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

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

[Nick Desaulniers]

Start of lore thread for context:
https://lore.kernel.org/intel-gfx/20221024210953.1572998-1-gwan-gyeong.mun@intel.com/

On Tue, Oct 25, 2022 at 2:25 AM Andi Shyti <andi.shyti@linux.intel.com> wrote:
>
> Hi Ashutosh,
>
> > > 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?
>
> yeah! I am not a clang user, and this must be some ugly error
> output. I don't think it makes sense to break it, though.
>
> > >         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?
>
> Because we are not compiling the builtin functions with gcc but
> gcc has support for them. The FIELD_PREP checks if the first
> parameter is a constant:
>
>         BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),
>
> where _mask was our field_mask, but we ignore it. Apparently
> clang doesn't.

So we've been in this code before. I'm having vague memories of
commit 444da3f52407 ("bitfield.h: don't compile-time validate _val in
FIELD_FIT")

But looking at the first __builtin_constant_p check in
__BF_FIELD_CHECK, I'm curious if that might need to be __is_constexpr
rather than __builtin_constant_p; a change like
commit 4d45bc82df66 ("coresight: etm4x: avoid build failure with
unrolled loops")

__builtin_constant_p is evaluated after most optimizations have run;
__is_constexpr must be evaluated by compiler front ends during
semantic analysis.

But reading through the full lore thread, it sounds like Jani has
another suggestion to try instead.
https://lore.kernel.org/intel-gfx/87eduwdllr.fsf@intel.com/

Please re-cc us and our list if that doesn't work out.

>
> If we want to stick to gcc only, then I still think the patch is
> correct for two reasons:
>
>   1. it's cleaner
>   2. we would get on with the job and if one day we will decide
>      to suppport builtin functions in gcc as well, we will sleep
>      peacefully :)
>
> > Copying llvm@lists.linux.dev too.
>
> maybe llvm folks have a better opinion.
>
> Thanks,
> Andi
>
> > 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
> > >
>


-- 
Thanks,
~Nick Desaulniers

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

[Dixit, Ashutosh]

On Tue, 25 Oct 2022 02:25:06 -0700, Andi Shyti wrote:
>
> Hi Ashutosh,

Hi Andi :)

> > > 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.

A "non-constant variable" does not seem to be the cause of the compile
error with clang, see below.

>
> > > 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?
>
> yeah! I am not a clang user, and this must be some ugly error
> output. I don't think it makes sense to break it, though.

18446744073709551615 == ~0ull (see use in __BF_FIELD_CHECK).

>
> > >         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) >     \

So clang seems to break here at this line in __BF_FIELD_CHECK (note ~0ull
also occurs here):

		BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >	\
				 __bf_cast_unsigned(_reg, ~0ull),	\
				 _pfx "type of reg too small for mask"); \

So it goes through previous checks including the "mask is not constant"
check. As Nick Desaulniers mentions "__builtin_constant_p is evaluated
after most optimizations have run" so by that time both compilers (gcc and
clang) have figured out that even though _mask is coming in as function
argument it is really the constant below:

#define   PKG_PWR_LIM_1		REG_GENMASK(14, 0)

But it is not clear why clang chokes on this "type of reg too small for
mask" check (and gcc doesn't) since everything is u32.

It is for this reason I want someone from llvm to chime in.

> > >                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
> > > ./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?
>
> Because we are not compiling the builtin functions with gcc but
> gcc has support for them. The FIELD_PREP checks if the first
> parameter is a constant:
>
>	BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),
>
> where _mask was our field_mask, but we ignore it. Apparently
> clang doesn't.

So we have debunked this above.

> If we want to stick to gcc only, then I still think the patch is
> correct for two reasons:
>
>   1. it's cleaner
>   2. we would get on with the job and if one day we will decide
>      to suppport builtin functions in gcc as well, we will sleep
>      peacefully :)

I disagree with the patch even if we need to fix this in i915 (rather than
say change the headers or something in clang).

Note that hwm_field_scale_and_write() pairs with hwm_field_read_and_scale()
(they are basically a set/get pair) so it is desirable they have identical
arguments. This patch breaks that symmetry.

If we have to fix this in i915, I prefer the following patch (so just skip
the checks in FIELD_PREP):

@@ -112,7 +112,7 @@ hwm_field_scale_and_write(struct hwm_drvdata *ddat, i915_reg_t rgadr,
        nval = DIV_ROUND_CLOSEST_ULL((u64)lval << nshift, scale_factor);

        bits_to_clear = field_msk;
-       bits_to_set = FIELD_PREP(field_msk, nval);
+       bits_to_set = (nval << __bf_shf(field_msk)) & field_msk;

        hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,

But I'd wait to hear from clang/llvm folks first.

> > Copying llvm@lists.linux.dev too.
>
> maybe llvm folks have a better opinion.
>

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
> > >

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

[Andi Shyti]

Hi Ashutosh,

> On Tue, 25 Oct 2022 02:25:06 -0700, Andi Shyti wrote:
> >
> > Hi Ashutosh,
> 
> Hi Andi :)
> 
> > > > 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.
> 
> A "non-constant variable" does not seem to be the cause of the compile
> error with clang, see below.
> 
> >
> > > > 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?
> >
> > yeah! I am not a clang user, and this must be some ugly error
> > output. I don't think it makes sense to break it, though.
> 
> 18446744073709551615 == ~0ull (see use in __BF_FIELD_CHECK).

I just wonder, then, where this number comes from, looks to me
like an ill formatted constant coming from the compiler
(definitely bigger than a ull).

> >
> > > >         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) >     \
> 
> So clang seems to break here at this line in __BF_FIELD_CHECK (note ~0ull
> also occurs here):
> 
> 		BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >	\
> 				 __bf_cast_unsigned(_reg, ~0ull),	\
> 				 _pfx "type of reg too small for mask"); \
> 
> So it goes through previous checks including the "mask is not constant"
> check. As Nick Desaulniers mentions "__builtin_constant_p is evaluated
> after most optimizations have run" so by that time both compilers (gcc and
> clang) have figured out that even though _mask is coming in as function
> argument it is really the constant below:
> 
> #define   PKG_PWR_LIM_1		REG_GENMASK(14, 0)

I also thought that the compiler should have figured it out, but
then why we got that error, and I don't see how
"bf_cast_unsigned(_reg, ~0ull)" could fail.

> But it is not clear why clang chokes on this "type of reg too small for
> mask" check (and gcc doesn't) since everything is u32.
> 
> It is for this reason I want someone from llvm to chime in.
> 
> > > >                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
> > > > ./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?
> >
> > Because we are not compiling the builtin functions with gcc but
> > gcc has support for them. The FIELD_PREP checks if the first
> > parameter is a constant:
> >
> >	BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),
> >
> > where _mask was our field_mask, but we ignore it. Apparently
> > clang doesn't.
> 
> So we have debunked this above.
> 
> > If we want to stick to gcc only, then I still think the patch is
> > correct for two reasons:
> >
> >   1. it's cleaner
> >   2. we would get on with the job and if one day we will decide
> >      to suppport builtin functions in gcc as well, we will sleep
> >      peacefully :)
> 
> I disagree with the patch even if we need to fix this in i915 (rather than
> say change the headers or something in clang).
> 
> Note that hwm_field_scale_and_write() pairs with hwm_field_read_and_scale()
> (they are basically a set/get pair) so it is desirable they have identical
> arguments. This patch breaks that symmetry.

OK, didn't see it! Makes sense.

> If we have to fix this in i915, I prefer the following patch (so just skip
> the checks in FIELD_PREP):
> 
> @@ -112,7 +112,7 @@ hwm_field_scale_and_write(struct hwm_drvdata *ddat, i915_reg_t rgadr,
>         nval = DIV_ROUND_CLOSEST_ULL((u64)lval << nshift, scale_factor);
> 
>         bits_to_clear = field_msk;
> -       bits_to_set = FIELD_PREP(field_msk, nval);
> +       bits_to_set = (nval << __bf_shf(field_msk)) & field_msk;
> 
>         hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,

doesn't look pretty, though! :/

> But I'd wait to hear from clang/llvm folks first.

Yeah! Looking forward to getting some ideas :)

Thanks, Ashutosh!
Andi

> > > Copying llvm@lists.linux.dev too.
> >
> > maybe llvm folks have a better opinion.
> >
> 
> 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
> > > >

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

[Nick Desaulniers]

On Tue, Oct 25, 2022 at 5:18 PM Andi Shyti <andi.shyti@linux.intel.com> wrote:
>
> Hi Ashutosh,
>
> > But I'd wait to hear from clang/llvm folks first.
>
> Yeah! Looking forward to getting some ideas :)

Gwan-gyeong, which tree and set of configs are necessary to reproduce
the observed warning?

