Re: [PATCH] drm/i915/hwmon: Fix a build error used with clang compiler

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Mon, 24 Oct 2022 14:09:53 -0700, Gwan-gyeong Mun wrote:
>

Hi GG,

> If a non-constant variable is used as the first argument of the FIELD_PREP
> macro, a build error occurs when using the clang compiler.
>
> Fix the following build error used with clang compiler:
>
> drivers/gpu/drm/i915/i915_hwmon.c:115:16: error: result of comparison of constant 18446744073709551615 with expression of type 'typeof (_Generic((field_msk), char: (unsigned char)0, unsigned char: (unsigned char)0, signed char: (unsigned char)0, unsigned short: (unsigned short)0, short: (unsigned short)0, unsigned int: (unsigned int)0, int: (unsigned int)0, unsigned long: (unsigned long)0, long: (unsigned long)0, unsigned long long: (unsigned long long)0, long long: (unsigned long long)0, default: (field_msk)))' (aka 'unsigned int') is always false [-Werror,-Wtautological-constant-out-of-range-compare]

What is 18446744073709551615? You may want to limit the length of this line
or checkpatch doesn't complain?

>         bits_to_set = FIELD_PREP(field_msk, nval);
>                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~
> ./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) >     \
>                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
> ./include/linux/build_bug.h:39:58: note: expanded from macro 'BUILD_BUG_ON_MSG'
>                                     ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~
> ./include/linux/compiler_types.h:357:22: note: expanded from macro 'compiletime_assert'
>         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
>         ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ./include/linux/compiler_types.h:345:23: note: expanded from macro '_compiletime_assert'
>         __compiletime_assert(condition, msg, prefix, suffix)
>         ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ./include/linux/compiler_types.h:337:9: note: expanded from macro '__compiletime_assert'
>                 if (!(condition))                                       \
>
> Fixes: 99f55efb7911 ("drm/i915/hwmon: Power PL1 limit and TDP setting")
> Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
> Cc: Anshuman Gupta <anshuman.gupta@intel.com>
> Cc: Andi Shyti <andi.shyti@linux.intel.com>
> Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_hwmon.c | 12 +++---------
>  1 file changed, 3 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_hwmon.c b/drivers/gpu/drm/i915/i915_hwmon.c
> index 9e9781493025..782a621b1928 100644
> --- a/drivers/gpu/drm/i915/i915_hwmon.c
> +++ b/drivers/gpu/drm/i915/i915_hwmon.c
> @@ -101,21 +101,16 @@ hwm_field_read_and_scale(struct hwm_drvdata *ddat, i915_reg_t rgadr,
>
>  static void
>  hwm_field_scale_and_write(struct hwm_drvdata *ddat, i915_reg_t rgadr,
> -			  u32 field_msk, int nshift,
> -			  unsigned int scale_factor, long lval)
> +			  int nshift, unsigned int scale_factor, long lval)
>  {
>	u32 nval;
> -	u32 bits_to_clear;
> -	u32 bits_to_set;
>
>	/* Computation in 64-bits to avoid overflow. Round to nearest. */
>	nval = DIV_ROUND_CLOSEST_ULL((u64)lval << nshift, scale_factor);
>
> -	bits_to_clear = field_msk;
> -	bits_to_set = FIELD_PREP(field_msk, nval);
> -
>	hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,
> -					    bits_to_clear, bits_to_set);
> +					    PKG_PWR_LIM_1,
> +					    FIELD_PREP(PKG_PWR_LIM_1, nval));

I don't want to give up so easily. We might have future uses for the
function where we want field_msk to be passed into the function (rather
than set inside the function as in this patch).

Do we understand what clang is complaining about? And why this compiles
with gcc?

Copying llvm@lists.linux.dev too.

Thanks.
--
Ashutosh


>  }
>
>  /*
> @@ -406,7 +401,6 @@ hwm_power_write(struct hwm_drvdata *ddat, u32 attr, int chan, long val)
>	case hwmon_power_max:
>		hwm_field_scale_and_write(ddat,
>					  hwmon->rg.pkg_rapl_limit,
> -					  PKG_PWR_LIM_1,
>					  hwmon->scl_shift_power,
>					  SF_POWER, val);
>		return 0;
> --
> 2.37.1
>