
>> +#define BIT_U8(b) (BIT_INPUT_CHECK(u8, b) + (unsigned int)BIT(b))
>> +#define BIT_U16(b) (BIT_INPUT_CHECK(u16, b) + (unsigned int)BIT(b))
> 
> Why not u8 and u16? This inconsistency needs to be well justified.

Because of the C integer promotion rules, if casted to u8 or u16, the
expression will immediately become a signed integer as soon as it is get
used. For example, if casted to u8

  BIT_U8(0) + BIT_U8(1)

would be a signed integer. And that may surprise people.

David also pointed this in the v3:

https://lore.kernel.org/intel-xe/d42dc197a15649e69d459362849a37f2@AcuMS.aculab.com/

and I agree with his comment.

I explained this in the changelog below the --- cutter, but it is
probably better to make the explanation more visible. I will add a
comment in the code to explain this.

>> +#define BIT_U32(b) (BIT_INPUT_CHECK(u32, b) + (u32)BIT(b))
>> +#define BIT_U64(b) (BIT_INPUT_CHECK(u64, b) + (u64)BIT_ULL(b))
> 
> Can you also use a TAB between the parentheses for better readability?
> E.g.,
> 
> #define BIT_U64(b)r	(BIT_INPUT_CHECK(u64, b) + (u64)BIT_ULL(b))

Sure. I prefer it with space, but no strong opinion. I will put tab in v5.

Yours sincerely,
Vincent Mailhol