Warnings are treated as errors, so I don't want this breaking our CI.
-- 
Thanks,
~Nick Desaulniers

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

[Dixit, Ashutosh]

On Thu, 27 Oct 2022 09:35:24 -0700, Nick Desaulniers wrote:
>

Hi Nick,

> On Tue, Oct 25, 2022 at 5:18 PM Andi Shyti <andi.shyti@linux.intel.com> wrote:
> >
> > Hi Ashutosh,
> >
> > > But I'd wait to hear from clang/llvm folks first.
> >
> > Yeah! Looking forward to getting some ideas :)
>
> Gwan-gyeong, which tree and set of configs are necessary to reproduce
> the observed warning?
>
> Warnings are treated as errors, so I don't want this breaking our CI.

The following or equivalent should do it:

git clone https://anongit.freedesktop.org/git/drm/drm-tip
git checkout drm-tip

Kernel config:
CONFIG_DRM_I915=m
CONFIG_HWMON=y

Files:
drivers/gpu/drm/i915/i915_hwmon.c/.h

Thanks for taking a look.
--
Ashutosh

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

[Nick Desaulniers]

On Thu, Oct 27, 2022 at 9:53 AM Dixit, Ashutosh
<ashutosh.dixit@intel.com> wrote:
>
> On Thu, 27 Oct 2022 09:35:24 -0700, Nick Desaulniers wrote:
> >
>
> Hi Nick,
>
> > On Tue, Oct 25, 2022 at 5:18 PM Andi Shyti <andi.shyti@linux.intel.com> wrote:
> > >
> > > Hi Ashutosh,
> > >
> > > > But I'd wait to hear from clang/llvm folks first.
> > >
> > > Yeah! Looking forward to getting some ideas :)
> >
> > Gwan-gyeong, which tree and set of configs are necessary to reproduce
> > the observed warning?
> >
> > Warnings are treated as errors, so I don't want this breaking our CI.
>
> The following or equivalent should do it:
>
> git clone https://anongit.freedesktop.org/git/drm/drm-tip
> git checkout drm-tip
>
> Kernel config:
> CONFIG_DRM_I915=m
> CONFIG_HWMON=y
>
> Files:
> drivers/gpu/drm/i915/i915_hwmon.c/.h
>
> Thanks for taking a look.

Thanks, I can repro now.

I haven't detangled the macro soup, but I noticed:

1. FIELD_PREP is defined in include/linux/bitfield.h which has the
following comment:
 18  * Mask must be a compilation time constant.

2. hwm_field_scale_and_write only has one callsite.

The following patch works:

```
diff --git a/drivers/gpu/drm/i915/i915_hwmon.c
b/drivers/gpu/drm/i915/i915_hwmon.c
index 9e9781493025..6ac29d90b92a 100644
--- a/drivers/gpu/drm/i915/i915_hwmon.c
+++ b/drivers/gpu/drm/i915/i915_hwmon.c
@@ -101,7 +101,7 @@ 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,
+                         int nshift,
                          unsigned int scale_factor, long lval)
 {
        u32 nval;
@@ -111,8 +111,8 @@ hwm_field_scale_and_write(struct hwm_drvdata
*ddat, i915_reg_t rgadr,
        /* 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);
+       bits_to_clear = PKG_PWR_LIM_1;
+       bits_to_set = FIELD_PREP(PKG_PWR_LIM_1, nval);

        hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,
                                            bits_to_clear, bits_to_set);
@@ -406,7 +406,6 @@ 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;
```
Though I'm not sure if you're planning to add further callsites of
hwm_field_scale_and_write with different field_masks?

Alternatively, (without the above diff),

```
diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index c9be1657f03d..6f40f12bcf89 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -8,6 +8,7 @@
 #define _LINUX_BITFIELD_H

 #include <linux/build_bug.h>
+#include <linux/const.h>
 #include <asm/byteorder.h>

 /*
@@ -62,7 +63,7 @@

 #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx)                      \
        ({                                                              \
-               BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),          \
+               BUILD_BUG_ON_MSG(!__is_constexpr(_mask),                \
                                 _pfx "mask is not constant");          \
                BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero");    \
                BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?           \
```
will produce:
error: call to __compiletime_assert_407 declared with 'error'
attribute: FIELD_PREP: mask is not constant

I haven't tested if that change is also feasible (on top of fixing
this specific instance), but I think it might help avoid more of these
subtleties wrt. __builtin_constant_p that depende heavily on compiler,
compiler version, optimization level.
-- 
Thanks,
~Nick Desaulniers

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

[Dixit, Ashutosh]

On Thu, 27 Oct 2022 10:16:47 -0700, Nick Desaulniers wrote:
>

Hi Nick,

> Thanks, I can repro now.
>
> I haven't detangled the macro soup, but I noticed:
>
> 1. FIELD_PREP is defined in include/linux/bitfield.h which has the
> following comment:
>  18  * Mask must be a compilation time constant.

I had comments about this here:

https://lore.kernel.org/intel-gfx/87ilk7pwrw.wl-ashutosh.dixit@intel.com/

The relevant part being:

---- {quote} ----
> > > ./include/linux/bitfield.h:71:53: note: expanded from macro '__BF_FIELD_CHECK'
> > >                 BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >     \

So clang seems to break here at this line in __BF_FIELD_CHECK (note ~0ull
also occurs here):

		BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >	\
				 __bf_cast_unsigned(_reg, ~0ull),	\
				 _pfx "type of reg too small for mask"); \

So it goes through previous checks including the "mask is not constant"
check. As Nick Desaulniers mentions "__builtin_constant_p is evaluated
after most optimizations have run" so by that time both compilers (gcc and
clang) have figured out that even though _mask is coming in as function
argument it is really the constant below:

#define   PKG_PWR_LIM_1		REG_GENMASK(14, 0)

But it is not clear why clang chokes on this "type of reg too small for
mask" check (and gcc doesn't) since everything is u32.
---- {end quote} ----

>
> 2. hwm_field_scale_and_write only has one callsite.
>
> The following patch works:

If we need to fix it at our end yes we can come up with one of these
patches. But we were hoping someone from clang/llvm can comment about the
"type of reg too small for mask" stuff. If this is something which needs to
be fixed in clang/llvm we probably don't want to hide the issue.

>
> ```
> diff --git a/drivers/gpu/drm/i915/i915_hwmon.c
> b/drivers/gpu/drm/i915/i915_hwmon.c
> index 9e9781493025..6ac29d90b92a 100644
> --- a/drivers/gpu/drm/i915/i915_hwmon.c
> +++ b/drivers/gpu/drm/i915/i915_hwmon.c
> @@ -101,7 +101,7 @@ 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,
> +                         int nshift,
>                           unsigned int scale_factor, long lval)
>  {
>         u32 nval;
> @@ -111,8 +111,8 @@ hwm_field_scale_and_write(struct hwm_drvdata
> *ddat, i915_reg_t rgadr,
>         /* 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);
> +       bits_to_clear = PKG_PWR_LIM_1;
> +       bits_to_set = FIELD_PREP(PKG_PWR_LIM_1, nval);
>
>         hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,
>                                             bits_to_clear, bits_to_set);
> @@ -406,7 +406,6 @@ 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;
> ```
> Though I'm not sure if you're planning to add further callsites of
> hwm_field_scale_and_write with different field_masks?

I have reasons for keeping it this way, it's there in the link above if you
are interested.

>
> Alternatively, (without the above diff),
>
> ```
> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> index c9be1657f03d..6f40f12bcf89 100644
> --- a/include/linux/bitfield.h
> +++ b/include/linux/bitfield.h
> @@ -8,6 +8,7 @@
>  #define _LINUX_BITFIELD_H
>
>  #include <linux/build_bug.h>
> +#include <linux/const.h>
>  #include <asm/byteorder.h>
>
>  /*
> @@ -62,7 +63,7 @@
>
>  #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx)                      \
>         ({                                                              \
> -               BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),          \
> +               BUILD_BUG_ON_MSG(!__is_constexpr(_mask),                \
>                                  _pfx "mask is not constant");          \
>                 BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero");    \
>                 BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?           \
> ```
> will produce:
> error: call to __compiletime_assert_407 declared with 'error'
> attribute: FIELD_PREP: mask is not constant
>
> I haven't tested if that change is also feasible (on top of fixing
> this specific instance), but I think it might help avoid more of these
> subtleties wrt. __builtin_constant_p that depende heavily on compiler,
> compiler version, optimization level.

Not disagreeing, can do something here if needed.

Thanks.
--
Ashutosh

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

[Gwan-gyeong Mun]

Hi all,

I should have written the commit message more accurately, but it seems 
that it was written inaccurately.

If the FIELD_PREP macro is expanded, the following macros are used.

#define FIELD_PREP(_mask, _val)						\
	({								\
		__BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: ");	\
		((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask);	\
	})


#define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx)			\
	({								\
		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))); \
	})

Among them, a build error is generated by the lower part of the 
__BF_FIELD_CHECK() macro.

		BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >	\
				 __bf_cast_unsigned(_reg, ~0ull),	\
				 _pfx "type of reg too small for mask"); \


Here, if you apply an argument to this macro, it will look like the 
following.

