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

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Message-ID: <5152101.mD2bWzUJ2V@wuerfel>
Date:	Wed, 07 Oct 2015 13:23:07 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org, linux-mips@...ux-mips.org
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	virtualization@...ts.linux-foundation.org,
	Gerd Hoffmann <kraxel@...hat.com>,
	Dave Airlie <airlied@...hat.com>,
	Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [PATCH] drm/virtio: use %llu format string form atomic64_t

On Wednesday 07 October 2015 13:04:06 Arnd Bergmann wrote:
> On Wednesday 07 October 2015 11:45:02 Russell King - ARM Linux wrote:
> > On Wed, Oct 07, 2015 at 12:41:21PM +0200, Arnd Bergmann wrote:
> > > The virtgpu driver prints the last_seq variable using the %ld or
> > > %lu format string, which does not work correctly on all architectures
> > > and causes this compiler warning on ARM:
> > > 
> > > drivers/gpu/drm/virtio/virtgpu_fence.c: In function 'virtio_timeline_value_str':
> > > drivers/gpu/drm/virtio/virtgpu_fence.c:64:22: warning: format '%lu' expects argument of type 'long unsigned int', but argument 4 has type 'long long int' [-Wformat=]
> > >   snprintf(str, size, "%lu", atomic64_read(&fence->drv->last_seq));
> > >                       ^
> > > drivers/gpu/drm/virtio/virtgpu_debugfs.c: In function 'virtio_gpu_debugfs_irq_info':
> > > drivers/gpu/drm/virtio/virtgpu_debugfs.c:37:16: warning: format '%ld' expects argument of type 'long int', but argument 3 has type 'long long int' [-Wformat=]
> > >   seq_printf(m, "fence %ld %lld\n",
> > >                 ^
> > > 
> > > In order to avoid the warnings, this changes the format strings to %llu
> > > and adds a cast to u64, which makes it work the same way everywhere.
> > 
> > You have to wonder why atomic64_* functions do not use u64 types.
> > If they're not reliant on manipulating 64-bit quantities, then what's
> > the point of calling them atomic _64_.
> 
> I haven't checked all architectures, but I assume what happens is that
> 64-bit ones just #define atomic64_t atomic_long_t, so they don't have
> to provide three sets of functions.

scratch that, I just looked at all the architectures and found that it's
just completely arbitrary, even within one architecture you get a mix
of 'long' and 'long long', plus this gem from MIPS:

static __inline__ int atomic64_add_unless(atomic64_t *v, long a, long u)

which truncates the result to 32 bit.

	Arnd
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