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

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Message-ID: <824dc1d1-14f0-433e-aa3f-679527b87077@wanadoo.fr>
Date: Thu, 6 Mar 2025 02:17:18 +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 06/03/2025 at 00:48, Andy Shevchenko wrote:
> On Wed, Mar 05, 2025 at 11:48:10PM +0900, Vincent Mailhol wrote:
>> 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:
> 
> ...
> 
>>>> +#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.
> 
> Yes, but wouldn't be better to put it more explicitly like
> 
> #define BIT_U8(b)	(BIT_INPUT_CHECK(u8, b) + (u8)BIT(b) + 0 + UL(0)) // + ULL(0) ?

OK, the final result would be unsigned. But, I do not follow how this is
more explicit.

Also, why doing:

  (u8)BIT(b) + 0 + UL(0)

and not just:

  (u8)BIT(b) + UL(0)

?

What is that intermediary '+ 0' for?

I am sorry, but I am having a hard time understanding how casting to u8
and then doing an addition with an unsigned long is more explicit than
directly doing a cast to the desired type.

As I mentioned in my answer to Yuri, I have a slight preference for the
unsigned int cast, but I am OK to go back to the u8/u16 cast as it was
in v3.

However, I really do not see how that '+ 0 + UL(0)' would be an improvement.

> Also, BIT_Uxx() gives different type at the end, shouldn't they all be promoted
> to unsigned long long at the end? Probably it won't work in real assembly.
> Can you add test cases which are written in assembly? (Yes, I understand that it will
> be architecture dependent, but still.)

No. I purposely guarded the definition of the BIT_Uxx() by a

  #if !defined(__ASSEMBLY__)

so that these are never visible in assembly. I actually put a comment to
explain why the GENMASK_U*() are not available in assembly. I can copy
paste the same comment to explain why why BIT_U*() are not made
available either:

  /*
   * Missing asm support
   *
   * BIT_U*() depends on BITS_PER_TYPE() which would not work in the asm
   * 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.
   */

I really believe that it would be a mistake to make the GENMASK_U*() or
the BIT_U*() available to assembly.

>> 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))
> 

Yours sincerely,
Vincent Mailhol