__bf_cast_unsigned(field_msk, field_msk) > __bf_cast_unsigned(0ULL, ~0ull)

The result is always false because an unsigned int value of type 
field_msk is not always greater than the maximum value of unsigned long 
long .
So, a build error occurs due to the following part of the clang compiler 
option.

[-Werror,-Wtautological-constant-out-of-range-compare]

You can simply override this warning in Clang by adding the build option 
below, but this seems like a bad attempt

i915/Makefile
CFLAGS_i915_hwmon.o += -Wno-tautological-constant-out-of-range-compare

The easiest way to solve this is to use a constant value, not a 
variable, as an argument to FIELD_PREP.

And since the REG_FIELD_PREP() macro suggested by Jani requires a const 
expression as the first argument, it cannot be changed with this macro 
alone in the existing code, it must be changed to input a constant value 
as shown below.

diff --git a/drivers/gpu/drm/i915/i915_hwmon.c 
b/drivers/gpu/drm/i915/i915_hwmon.c
index 08c921421a5f..abb3a194c548 100644
--- a/drivers/gpu/drm/i915/i915_hwmon.c
+++ b/drivers/gpu/drm/i915/i915_hwmon.c
@@ -101,7 +101,7 @@ 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,
-                         const u32 field_msk, int nshift,
+                         int nshift,
                           unsigned int scale_factor, long lval)
  {
         u32 nval;
@@ -111,8 +111,8 @@ hwm_field_scale_and_write(struct hwm_drvdata *ddat, 
i915_reg_t rgadr,
         /* 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 = REG_FIELD_PREP(field_msk, nval);
+       bits_to_clear = PKG_PWR_LIM_1;
+       bits_to_set = REG_FIELD_PREP(PKG_PWR_LIM_1, nval);

         hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,
                                             bits_to_clear, bits_to_set);
@@ -406,7 +406,6 @@ 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;



In addition, if there is no build problem regardless of the size of the 
type as the first argument in FIELD_PREP, it is possible through the 
following modification.
(Since this modification modifies include/linux/bitfield.h , I will send 
it as a separate patch.
   )

However, it seems that we need to have Jani's confirm whether it is okay 
to use FIELD_PREP() instead of REG_FIELD_PREP() which is forced to u32 
return type.

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index c9be1657f03d..6e96799b6f38 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -9,7 +9,7 @@

  #include <linux/build_bug.h>
  #include <asm/byteorder.h>
-
+#include <linux/overflow.h>
  /*
   * Bitfield access macros
   *
@@ -69,7 +69,7 @@
                                  ~((_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),       \
+                                __bf_cast_unsigned(_reg, 
type_max(__unsigned_scalar_typeof(_reg))),    \
                                  _pfx "type of reg too small for mask"); \
                 __BUILD_BUG_ON_NOT_POWER_OF_2((_mask) +                 \
                                               (1ULL << __bf_shf(_mask))); \
@@ -84,7 +84,7 @@
   */
  #define FIELD_MAX(_mask)                                               \
         ({                                                              \
-               __BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_MAX: ");     \
+               __BF_FIELD_CHECK(_mask, 
type_min(__unsigned_scalar_typeof(_mask)), 
type_min(__unsigned_scalar_typeof(_mask)), "FIELD_MAX: ");   \
                 (typeof(_mask))((_mask) >> __bf_shf(_mask));            \
         })

@@ -97,7 +97,7 @@
   */
  #define FIELD_FIT(_mask, _val)                                         \
         ({                                                              \
-               __BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_FIT: ");     \
+               __BF_FIELD_CHECK(_mask, 
type_min(__unsigned_scalar_typeof(_mask)), 
type_min(__unsigned_scalar_typeof(_val)), "FIELD_FIT: ");    \
                 !((((typeof(_mask))_val) << __bf_shf(_mask)) & ~(_mask)); \
         })

@@ -111,7 +111,7 @@
   */
  #define FIELD_PREP(_mask, _val) 
          \
         ({                                                              \
-               __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: ");    \
+               __BF_FIELD_CHECK(_mask, 
type_min(__unsigned_scalar_typeof(_mask)), _val, "FIELD_PREP: ");       \
                 ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask);   \
         })

@@ -125,7 +125,7 @@
   */
  #define FIELD_GET(_mask, _reg)                                         \
         ({                                                              \
-               __BF_FIELD_CHECK(_mask, _reg, 0U, "FIELD_GET: ");       \
+               __BF_FIELD_CHECK(_mask, _reg, 
type_min(__unsigned_scalar_typeof(_reg)), "FIELD_GET: "); \
                 (typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask)); \
         })


Br,

G.G.


On 10/27/22 9:32 PM, Dixit, Ashutosh wrote:
> On Thu, 27 Oct 2022 10:16:47 -0700, Nick Desaulniers wrote:
>>
> 
> Hi Nick,
> 
>> Thanks, I can repro now.
>>
>> I haven't detangled the macro soup, but I noticed:
>>
>> 1. FIELD_PREP is defined in include/linux/bitfield.h which has the
>> following comment:
>>   18  * Mask must be a compilation time constant.
> 
> I had comments about this here:
> 
> https://lore.kernel.org/intel-gfx/87ilk7pwrw.wl-ashutosh.dixit@intel.com/
> 
> The relevant part being:
> 
> ---- {quote} ----
>>>> ./include/linux/bitfield.h:71:53: note: expanded from macro '__BF_FIELD_CHECK'
>>>>                  BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >     \
> 
> So clang seems to break here at this line in __BF_FIELD_CHECK (note ~0ull
> also occurs here):
> 
> 		BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >	\
> 				 __bf_cast_unsigned(_reg, ~0ull),	\
> 				 _pfx "type of reg too small for mask"); \
> 
> So it goes through previous checks including the "mask is not constant"
> check. As Nick Desaulniers mentions "__builtin_constant_p is evaluated
> after most optimizations have run" so by that time both compilers (gcc and
> clang) have figured out that even though _mask is coming in as function
> argument it is really the constant below:
> 
> #define   PKG_PWR_LIM_1		REG_GENMASK(14, 0)
> 
> But it is not clear why clang chokes on this "type of reg too small for
> mask" check (and gcc doesn't) since everything is u32.
> ---- {end quote} ----
> 
>>
>> 2. hwm_field_scale_and_write only has one callsite.
>>
>> The following patch works:
> 
> If we need to fix it at our end yes we can come up with one of these
> patches. But we were hoping someone from clang/llvm can comment about the
> "type of reg too small for mask" stuff. If this is something which needs to
> be fixed in clang/llvm we probably don't want to hide the issue.
> 
>>
>> ```
>> diff --git a/drivers/gpu/drm/i915/i915_hwmon.c
>> b/drivers/gpu/drm/i915/i915_hwmon.c
>> index 9e9781493025..6ac29d90b92a 100644
>> --- a/drivers/gpu/drm/i915/i915_hwmon.c
>> +++ b/drivers/gpu/drm/i915/i915_hwmon.c
>> @@ -101,7 +101,7 @@ 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,
>> +                         int nshift,
>>                            unsigned int scale_factor, long lval)
>>   {
>>          u32 nval;
>> @@ -111,8 +111,8 @@ hwm_field_scale_and_write(struct hwm_drvdata
>> *ddat, i915_reg_t rgadr,
>>          /* 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);
>> +       bits_to_clear = PKG_PWR_LIM_1;
>> +       bits_to_set = FIELD_PREP(PKG_PWR_LIM_1, nval);
>>
>>          hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,
>>                                              bits_to_clear, bits_to_set);
>> @@ -406,7 +406,6 @@ 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;
>> ```
>> Though I'm not sure if you're planning to add further callsites of
>> hwm_field_scale_and_write with different field_masks?
> 
> I have reasons for keeping it this way, it's there in the link above if you
> are interested.
> 
>>
>> Alternatively, (without the above diff),
>>
>> ```
>> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
>> index c9be1657f03d..6f40f12bcf89 100644
>> --- a/include/linux/bitfield.h
>> +++ b/include/linux/bitfield.h
>> @@ -8,6 +8,7 @@
>>   #define _LINUX_BITFIELD_H
>>
>>   #include <linux/build_bug.h>
>> +#include <linux/const.h>
>>   #include <asm/byteorder.h>
>>
>>   /*
>> @@ -62,7 +63,7 @@
>>
>>   #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx)                      \
>>          ({                                                              \
>> -               BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),          \
>> +               BUILD_BUG_ON_MSG(!__is_constexpr(_mask),                \
>>                                   _pfx "mask is not constant");          \
>>                  BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero");    \
>>                  BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?           \
>> ```
>> will produce:
>> error: call to __compiletime_assert_407 declared with 'error'
>> attribute: FIELD_PREP: mask is not constant
>>
>> I haven't tested if that change is also feasible (on top of fixing
>> this specific instance), but I think it might help avoid more of these
>> subtleties wrt. __builtin_constant_p that depende heavily on compiler,
>> compiler version, optimization level.
> 
> Not disagreeing, can do something here if needed.
> 
> Thanks.
> --
> Ashutosh

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

[Gwan-gyeong Mun]

Resend, because some content was accidentally omitted from the previous 
reply.
Please ignore the previous email.

