Re: [PATCH] drm/virtio: use %llu format string form atomic64_t

[Geert Uytterhoeven]

On Wed, Oct 7, 2015 at 1:23 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> 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.

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

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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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