[PATCH] clk: sp7021: Adjust width of _m in HWM_FIELD_PREP()
Nick Desaulniers
ndesaulniers at google.com
Mon May 1 15:15:22 PDT 2023
On Mon, May 1, 2023 at 3:03 PM Nick Desaulniers <ndesaulniers at google.com> wrote:
>
> On Mon, May 1, 2023 at 2:37 PM Nathan Chancellor <nathan at kernel.org> wrote:
> >
> > On Mon, May 01, 2023 at 02:34:47PM -0700, Nathan Chancellor wrote:
> > > When building with clang + W=1, there is a warning around an internal
> > > comparison check within the FIELD_PREP() macro, due to a 32-bit variable
> > > comparison against ~0ull:
> > >
> > > drivers/clk/clk-sp7021.c:316:8: error: result of comparison of constant 18446744073709551615 with expression of type 'typeof (_Generic((_m), ...' (aka 'unsigned int') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
> > > r0 |= HWM_FIELD_PREP(MASK_SEL_FRA, clk->p[SEL_FRA]);
> > > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > drivers/clk/clk-sp7021.c:45:15: note: expanded from macro 'HWM_FIELD_PREP'
> > > (_m << 16) | FIELD_PREP(_m, value); \
> > > ^~~~~~~~~~~~~~~~~~~~~
> > > include/linux/bitfield.h:114:3: note: expanded from macro 'FIELD_PREP'
> > > __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: "); \
> > > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > include/linux/bitfield.h:71:53: note: expanded from macro '__BF_FIELD_CHECK'
> > > BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) > \
> > > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
> > > note: (skipping 1 expansions in backtrace; use -fmacro-backtrace-limit=0 to see all)
> > > include/linux/compiler_types.h:397:22: note: expanded from macro 'compiletime_assert'
> > > _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> > > ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > include/linux/compiler_types.h:385:23: note: expanded from macro '_compiletime_assert'
> > > __compiletime_assert(condition, msg, prefix, suffix)
> > > ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > include/linux/compiler_types.h:377:9: note: expanded from macro '__compiletime_assert'
> > > if (!(condition)) \
> > > ^~~~~~~~~
> > >
> > > This is expected given the tyoes of the input. Increase the size of the
>
> s/tyoes/types/
>
> Thanks for the patch!
>
> Reviewed-by: Nick Desaulniers <ndesaulniers at google.com>
>
> Nathan, do you think anything can be done in FIELD_PREP or even
> __BF_FIELD_CHECK using _Static_assert (or static_assert from
> include/linux/build_bug.h) to help elucidate to developers that their
> mask is too small? I guess the line 71 of include/linux/bitfield.h
> that is trying to warn via BUILD_BUG_ON is itself triggering a
> -Wtautological-constant-out-of-range-compare warning that is perhaps
> distracting from what the issue is?
>
> If you're able to reproduce the issue locally still, does this seem to work?
> ```
> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> index ebfa12f69501..762a27b5ae15 100644
> --- a/include/linux/bitfield.h
> +++ b/include/linux/bitfield.h
> @@ -62,15 +62,15 @@
>
> #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx) \
> ({ \
> + static_assert(__bf_cast_unsigned(_mask, _mask) <= \
> + __bf_cast_unsigned(_reg, ~0ull), \
> + _pfx "type of reg too small for mask"); \
> BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask), \
> _pfx "mask is not constant"); \
> BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero"); \
> BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ? \
> ~((_mask) >> __bf_shf(_mask)) & (_val) : 0, \
> _pfx "value too large for the field"); \
> - BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) > \
> - __bf_cast_unsigned(_reg, ~0ull), \
> - _pfx "type of reg too small for mask"); \
> __BUILD_BUG_ON_NOT_POWER_OF_2((_mask) + \
> (1ULL << __bf_shf(_mask))); \
> })
>
> ```
Hmmm....maybe not:
```
drivers/gpu/drm/i915/i915_hwmon.c:97:14: error: static assertion
expression is not an integral constant expression
reg_value = REG_FIELD_GET(field_msk, reg_value);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./drivers/gpu/drm/i915/i915_reg_defs.h:87:44: note: expanded from
macro 'REG_FIELD_GET'
#define REG_FIELD_GET(__mask, __val) ((u32)FIELD_GET(__mask, __val))
^~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/bitfield.h:154:3: note: expanded from macro 'FIELD_GET'
__BF_FIELD_CHECK(_mask, _reg, 0U, "FIELD_GET: "); \
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/bitfield.h:65:17: note: expanded from macro '__BF_FIELD_CHECK'
static_assert(__bf_cast_unsigned(_mask, _mask) <= \
~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/bitfield.h:61:70: note: expanded from macro '__bf_cast_unsigned'
#define __bf_cast_unsigned(type, x) ((__unsigned_scalar_typeof(type))(x))
^
./include/linux/build_bug.h:77:50: note: expanded from macro 'static_assert'
#define static_assert(expr, ...) __static_assert(expr, ##__VA_ARGS__, #expr)
~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:78:56: note: expanded from macro '__static_assert'
#define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
^~~~
```
so there's probably a use of REG_FIELD_GET that is
__builtin_constant_p but not an integral constant expression. That's
too bad...
>
>
> > > temporary variable in HWM_FIELD_PREP() to eliminate the warning, which
> > > follows the logic of commit cfd6fb45cfaf ("crypto: ccree - avoid
> > > out-of-range warnings from clang") for the same reasons.
> >
> > Gah, I forgot:
> >
> > Reported-by: kernel test robot <lkp at intel.com>
> > Link: https://lore.kernel.org/202303221947.pXP2v4xJ-lkp@intel.com/
> >
> > > Signed-off-by: Nathan Chancellor <nathan at kernel.org>
> > > ---
> > > drivers/clk/clk-sp7021.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/clk/clk-sp7021.c b/drivers/clk/clk-sp7021.c
> > > index 8fec14120105..11d22043ddd7 100644
> > > --- a/drivers/clk/clk-sp7021.c
> > > +++ b/drivers/clk/clk-sp7021.c
> > > @@ -41,7 +41,7 @@ enum {
> > > /* HIWORD_MASK FIELD_PREP */
> > > #define HWM_FIELD_PREP(mask, value) \
> > > ({ \
> > > - u32 _m = mask; \
> > > + u64 _m = mask; \
> > > (_m << 16) | FIELD_PREP(_m, value); \
> > > })
> > >
> > >
> > > ---
> > > base-commit: d54c1fd4a51e8fbc7f9da86b0cd338a4f7cd2bb2
> > > change-id: 20230501-sp7021-field_prep-warning-223f17aeea8e
> > >
> > > Best regards,
> > > --
> > > Nathan Chancellor <nathan at kernel.org>
> > >
>
>
>
> --
> Thanks,
> ~Nick Desaulniers
--
Thanks,
~Nick Desaulniers
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