Hi all,

I should have written the original commit message more accurately, but 
it seems that it was written inaccurately.

If the FIELD_PREP macro is expanded, the following macros are used.

#define FIELD_PREP(_mask, _val)						\
	({								\
		__BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: ");	\
		((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask);	\
	})


#define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx)			\
	({								\
		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))); \
	})

Among them, a build error is generated by the lower part of the 
__BF_FIELD_CHECK() macro.

		BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >	\
				 __bf_cast_unsigned(_reg, ~0ull),	\
				 _pfx "type of reg too small for mask"); \


Here, if you apply an argument to this macro, it will look like the 
following.

__bf_cast_unsigned(field_msk, field_msk) > __bf_cast_unsigned(0ULL, ~0ull)

The result is always false because an unsigned int value of type 
field_msk is not always greater than the maximum value of unsigned long 
long .
So, a build error occurs due to the following part of the clang compiler 
option.

[-Werror,-Wtautological-constant-out-of-range-compare]

You can simply override this warning in Clang by adding the build option 
below, but this seems like a bad attempt

i915/Makefile
CFLAGS_i915_hwmon.o += -Wno-tautological-constant-out-of-range-compare

The easiest way to solve this is to use a constant value, not a 
variable, as an argument to FIELD_PREP.

And since the REG_FIELD_PREP() macro suggested by Jani requires a const 
expression as the first argument, it cannot be changed with this macro 
alone in the existing code, it must be changed to input a constant value 
as shown below.

diff --git a/drivers/gpu/drm/i915/i915_hwmon.c 
b/drivers/gpu/drm/i915/i915_hwmon.c
index 08c921421a5f..abb3a194c548 100644
--- a/drivers/gpu/drm/i915/i915_hwmon.c
+++ b/drivers/gpu/drm/i915/i915_hwmon.c
@@ -101,7 +101,7 @@ 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,
-                         const u32 field_msk, int nshift,
+                         int nshift,
                           unsigned int scale_factor, long lval)
  {
         u32 nval;
@@ -111,8 +111,8 @@ hwm_field_scale_and_write(struct hwm_drvdata *ddat, 
i915_reg_t rgadr,
         /* 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 = REG_FIELD_PREP(field_msk, nval);
+       bits_to_clear = PKG_PWR_LIM_1;
+       bits_to_set = REG_FIELD_PREP(PKG_PWR_LIM_1, nval);

         hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,
                                             bits_to_clear, bits_to_set);
@@ -406,7 +406,6 @@ 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;



In addition, if there is no build problem regardless of the size of the 
type as the first argument in FIELD_PREP, it is possible through the 
following modification.
(Since this modification modifies include/linux/bitfield.h , I will send 
it as a separate patch.
   )

However, it seems that we need to have Jani's confirm whether it is okay 
to use FIELD_PREP() instead of REG_FIELD_PREP() which is forced to u32 
return type in i915.

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index c9be1657f03d..6e96799b6f38 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -9,7 +9,7 @@

  #include <linux/build_bug.h>
  #include <asm/byteorder.h>
-
+#include <linux/overflow.h>
  /*
   * Bitfield access macros
   *
@@ -69,7 +69,7 @@
                                  ~((_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),       \
+                                __bf_cast_unsigned(_reg, 
type_max(__unsigned_scalar_typeof(_reg))),    \
                                  _pfx "type of reg too small for mask"); \
                 __BUILD_BUG_ON_NOT_POWER_OF_2((_mask) +                 \
                                               (1ULL << __bf_shf(_mask))); \
@@ -84,7 +84,7 @@
   */
  #define FIELD_MAX(_mask)                                               \
         ({                                                              \
-               __BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_MAX: ");     \
+               __BF_FIELD_CHECK(_mask, 
type_min(__unsigned_scalar_typeof(_mask)), 
type_min(__unsigned_scalar_typeof(_mask)), "FIELD_MAX: ");   \
                 (typeof(_mask))((_mask) >> __bf_shf(_mask));            \
         })

@@ -97,7 +97,7 @@
   */
  #define FIELD_FIT(_mask, _val)                                         \
         ({                                                              \
-               __BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_FIT: ");     \
+               __BF_FIELD_CHECK(_mask, 
type_min(__unsigned_scalar_typeof(_mask)), 
type_min(__unsigned_scalar_typeof(_val)), "FIELD_FIT: ");    \
                 !((((typeof(_mask))_val) << __bf_shf(_mask)) & ~(_mask)); \
         })

@@ -111,7 +111,7 @@
   */
  #define FIELD_PREP(_mask, _val) 
          \
         ({                                                              \
-               __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: ");    \
+               __BF_FIELD_CHECK(_mask, 
type_min(__unsigned_scalar_typeof(_mask)), _val, "FIELD_PREP: ");       \
                 ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask);   \
         })

@@ -125,7 +125,7 @@
   */
  #define FIELD_GET(_mask, _reg)                                         \
         ({                                                              \
-               __BF_FIELD_CHECK(_mask, _reg, 0U, "FIELD_GET: ");       \
+               __BF_FIELD_CHECK(_mask, _reg, 
type_min(__unsigned_scalar_typeof(_reg)), "FIELD_GET: "); \
                 (typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask)); \
         })


Br,

G.G.

On 10/27/22 9:32 PM, Dixit, Ashutosh wrote:
> On Thu, 27 Oct 2022 10:16:47 -0700, Nick Desaulniers wrote:
>>
> 
> Hi Nick,
> 
>> Thanks, I can repro now.
>>
>> I haven't detangled the macro soup, but I noticed:
>>
>> 1. FIELD_PREP is defined in include/linux/bitfield.h which has the
>> following comment:
>>   18  * Mask must be a compilation time constant.
> 
> I had comments about this here:
> 
> https://lore.kernel.org/intel-gfx/87ilk7pwrw.wl-ashutosh.dixit@intel.com/
> 
> The relevant part being:
> 
> ---- {quote} ----
>>>> ./include/linux/bitfield.h:71:53: note: expanded from macro '__BF_FIELD_CHECK'
>>>>                  BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >     \
> 
> So clang seems to break here at this line in __BF_FIELD_CHECK (note ~0ull
> also occurs here):
> 
> 		BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >	\
> 				 __bf_cast_unsigned(_reg, ~0ull),	\
> 				 _pfx "type of reg too small for mask"); \
> 
> So it goes through previous checks including the "mask is not constant"
> check. As Nick Desaulniers mentions "__builtin_constant_p is evaluated
> after most optimizations have run" so by that time both compilers (gcc and
> clang) have figured out that even though _mask is coming in as function
> argument it is really the constant below:
> 
> #define   PKG_PWR_LIM_1		REG_GENMASK(14, 0)
> 
> But it is not clear why clang chokes on this "type of reg too small for
> mask" check (and gcc doesn't) since everything is u32.
> ---- {end quote} ----
> 
>>
>> 2. hwm_field_scale_and_write only has one callsite.
>>
>> The following patch works:
> 
> If we need to fix it at our end yes we can come up with one of these
> patches. But we were hoping someone from clang/llvm can comment about the
> "type of reg too small for mask" stuff. If this is something which needs to
> be fixed in clang/llvm we probably don't want to hide the issue.
> 
>>
>> ```
>> diff --git a/drivers/gpu/drm/i915/i915_hwmon.c
>> b/drivers/gpu/drm/i915/i915_hwmon.c
>> index 9e9781493025..6ac29d90b92a 100644
>> --- a/drivers/gpu/drm/i915/i915_hwmon.c
>> +++ b/drivers/gpu/drm/i915/i915_hwmon.c
>> @@ -101,7 +101,7 @@ 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,
>> +                         int nshift,
>>                            unsigned int scale_factor, long lval)
>>   {
>>          u32 nval;
>> @@ -111,8 +111,8 @@ hwm_field_scale_and_write(struct hwm_drvdata
>> *ddat, i915_reg_t rgadr,
>>          /* 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);
>> +       bits_to_clear = PKG_PWR_LIM_1;
>> +       bits_to_set = FIELD_PREP(PKG_PWR_LIM_1, nval);
>>
>>          hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,
>>                                              bits_to_clear, bits_to_set);
>> @@ -406,7 +406,6 @@ 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;
>> ```
>> Though I'm not sure if you're planning to add further callsites of
>> hwm_field_scale_and_write with different field_masks?
> 
> I have reasons for keeping it this way, it's there in the link above if you
> are interested.
> 
>>
>> Alternatively, (without the above diff),
>>
>> ```
>> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
>> index c9be1657f03d..6f40f12bcf89 100644
>> --- a/include/linux/bitfield.h
>> +++ b/include/linux/bitfield.h
>> @@ -8,6 +8,7 @@
>>   #define _LINUX_BITFIELD_H
>>
>>   #include <linux/build_bug.h>
>> +#include <linux/const.h>
>>   #include <asm/byteorder.h>
>>
>>   /*
>> @@ -62,7 +63,7 @@
>>
>>   #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx)                      \
>>          ({                                                              \
>> -               BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),          \
>> +               BUILD_BUG_ON_MSG(!__is_constexpr(_mask),                \
>>                                   _pfx "mask is not constant");          \
>>                  BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero");    \
>>                  BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?           \
>> ```
>> will produce:
>> error: call to __compiletime_assert_407 declared with 'error'
>> attribute: FIELD_PREP: mask is not constant
>>
>> I haven't tested if that change is also feasible (on top of fixing
>> this specific instance), but I think it might help avoid more of these
>> subtleties wrt. __builtin_constant_p that depende heavily on compiler,
>> compiler version, optimization level.
> 
> Not disagreeing, can do something here if needed.
> 
> Thanks.
> --
> Ashutosh

