Linux Virtualization - Oct 2015 - [PATCH] drm/virtio: use %llu format string form atomic64_t

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.

	Arnd

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

On Wed, Oct 7, 2015 at 1:23 PM, Arnd Bergmann <arnd at arndb.de>
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 at
linux-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

On Wed, Oct 07, 2015 at 01:23:07PM +0200, Arnd Bergmann wrote:
> > 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.
Eh...  The result is 0/1 so nothing is truncated.  Alpha, MIPS,
PARISC and PowerPC are using the same prototype and x86 only differs
in the use of inline instead __inline__.  And anyway, that function on
MIPS is only built for CONFIG_64BIT.

What's wrong on MIPS is the comment describing the function's return
value
which was changed by f24219b4e90cf70ec4a211b17fbabc725a0ddf3c (atomic: move
atomic_add_unless to generic code) and I've queued up a patch to fix that
since a few days.  I guess that was a cut and paste error from
__atomic_add_unless which indeed does return the old value.

  Ralf