linux-kernel - Re: [PATCH v2 1/8] minmax: Put all the clamp() definitions together

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Message-Id: <f88a19d1-c374-43d1-a905-1e973fb6ce5a@app.fastmail.com>
Date: Tue, 30 Jul 2024 12:10:41 +0200
From: "Arnd Bergmann" <arnd@...nel.org>
To: "Linus Torvalds" <torvalds@...uxfoundation.org>
Cc: "David Laight" <David.Laight@...lab.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "Jens Axboe" <axboe@...nel.dk>, "Matthew Wilcox" <willy@...radead.org>,
 "Christoph Hellwig" <hch@...radead.org>,
 "Andrew Morton" <akpm@...ux-foundation.org>,
 "Andy Shevchenko" <andriy.shevchenko@...ux.intel.com>,
 "Dan Carpenter" <dan.carpenter@...aro.org>,
 "Jason A . Donenfeld" <Jason@...c4.com>,
 "pedro.falcato@...il.com" <pedro.falcato@...il.com>,
 "Mateusz Guzik" <mjguzik@...il.com>,
 "linux-mm@...ck.org" <linux-mm@...ck.org>,
 "Lorenzo Stoakes" <lorenzo.stoakes@...cle.com>
Subject: Re: [PATCH v2 1/8] minmax: Put all the clamp() definitions together

On Tue, Jul 30, 2024, at 05:59, Linus Torvalds wrote:
> On Mon, 29 Jul 2024 at 16:21, Linus Torvalds <torvalds@...uxfoundation.org> wrote:
>>
>> Attached is the patch I have in my tree right now - it complains about
>> a 'bcachefs' comparison between an 'u16' and a 's64', because I also
>> removed the 'implicit integer promotion is ok' logic, because I think
>> it's wrong.

I'm giving this a spin on the randconfig test setup now to see
if there are some other cases like the bcachefs one. So far I've
seen one failure, but I can't make sense of it yet:

drivers/gpu/drm/i915/display/intel_backlight.c: In function 'scale':
include/linux/compiler_types.h:510:45: error: call to '__compiletime_assert_905' declared with attribute error: clamp() low limit source_min greater than high limit source_max
include/linux/minmax.h:107:9: note: in expansion of macro 'BUILD_BUG_ON_MSG'
  107 |         BUILD_BUG_ON_MSG(statically_true(ulo > uhi),                            \
drivers/gpu/drm/i915/display/intel_backlight.c:47:22: note: in expansion of macro 'clamp'
   47 |         source_val = clamp(source_val, source_min, source_max);

See https://pastebin.com/raw/yLJ5ZqVw for the x86-64 .config
that triggered this.

>> I don't think a min(u16,s64) is a valid minimum, for exactly the same
>> reason a min(u32,s64) is not valid.
>
> Oh, and I noticed that it screws up the 32-bit case, and that does
> need a workaround for that.
>
> So here's a better version. The patch contains one possible fix to
> bcachefs for the type confusion there, but I'll wait for Kent to
> respond on that.

That's still a typo in the 32-bit case, right?
I've changed

 __builtin_choose_expr(sizeof(ux)>32,1LL,1L))

to check for sizeof(ux)>4 for my testing.

    Arnd