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

[Jani Nikula]

On Fri, 28 Oct 2022, Gwan-gyeong Mun <gwan-gyeong.mun@intel.com> wrote:
> Resend, because some content was accidentally omitted from the previous 
> reply.
> Please ignore the previous email.
>
> Hi all,
>
> I should have written the original commit message more accurately, but 
> it seems that it was written inaccurately.
>
> If the FIELD_PREP macro is expanded, the following macros are used.
>
> #define FIELD_PREP(_mask, _val)						\
> 	({								\
> 		__BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: ");	\
> 		((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask);	\
> 	})
>
>
> #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx)			\
> 	({								\
> 		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))); \
> 	})
>
> Among them, a build error is generated by the lower part of the 
> __BF_FIELD_CHECK() macro.
>
> 		BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >	\
> 				 __bf_cast_unsigned(_reg, ~0ull),	\
> 				 _pfx "type of reg too small for mask"); \
>
>
> Here, if you apply an argument to this macro, it will look like the 
> following.
>
> __bf_cast_unsigned(field_msk, field_msk) > __bf_cast_unsigned(0ULL, ~0ull)
>
> The result is always false because an unsigned int value of type 
> field_msk is not always greater than the maximum value of unsigned long 
> long .
> So, a build error occurs due to the following part of the clang compiler 
> option.
>
> [-Werror,-Wtautological-constant-out-of-range-compare]
>
> You can simply override this warning in Clang by adding the build option 
> below, but this seems like a bad attempt
>
> i915/Makefile
> CFLAGS_i915_hwmon.o += -Wno-tautological-constant-out-of-range-compare
>
> The easiest way to solve this is to use a constant value, not a 
> variable, as an argument to FIELD_PREP.
>
> And since the REG_FIELD_PREP() macro suggested by Jani requires a const 
> expression as the first argument, it cannot be changed with this macro 
> alone in the existing code, it must be changed to input a constant value 
> as shown below.

We've added REG_FIELD_PREP() precisely to avoid the problems with the
types and ranges, as we want it to operate on u32. It also uses
__is_constexpr() to avoid dependencies on compiler implementation and
optimizations.

Please use REG_FIELD_PREP() and a constant value. Maybe rethink the
interface if needed.

BR,
Jani.




>
> diff --git a/drivers/gpu/drm/i915/i915_hwmon.c 
> b/drivers/gpu/drm/i915/i915_hwmon.c
> index 08c921421a5f..abb3a194c548 100644
> --- a/drivers/gpu/drm/i915/i915_hwmon.c
> +++ b/drivers/gpu/drm/i915/i915_hwmon.c
> @@ -101,7 +101,7 @@ 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,
> -                         const u32 field_msk, int nshift,
> +                         int nshift,
>                            unsigned int scale_factor, long lval)
>   {
>          u32 nval;
> @@ -111,8 +111,8 @@ hwm_field_scale_and_write(struct hwm_drvdata *ddat, 
> i915_reg_t rgadr,
>          /* 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 = REG_FIELD_PREP(field_msk, nval);
> +       bits_to_clear = PKG_PWR_LIM_1;
> +       bits_to_set = REG_FIELD_PREP(PKG_PWR_LIM_1, nval);
>
>          hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,
>                                              bits_to_clear, bits_to_set);
> @@ -406,7 +406,6 @@ 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;
>
>
>
> In addition, if there is no build problem regardless of the size of the 
> type as the first argument in FIELD_PREP, it is possible through the 
> following modification.
> (Since this modification modifies include/linux/bitfield.h , I will send 
> it as a separate patch.
>    )
>
> However, it seems that we need to have Jani's confirm whether it is okay 
> to use FIELD_PREP() instead of REG_FIELD_PREP() which is forced to u32 
> return type in i915.
>
> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> index c9be1657f03d..6e96799b6f38 100644
> --- a/include/linux/bitfield.h
> +++ b/include/linux/bitfield.h
> @@ -9,7 +9,7 @@
>
>   #include <linux/build_bug.h>
>   #include <asm/byteorder.h>
> -
> +#include <linux/overflow.h>
>   /*
>    * Bitfield access macros
>    *
> @@ -69,7 +69,7 @@
>                                   ~((_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),       \
> +                                __bf_cast_unsigned(_reg, 
> type_max(__unsigned_scalar_typeof(_reg))),    \
>                                   _pfx "type of reg too small for mask"); \
>                  __BUILD_BUG_ON_NOT_POWER_OF_2((_mask) +                 \
>                                                (1ULL << __bf_shf(_mask))); \
> @@ -84,7 +84,7 @@
>    */
>   #define FIELD_MAX(_mask)                                               \
>          ({                                                              \
> -               __BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_MAX: ");     \
> +               __BF_FIELD_CHECK(_mask, 
> type_min(__unsigned_scalar_typeof(_mask)), 
> type_min(__unsigned_scalar_typeof(_mask)), "FIELD_MAX: ");   \
>                  (typeof(_mask))((_mask) >> __bf_shf(_mask));            \
>          })
>
> @@ -97,7 +97,7 @@
>    */
>   #define FIELD_FIT(_mask, _val)                                         \
>          ({                                                              \
> -               __BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_FIT: ");     \
> +               __BF_FIELD_CHECK(_mask, 
> type_min(__unsigned_scalar_typeof(_mask)), 
> type_min(__unsigned_scalar_typeof(_val)), "FIELD_FIT: ");    \
>                  !((((typeof(_mask))_val) << __bf_shf(_mask)) & ~(_mask)); \
>          })
>
> @@ -111,7 +111,7 @@
>    */
>   #define FIELD_PREP(_mask, _val) 
>           \
>          ({                                                              \
> -               __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: ");    \
> +               __BF_FIELD_CHECK(_mask, 
> type_min(__unsigned_scalar_typeof(_mask)), _val, "FIELD_PREP: ");       \
>                  ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask);   \
>          })
>
> @@ -125,7 +125,7 @@
>    */
>   #define FIELD_GET(_mask, _reg)                                         \
>          ({                                                              \
> -               __BF_FIELD_CHECK(_mask, _reg, 0U, "FIELD_GET: ");       \
> +               __BF_FIELD_CHECK(_mask, _reg, 
> type_min(__unsigned_scalar_typeof(_reg)), "FIELD_GET: "); \
>                  (typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask)); \
>          })
>
>
> Br,
>
> G.G.
>
> On 10/27/22 9:32 PM, Dixit, Ashutosh wrote:
>> On Thu, 27 Oct 2022 10:16:47 -0700, Nick Desaulniers wrote:
>>>
>> 
>> Hi Nick,
>> 
>>> Thanks, I can repro now.
>>>
>>> I haven't detangled the macro soup, but I noticed:
>>>
>>> 1. FIELD_PREP is defined in include/linux/bitfield.h which has the
>>> following comment:
>>>   18  * Mask must be a compilation time constant.
>> 
>> I had comments about this here:
>> 
>> https://lore.kernel.org/intel-gfx/87ilk7pwrw.wl-ashutosh.dixit@intel.com/
>> 
>> The relevant part being:
>> 
>> ---- {quote} ----
>>>>> ./include/linux/bitfield.h:71:53: note: expanded from macro '__BF_FIELD_CHECK'
>>>>>                  BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >     \
>> 
>> So clang seems to break here at this line in __BF_FIELD_CHECK (note ~0ull
>> also occurs here):
>> 
>> 		BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >	\
>> 				 __bf_cast_unsigned(_reg, ~0ull),	\
>> 				 _pfx "type of reg too small for mask"); \
>> 
>> So it goes through previous checks including the "mask is not constant"
>> check. As Nick Desaulniers mentions "__builtin_constant_p is evaluated
>> after most optimizations have run" so by that time both compilers (gcc and
>> clang) have figured out that even though _mask is coming in as function
>> argument it is really the constant below:
>> 
>> #define   PKG_PWR_LIM_1		REG_GENMASK(14, 0)
>> 
>> But it is not clear why clang chokes on this "type of reg too small for
>> mask" check (and gcc doesn't) since everything is u32.
>> ---- {end quote} ----
>> 
>>>
>>> 2. hwm_field_scale_and_write only has one callsite.
>>>
>>> The following patch works:
>> 
>> If we need to fix it at our end yes we can come up with one of these
>> patches. But we were hoping someone from clang/llvm can comment about the
>> "type of reg too small for mask" stuff. If this is something which needs to
>> be fixed in clang/llvm we probably don't want to hide the issue.
>> 
>>>
>>> ```
>>> diff --git a/drivers/gpu/drm/i915/i915_hwmon.c
>>> b/drivers/gpu/drm/i915/i915_hwmon.c
>>> index 9e9781493025..6ac29d90b92a 100644
>>> --- a/drivers/gpu/drm/i915/i915_hwmon.c
>>> +++ b/drivers/gpu/drm/i915/i915_hwmon.c
>>> @@ -101,7 +101,7 @@ 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,
>>> +                         int nshift,
>>>                            unsigned int scale_factor, long lval)
>>>   {
>>>          u32 nval;
>>> @@ -111,8 +111,8 @@ hwm_field_scale_and_write(struct hwm_drvdata
>>> *ddat, i915_reg_t rgadr,
>>>          /* 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);
>>> +       bits_to_clear = PKG_PWR_LIM_1;
>>> +       bits_to_set = FIELD_PREP(PKG_PWR_LIM_1, nval);
>>>
>>>          hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,
>>>                                              bits_to_clear, bits_to_set);
>>> @@ -406,7 +406,6 @@ 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;
>>> ```
>>> Though I'm not sure if you're planning to add further callsites of
>>> hwm_field_scale_and_write with different field_masks?
>> 
>> I have reasons for keeping it this way, it's there in the link above if you
>> are interested.
>> 
>>>
>>> Alternatively, (without the above diff),
>>>
>>> ```
>>> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
>>> index c9be1657f03d..6f40f12bcf89 100644
>>> --- a/include/linux/bitfield.h
>>> +++ b/include/linux/bitfield.h
>>> @@ -8,6 +8,7 @@
>>>   #define _LINUX_BITFIELD_H
>>>
>>>   #include <linux/build_bug.h>
>>> +#include <linux/const.h>
>>>   #include <asm/byteorder.h>
>>>
>>>   /*
>>> @@ -62,7 +63,7 @@
>>>
>>>   #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx)                      \
>>>          ({                                                              \
>>> -               BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),          \
>>> +               BUILD_BUG_ON_MSG(!__is_constexpr(_mask),                \
>>>                                   _pfx "mask is not constant");          \
>>>                  BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero");    \
>>>                  BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?           \
>>> ```
>>> will produce:
>>> error: call to __compiletime_assert_407 declared with 'error'
>>> attribute: FIELD_PREP: mask is not constant
>>>
>>> I haven't tested if that change is also feasible (on top of fixing
>>> this specific instance), but I think it might help avoid more of these
>>> subtleties wrt. __builtin_constant_p that depende heavily on compiler,
>>> compiler version, optimization level.
>> 
>> Not disagreeing, can do something here if needed.
>> 
>> Thanks.
>> --
>> Ashutosh

