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

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Date:	Mon, 19 Oct 2015 12:11 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Geert Uytterhoeven <geert@...ux-m68k.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 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.

	Arnd
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