linux-kernel - Re: [PATCH] drm/virtio: use %llu format string form atomic64

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Date:	Mon, 19 Oct 2015 12:39:09 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Ralf Baechle <ralf@...ux-mips.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	DRI Development <dri-devel@...ts.freedesktop.org>,
	virtualization@...ts.linux-foundation.org,
	Gerd Hoffmann <kraxel@...hat.com>,
	Dave Airlie <airlied@...hat.com>
Subject: Re: [PATCH] drm/virtio: use %llu format string form atomic64_t

On Mon, Oct 19, 2015 at 12:11 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Monday 19 October 2015 09:34:15 Geert Uytterhoeven wrote:
>> On Wed, Oct 7, 2015 at 1:23 PM, Arnd Bergmann <arnd@...db.de> wrote:
>> > static __inline__ int atomic64_add_unless(atomic64_t *v, long a, long u)
>> >
>> > which truncates the result to 32 bit.
>>
>> Woops.
>>
>> See also my unanswered question in "atomic64 on 32-bit vs 64-bit (was:
>> Re: Add virtio gpu driver.)", which is still valid:
>> https://lkml.org/lkml/2015/6/28/18
>>
>
> Regarding your question of
>
>> Instead of sprinkling casts, is there any good reason why atomic64_read()
>> and atomic64_t aren't "long long" everywhere, cfr. u64?
>
>
> I assume the answer is that some (all?) 64-bit architectures intentionally
> return 'long' here, in order for atomic_long_read() to return 'long' on
> all architectures, given the definitions from
> include/asm-generic/atomic-long.h
>
> We would have to either change those, or we have to pick between
> atomic_long_* or atomic64_* to have a consistent return type.

I guess the main reason is this comment in include/asm-generic/atomic-long.h,
which I hadn't noticed before:

 * Casts for parameters are avoided for existing atomic functions in order to
 * avoid issues with cast-as-lval under gcc 4.x and other limitations that the
 * macros of a platform may have.

Still, it's a pity, as printing atomic_64 is one more place where casts are
needed in callers.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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