-- 
Jani Nikula, Intel Open Source Graphics Center

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

[Joonas Lahtinen]

Quoting Jani Nikula (2022-10-28 11:46:21)
> On Fri, 28 Oct 2022, Gwan-gyeong Mun <gwan-gyeong.mun@intel.com> wrote:
> > Resend, because some content was accidentally omitted from the previous 
> > reply.
> > Please ignore the previous email.
> >
> > Hi all,
> >
> > I should have written the original commit message more accurately, but 
> > it seems that it was written inaccurately.
> >
> > If the FIELD_PREP macro is expanded, the following macros are used.
> >
> > #define FIELD_PREP(_mask, _val)                                               \
> >       ({                                                              \
> >               __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: ");    \
> >               ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask);   \
> >       })
> >
> >
> > #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx)                     \
> >       ({                                                              \
> >               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))); \
> >       })
> >
> > Among them, a build error is generated by the lower part of the 
> > __BF_FIELD_CHECK() macro.
> >
> >               BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >     \
> >                                __bf_cast_unsigned(_reg, ~0ull),       \
> >                                _pfx "type of reg too small for mask"); \
> >
> >
> > Here, if you apply an argument to this macro, it will look like the 
> > following.
> >
> > __bf_cast_unsigned(field_msk, field_msk) > __bf_cast_unsigned(0ULL, ~0ull)
> >
> > The result is always false because an unsigned int value of type 
> > field_msk is not always greater than the maximum value of unsigned long 
> > long .
> > So, a build error occurs due to the following part of the clang compiler 
> > option.
> >
> > [-Werror,-Wtautological-constant-out-of-range-compare]
> >
> > You can simply override this warning in Clang by adding the build option 
> > below, but this seems like a bad attempt
> >
> > i915/Makefile
> > CFLAGS_i915_hwmon.o += -Wno-tautological-constant-out-of-range-compare
> >
> > The easiest way to solve this is to use a constant value, not a 
> > variable, as an argument to FIELD_PREP.
> >
> > And since the REG_FIELD_PREP() macro suggested by Jani requires a const 
> > expression as the first argument, it cannot be changed with this macro 
> > alone in the existing code, it must be changed to input a constant value 
> > as shown below.
> 
> We've added REG_FIELD_PREP() precisely to avoid the problems with the
> types and ranges, as we want it to operate on u32. It also uses
> __is_constexpr() to avoid dependencies on compiler implementation and
> optimizations.
> 
> Please use REG_FIELD_PREP() and a constant value. Maybe rethink the
> interface if needed.

Ashutosh and GG, can we get a fix for this merged ASAP. It's currently
blocking the drm-intel-gt-next pull request.

