linux-kernel - Re: [PATCH v6 1/3] phy: mediatek: fix build warning caused by clang

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Date:   Tue, 31 Jan 2023 08:57:11 +0000
From:   Chunfeng Yun (云春峰) 
        <Chunfeng.Yun@...iatek.com>
To:     "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>
CC:     "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "lkp@...el.com" <lkp@...el.com>,
        "nathan@...nel.org" <nathan@...nel.org>,
        "kishon@...nel.org" <kishon@...nel.org>,
        Eddie Hung (洪正鑫) 
        <Eddie.Hung@...iatek.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "vkoul@...nel.org" <vkoul@...nel.org>,
        "matthias.bgg@...il.com" <matthias.bgg@...il.com>,
        "linux-phy@...ts.infradead.org" <linux-phy@...ts.infradead.org>,
        "arnd@...db.de" <arnd@...db.de>,
        "trix@...hat.com" <trix@...hat.com>,
        "angelogioacchino.delregno@...labora.com" 
        <angelogioacchino.delregno@...labora.com>
Subject: Re: [PATCH v6 1/3] phy: mediatek: fix build warning caused by clang

On Fri, 2023-01-13 at 10:36 -0800, Nick Desaulniers wrote:
> On Wed, Jan 4, 2023 at 5:26 AM Chunfeng Yun <
> chunfeng.yun@...iatek.com> wrote:
> > 
> > Remove the temporary @mask_, this may cause build warning when use
> > clang
> > compiler for powerpc, but can't reproduce it when compile for
> > arm64.
> > the build warning is -Wtautological-constant-out-of-range-compare,
> > and
> > caused by
> > "BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask)"
> 
> Can you please include the text of the observed warning?
> 
> > 
> > After removing @mask_, there is a "CHECK:MACRO_ARG_REUSE" when run
> > checkpatch.pl, due to @mask is constant, no reuse problem will
> > happen.
> > 
> > Fixes: 84513eccd678 ("phy: mediatek: fix build warning of
> > FIELD_PREP()")
> 
> Uh, why was 84513eccd678 sent/reviewed/merged if it didn't fix the
> issue correctly in the first place?
> 
> Is the issue perhaps that your mask isn't wide enough in the first
> place, and should be?  See:
> commit cfd6fb45cfaf ("crypto: ccree - avoid out-of-range warnings
> from clang")
> for inspiration.
I look at this patch, it said that "
This does not happen in other places that just pass a constant here.
"
That's indeed true, no such warning before using FIELD_PREP() due to
the masks are always constant.

So seems that it can fix the waring if avoid using a temporary variable
@mask_.

Thanks a lot

> 
> > Reported-by: kernel test robot <lkp@...el.com>
> 
> Can you please include the Link: tag to the lore URL of the report?
> 
> > Signed-off-by: Chunfeng Yun <chunfeng.yun@...iatek.com>
> > ---
> > v6: modify the title
> > v5: no changes
> > v4 new patch, I'm not sure it can fix build warning, due to I don't
> > cross compile
> >     it for powerpc using clang in office.
> > ---
> >  drivers/phy/mediatek/phy-mtk-io.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/phy/mediatek/phy-mtk-io.h
> > b/drivers/phy/mediatek/phy-mtk-io.h
> > index d20ad5e5be81..58f06db822cb 100644
> > --- a/drivers/phy/mediatek/phy-mtk-io.h
> > +++ b/drivers/phy/mediatek/phy-mtk-io.h
> > @@ -39,8 +39,8 @@ static inline void mtk_phy_update_bits(void
> > __iomem *reg, u32 mask, u32 val)
> >  /* field @mask shall be constant and continuous */
> >  #define mtk_phy_update_field(reg, mask, val) \
> >  ({ \
> > -       typeof(mask) mask_ = (mask);    \
> > -       mtk_phy_update_bits(reg, mask_, FIELD_PREP(mask_, val)); \
> > +       BUILD_BUG_ON_MSG(!__builtin_constant_p(mask), "mask is not
> > constant"); \
> > +       mtk_phy_update_bits(reg, mask, FIELD_PREP(mask, val)); \
> >  })
> > 
> >  #endif
> > --
> > 2.18.0
> > 
> 
>