linux-kernel - Re: [PATCH] iavf: Use printf instead of gnu_printf for iavf_debug

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Date:   Thu, 10 Jan 2019 12:07:15 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Subject: Re: [PATCH] iavf: Use printf instead of gnu_printf for iavf_debug_d

On Thu, Jan 10, 2019 at 10:54:46AM -0800, Nick Desaulniers wrote:
> On Wed, Jan 9, 2019 at 8:22 PM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > Clang warns:
> >
> > In file included from drivers/net/ethernet/intel/iavf/iavf_main.c:4:
> > In file included from drivers/net/ethernet/intel/iavf/iavf.h:37:
> > In file included from drivers/net/ethernet/intel/iavf/iavf_type.h:8:
> > drivers/net/ethernet/intel/iavf/iavf_osdep.h:49:18: warning: 'format' attribute argument not supported: gnu_printf [-Wignored-attributes]
> >         __attribute__ ((format(gnu_printf, 3, 4)));
> >                         ^
> > 1 warning generated.
> >
> > We can convert from gnu_printf to printf without any side effects for
> > two reasons:
> >
> > 1. All iavf_debug instances use standard printf formats, as pointed out
> >    by Miguel Ojeda at the below link, meaning gnu_printf is not strictly
> >    required.
> >
> > 2. However, GCC has aliased printf to gnu_printf on Linux since at least
> >    2010 based on git history.
> 
> Thanks for this fix!
> FWIW, using godbolt to see which version something was added in is
> better than giving a year of the commit.  I don't think it matters for
> this case, as the kernel already makes use of `printf` as you do in
> this change, but food for thought for next time.
> 

Yes that is a good point, I need to be taking advantage of that site
more.

Thank you for the review!
Nathan

> >
> >    From gcc/c-family/c-format.c:
> >
> >    /* Attributes such as "printf" are equivalent to those such as
> >       "gnu_printf" unless this is overridden by a target.  */
> >    static const target_ovr_attr gnu_target_overrides_format_attributes[] =
> >    {
> >      { "gnu_printf",   "printf" },
> >      { "gnu_scanf",    "scanf" },
> >      { "gnu_strftime", "strftime" },
> >      { "gnu_strfmon",  "strfmon" },
> >      { NULL,           NULL }
> >    };
> >
> > The mentioned override only happens on Windows (mingw32). Changing from
> > gnu_printf to printf is a no-op for GCC and stops Clang from warning.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/111
> > Suggested-by: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
> > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> > ---
> >  drivers/net/ethernet/intel/iavf/iavf_osdep.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/ethernet/intel/iavf/iavf_osdep.h b/drivers/net/ethernet/intel/iavf/iavf_osdep.h
> > index e6e0b0328706..c90cafb526d0 100644
> > --- a/drivers/net/ethernet/intel/iavf/iavf_osdep.h
> > +++ b/drivers/net/ethernet/intel/iavf/iavf_osdep.h
> > @@ -46,7 +46,7 @@ struct iavf_virt_mem {
> >
> >  #define iavf_debug(h, m, s, ...)  iavf_debug_d(h, m, s, ##__VA_ARGS__)
> >  extern void iavf_debug_d(void *hw, u32 mask, char *fmt_str, ...)
> > -       __attribute__ ((format(gnu_printf, 3, 4)));
> > +       __printf(3, 4);
> 
> And you make use of the __attribute__ wrapper from
> include/linux/compiler_attributes.h, cool.
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> 
> -- 
> Thanks,
> ~Nick Desaulniers