Regards, Joonas

> 
> BR,
> Jani.
> 
> 
> 
> 
> >
> > diff --git a/drivers/gpu/drm/i915/i915_hwmon.c 
> > b/drivers/gpu/drm/i915/i915_hwmon.c
> > index 08c921421a5f..abb3a194c548 100644
> > --- a/drivers/gpu/drm/i915/i915_hwmon.c
> > +++ b/drivers/gpu/drm/i915/i915_hwmon.c
> > @@ -101,7 +101,7 @@ 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,
> > -                         const u32 field_msk, int nshift,
> > +                         int nshift,
> >                            unsigned int scale_factor, long lval)
> >   {
> >          u32 nval;
> > @@ -111,8 +111,8 @@ hwm_field_scale_and_write(struct hwm_drvdata *ddat, 
> > i915_reg_t rgadr,
> >          /* 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 = REG_FIELD_PREP(field_msk, nval);
> > +       bits_to_clear = PKG_PWR_LIM_1;
> > +       bits_to_set = REG_FIELD_PREP(PKG_PWR_LIM_1, nval);
> >
> >          hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,
> >                                              bits_to_clear, bits_to_set);
> > @@ -406,7 +406,6 @@ 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;
> >
> >
> >
> > In addition, if there is no build problem regardless of the size of the 
> > type as the first argument in FIELD_PREP, it is possible through the 
> > following modification.
> > (Since this modification modifies include/linux/bitfield.h , I will send 
> > it as a separate patch.
> >    )
> >
> > However, it seems that we need to have Jani's confirm whether it is okay 
> > to use FIELD_PREP() instead of REG_FIELD_PREP() which is forced to u32 
> > return type in i915.
> >
> > diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> > index c9be1657f03d..6e96799b6f38 100644
> > --- a/include/linux/bitfield.h
> > +++ b/include/linux/bitfield.h
> > @@ -9,7 +9,7 @@
> >
> >   #include <linux/build_bug.h>
> >   #include <asm/byteorder.h>
> > -
> > +#include <linux/overflow.h>
> >   /*
> >    * Bitfield access macros
> >    *
> > @@ -69,7 +69,7 @@
> >                                   ~((_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),       \
> > +                                __bf_cast_unsigned(_reg, 
> > type_max(__unsigned_scalar_typeof(_reg))),    \
> >                                   _pfx "type of reg too small for mask"); \
> >                  __BUILD_BUG_ON_NOT_POWER_OF_2((_mask) +                 \
> >                                                (1ULL << __bf_shf(_mask))); \
> > @@ -84,7 +84,7 @@
> >    */
> >   #define FIELD_MAX(_mask)                                               \
> >          ({                                                              \
> > -               __BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_MAX: ");     \
> > +               __BF_FIELD_CHECK(_mask, 
> > type_min(__unsigned_scalar_typeof(_mask)), 
> > type_min(__unsigned_scalar_typeof(_mask)), "FIELD_MAX: ");   \
> >                  (typeof(_mask))((_mask) >> __bf_shf(_mask));            \
> >          })
> >
> > @@ -97,7 +97,7 @@
> >    */
> >   #define FIELD_FIT(_mask, _val)                                         \
> >          ({                                                              \
> > -               __BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_FIT: ");     \
> > +               __BF_FIELD_CHECK(_mask, 
> > type_min(__unsigned_scalar_typeof(_mask)), 
> > type_min(__unsigned_scalar_typeof(_val)), "FIELD_FIT: ");    \
> >                  !((((typeof(_mask))_val) << __bf_shf(_mask)) & ~(_mask)); \
> >          })
> >
> > @@ -111,7 +111,7 @@
> >    */
> >   #define FIELD_PREP(_mask, _val) 
> >           \
> >          ({                                                              \
> > -               __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: ");    \
> > +               __BF_FIELD_CHECK(_mask, 
> > type_min(__unsigned_scalar_typeof(_mask)), _val, "FIELD_PREP: ");       \
> >                  ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask);   \
> >          })
> >
> > @@ -125,7 +125,7 @@
> >    */
> >   #define FIELD_GET(_mask, _reg)                                         \
> >          ({                                                              \
> > -               __BF_FIELD_CHECK(_mask, _reg, 0U, "FIELD_GET: ");       \
> > +               __BF_FIELD_CHECK(_mask, _reg, 
> > type_min(__unsigned_scalar_typeof(_reg)), "FIELD_GET: "); \
> >                  (typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask)); \
> >          })
> >
> >
> > Br,
> >
> > G.G.
> >
> > On 10/27/22 9:32 PM, Dixit, Ashutosh wrote:
> >> On Thu, 27 Oct 2022 10:16:47 -0700, Nick Desaulniers wrote:
> >>>
> >> 
> >> Hi Nick,
> >> 
> >>> Thanks, I can repro now.
> >>>
> >>> I haven't detangled the macro soup, but I noticed:
> >>>
> >>> 1. FIELD_PREP is defined in include/linux/bitfield.h which has the
> >>> following comment:
> >>>   18  * Mask must be a compilation time constant.
> >> 
> >> I had comments about this here:
> >> 
> >> https://lore.kernel.org/intel-gfx/87ilk7pwrw.wl-ashutosh.dixit@intel.com/
> >> 
> >> The relevant part being:
> >> 
> >> ---- {quote} ----
> >>>>> ./include/linux/bitfield.h:71:53: note: expanded from macro '__BF_FIELD_CHECK'
> >>>>>                  BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >     \
> >> 
> >> So clang seems to break here at this line in __BF_FIELD_CHECK (note ~0ull
> >> also occurs here):
> >> 
> >>              BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >     \
> >>                               __bf_cast_unsigned(_reg, ~0ull),       \
> >>                               _pfx "type of reg too small for mask"); \
> >> 
> >> So it goes through previous checks including the "mask is not constant"
> >> check. As Nick Desaulniers mentions "__builtin_constant_p is evaluated
> >> after most optimizations have run" so by that time both compilers (gcc and
> >> clang) have figured out that even though _mask is coming in as function
> >> argument it is really the constant below:
> >> 
> >> #define   PKG_PWR_LIM_1              REG_GENMASK(14, 0)
> >> 
> >> But it is not clear why clang chokes on this "type of reg too small for
> >> mask" check (and gcc doesn't) since everything is u32.
> >> ---- {end quote} ----
> >> 
> >>>
> >>> 2. hwm_field_scale_and_write only has one callsite.
> >>>
> >>> The following patch works:
> >> 
> >> If we need to fix it at our end yes we can come up with one of these
> >> patches. But we were hoping someone from clang/llvm can comment about the
> >> "type of reg too small for mask" stuff. If this is something which needs to
> >> be fixed in clang/llvm we probably don't want to hide the issue.
> >> 
> >>>
> >>> ```
> >>> diff --git a/drivers/gpu/drm/i915/i915_hwmon.c
> >>> b/drivers/gpu/drm/i915/i915_hwmon.c
> >>> index 9e9781493025..6ac29d90b92a 100644
> >>> --- a/drivers/gpu/drm/i915/i915_hwmon.c
> >>> +++ b/drivers/gpu/drm/i915/i915_hwmon.c
> >>> @@ -101,7 +101,7 @@ 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,
> >>> +                         int nshift,
> >>>                            unsigned int scale_factor, long lval)
> >>>   {
> >>>          u32 nval;
> >>> @@ -111,8 +111,8 @@ hwm_field_scale_and_write(struct hwm_drvdata
> >>> *ddat, i915_reg_t rgadr,
> >>>          /* 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);
> >>> +       bits_to_clear = PKG_PWR_LIM_1;
> >>> +       bits_to_set = FIELD_PREP(PKG_PWR_LIM_1, nval);
> >>>
> >>>          hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,
> >>>                                              bits_to_clear, bits_to_set);
> >>> @@ -406,7 +406,6 @@ 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;
> >>> ```
> >>> Though I'm not sure if you're planning to add further callsites of
> >>> hwm_field_scale_and_write with different field_masks?
> >> 
> >> I have reasons for keeping it this way, it's there in the link above if you
> >> are interested.
> >> 
> >>>
> >>> Alternatively, (without the above diff),
> >>>
> >>> ```
> >>> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> >>> index c9be1657f03d..6f40f12bcf89 100644
> >>> --- a/include/linux/bitfield.h
> >>> +++ b/include/linux/bitfield.h
> >>> @@ -8,6 +8,7 @@
> >>>   #define _LINUX_BITFIELD_H
> >>>
> >>>   #include <linux/build_bug.h>
> >>> +#include <linux/const.h>
> >>>   #include <asm/byteorder.h>
> >>>
> >>>   /*
> >>> @@ -62,7 +63,7 @@
> >>>
> >>>   #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx)                      \
> >>>          ({                                                              \
> >>> -               BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),          \
> >>> +               BUILD_BUG_ON_MSG(!__is_constexpr(_mask),                \
> >>>                                   _pfx "mask is not constant");          \
> >>>                  BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero");    \
> >>>                  BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?           \
> >>> ```
> >>> will produce:
> >>> error: call to __compiletime_assert_407 declared with 'error'
> >>> attribute: FIELD_PREP: mask is not constant
> >>>
> >>> I haven't tested if that change is also feasible (on top of fixing
> >>> this specific instance), but I think it might help avoid more of these
> >>> subtleties wrt. __builtin_constant_p that depende heavily on compiler,
> >>> compiler version, optimization level.
> >> 
> >> Not disagreeing, can do something here if needed.
> >> 
> >> Thanks.
> >> --
> >> Ashutosh
> 
> -- 
> Jani Nikula, Intel Open Source Graphics Center

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

[Gwan-gyeong Mun]

On 11/2/22 8:32 AM, Joonas Lahtinen wrote:
> Quoting Jani Nikula (2022-10-28 11:46:21)
>> On Fri, 28 Oct 2022, Gwan-gyeong Mun <gwan-gyeong.mun@intel.com> wrote:
>>> Resend, because some content was accidentally omitted from the previous
>>> reply.
>>> Please ignore the previous email.
>>>
>>> Hi all,
>>>
>>> I should have written the original commit message more accurately, but
>>> it seems that it was written inaccurately.
>>>
>>> If the FIELD_PREP macro is expanded, the following macros are used.
>>>
>>> #define FIELD_PREP(_mask, _val)                                               \
>>>        ({                                                              \
>>>                __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: ");    \
>>>                ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask);   \
>>>        })
>>>
>>>
>>> #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx)                     \
>>>        ({                                                              \
>>>                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))); \
>>>        })
>>>
>>> Among them, a build error is generated by the lower part of the
>>> __BF_FIELD_CHECK() macro.
>>>
>>>                BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >     \
>>>                                 __bf_cast_unsigned(_reg, ~0ull),       \
>>>                                 _pfx "type of reg too small for mask"); \
>>>
>>>
>>> Here, if you apply an argument to this macro, it will look like the
>>> following.
>>>
>>> __bf_cast_unsigned(field_msk, field_msk) > __bf_cast_unsigned(0ULL, ~0ull)
>>>
>>> The result is always false because an unsigned int value of type
>>> field_msk is not always greater than the maximum value of unsigned long
>>> long .
>>> So, a build error occurs due to the following part of the clang compiler
>>> option.
>>>
>>> [-Werror,-Wtautological-constant-out-of-range-compare]
>>>
>>> You can simply override this warning in Clang by adding the build option
>>> below, but this seems like a bad attempt
>>>
>>> i915/Makefile
>>> CFLAGS_i915_hwmon.o += -Wno-tautological-constant-out-of-range-compare
>>>
>>> The easiest way to solve this is to use a constant value, not a
>>> variable, as an argument to FIELD_PREP.
>>>
>>> And since the REG_FIELD_PREP() macro suggested by Jani requires a const
>>> expression as the first argument, it cannot be changed with this macro
>>> alone in the existing code, it must be changed to input a constant value
>>> as shown below.
>>
>> We've added REG_FIELD_PREP() precisely to avoid the problems with the
>> types and ranges, as we want it to operate on u32. It also uses
>> __is_constexpr() to avoid dependencies on compiler implementation and
>> optimizations.
>>
>> Please use REG_FIELD_PREP() and a constant value. Maybe rethink the
>> interface if needed.
> 
> Ashutosh and GG, can we get a fix for this merged ASAP. It's currently
> blocking the drm-intel-gt-next pull request.
> 
> Regards, Joonas
> 
Hi Joonas,
As a workaround patch, this patch[1] was reviewed by Ashutoshr and acked 
by Jani.

[1] https://patchwork.freedesktop.org/patch/509248/?series=110094&rev=5


Br,

G.G.
>>
>> BR,
>> Jani.
>>
>>
>>
>>
>>>
>>> diff --git a/drivers/gpu/drm/i915/i915_hwmon.c
>>> b/drivers/gpu/drm/i915/i915_hwmon.c
>>> index 08c921421a5f..abb3a194c548 100644
>>> --- a/drivers/gpu/drm/i915/i915_hwmon.c
>>> +++ b/drivers/gpu/drm/i915/i915_hwmon.c
>>> @@ -101,7 +101,7 @@ 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,
>>> -                         const u32 field_msk, int nshift,
>>> +                         int nshift,
>>>                             unsigned int scale_factor, long lval)
>>>    {
>>>           u32 nval;
>>> @@ -111,8 +111,8 @@ hwm_field_scale_and_write(struct hwm_drvdata *ddat,
>>> i915_reg_t rgadr,
>>>           /* 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 = REG_FIELD_PREP(field_msk, nval);
>>> +       bits_to_clear = PKG_PWR_LIM_1;
>>> +       bits_to_set = REG_FIELD_PREP(PKG_PWR_LIM_1, nval);
>>>
>>>           hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,
>>>                                               bits_to_clear, bits_to_set);
>>> @@ -406,7 +406,6 @@ 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;
>>>
>>>
>>>
>>> In addition, if there is no build problem regardless of the size of the
>>> type as the first argument in FIELD_PREP, it is possible through the
>>> following modification.
>>> (Since this modification modifies include/linux/bitfield.h , I will send
>>> it as a separate patch.
>>>     )
>>>
>>> However, it seems that we need to have Jani's confirm whether it is okay
>>> to use FIELD_PREP() instead of REG_FIELD_PREP() which is forced to u32
>>> return type in i915.
>>>
>>> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
>>> index c9be1657f03d..6e96799b6f38 100644
>>> --- a/include/linux/bitfield.h
>>> +++ b/include/linux/bitfield.h
>>> @@ -9,7 +9,7 @@
>>>
>>>    #include <linux/build_bug.h>
>>>    #include <asm/byteorder.h>
>>> -
>>> +#include <linux/overflow.h>
>>>    /*
>>>     * Bitfield access macros
>>>     *
>>> @@ -69,7 +69,7 @@
>>>                                    ~((_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),       \
>>> +                                __bf_cast_unsigned(_reg,
>>> type_max(__unsigned_scalar_typeof(_reg))),    \
>>>                                    _pfx "type of reg too small for mask"); \
>>>                   __BUILD_BUG_ON_NOT_POWER_OF_2((_mask) +                 \
>>>                                                 (1ULL << __bf_shf(_mask))); \
>>> @@ -84,7 +84,7 @@
>>>     */
>>>    #define FIELD_MAX(_mask)                                               \
>>>           ({                                                              \
>>> -               __BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_MAX: ");     \
>>> +               __BF_FIELD_CHECK(_mask,
>>> type_min(__unsigned_scalar_typeof(_mask)),
>>> type_min(__unsigned_scalar_typeof(_mask)), "FIELD_MAX: ");   \
>>>                   (typeof(_mask))((_mask) >> __bf_shf(_mask));            \
>>>           })
>>>
>>> @@ -97,7 +97,7 @@
>>>     */
>>>    #define FIELD_FIT(_mask, _val)                                         \
>>>           ({                                                              \
>>> -               __BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_FIT: ");     \
>>> +               __BF_FIELD_CHECK(_mask,
>>> type_min(__unsigned_scalar_typeof(_mask)),
>>> type_min(__unsigned_scalar_typeof(_val)), "FIELD_FIT: ");    \
>>>                   !((((typeof(_mask))_val) << __bf_shf(_mask)) & ~(_mask)); \
>>>           })
>>>
>>> @@ -111,7 +111,7 @@
>>>     */
>>>    #define FIELD_PREP(_mask, _val)
>>>            \
>>>           ({                                                              \
>>> -               __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: ");    \
>>> +               __BF_FIELD_CHECK(_mask,
>>> type_min(__unsigned_scalar_typeof(_mask)), _val, "FIELD_PREP: ");       \
>>>                   ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask);   \
>>>           })
>>>
>>> @@ -125,7 +125,7 @@
>>>     */
>>>    #define FIELD_GET(_mask, _reg)                                         \
>>>           ({                                                              \
>>> -               __BF_FIELD_CHECK(_mask, _reg, 0U, "FIELD_GET: ");       \
>>> +               __BF_FIELD_CHECK(_mask, _reg,
>>> type_min(__unsigned_scalar_typeof(_reg)), "FIELD_GET: "); \
>>>                   (typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask)); \
>>>           })
>>>
>>>
>>> Br,
>>>
>>> G.G.
>>>
>>> On 10/27/22 9:32 PM, Dixit, Ashutosh wrote:
>>>> On Thu, 27 Oct 2022 10:16:47 -0700, Nick Desaulniers wrote:
>>>>>
>>>>
>>>> Hi Nick,
>>>>
>>>>> Thanks, I can repro now.
>>>>>
>>>>> I haven't detangled the macro soup, but I noticed:
>>>>>
>>>>> 1. FIELD_PREP is defined in include/linux/bitfield.h which has the
>>>>> following comment:
>>>>>    18  * Mask must be a compilation time constant.
>>>>
>>>> I had comments about this here:
>>>>
>>>> https://lore.kernel.org/intel-gfx/87ilk7pwrw.wl-ashutosh.dixit@intel.com/
>>>>
>>>> The relevant part being:
>>>>
>>>> ---- {quote} ----
>>>>>>> ./include/linux/bitfield.h:71:53: note: expanded from macro '__BF_FIELD_CHECK'
>>>>>>>                   BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >     \
>>>>
>>>> So clang seems to break here at this line in __BF_FIELD_CHECK (note ~0ull
>>>> also occurs here):
>>>>
>>>>               BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >     \
>>>>                                __bf_cast_unsigned(_reg, ~0ull),       \
>>>>                                _pfx "type of reg too small for mask"); \
>>>>
>>>> So it goes through previous checks including the "mask is not constant"
>>>> check. As Nick Desaulniers mentions "__builtin_constant_p is evaluated
>>>> after most optimizations have run" so by that time both compilers (gcc and
>>>> clang) have figured out that even though _mask is coming in as function
>>>> argument it is really the constant below:
>>>>
>>>> #define   PKG_PWR_LIM_1              REG_GENMASK(14, 0)
>>>>
>>>> But it is not clear why clang chokes on this "type of reg too small for
>>>> mask" check (and gcc doesn't) since everything is u32.
>>>> ---- {end quote} ----
>>>>
>>>>>
>>>>> 2. hwm_field_scale_and_write only has one callsite.
>>>>>
>>>>> The following patch works:
>>>>
>>>> If we need to fix it at our end yes we can come up with one of these
>>>> patches. But we were hoping someone from clang/llvm can comment about the
>>>> "type of reg too small for mask" stuff. If this is something which needs to
>>>> be fixed in clang/llvm we probably don't want to hide the issue.
>>>>
>>>>>
>>>>> ```
>>>>> diff --git a/drivers/gpu/drm/i915/i915_hwmon.c
>>>>> b/drivers/gpu/drm/i915/i915_hwmon.c
>>>>> index 9e9781493025..6ac29d90b92a 100644
>>>>> --- a/drivers/gpu/drm/i915/i915_hwmon.c
>>>>> +++ b/drivers/gpu/drm/i915/i915_hwmon.c
>>>>> @@ -101,7 +101,7 @@ 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,
>>>>> +                         int nshift,
>>>>>                             unsigned int scale_factor, long lval)
>>>>>    {
>>>>>           u32 nval;
>>>>> @@ -111,8 +111,8 @@ hwm_field_scale_and_write(struct hwm_drvdata
>>>>> *ddat, i915_reg_t rgadr,
>>>>>           /* 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);
>>>>> +       bits_to_clear = PKG_PWR_LIM_1;
>>>>> +       bits_to_set = FIELD_PREP(PKG_PWR_LIM_1, nval);
>>>>>
>>>>>           hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,
>>>>>                                               bits_to_clear, bits_to_set);
>>>>> @@ -406,7 +406,6 @@ 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;
>>>>> ```
>>>>> Though I'm not sure if you're planning to add further callsites of
>>>>> hwm_field_scale_and_write with different field_masks?
>>>>
>>>> I have reasons for keeping it this way, it's there in the link above if you
>>>> are interested.
>>>>
>>>>>
>>>>> Alternatively, (without the above diff),
>>>>>
>>>>> ```
>>>>> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
>>>>> index c9be1657f03d..6f40f12bcf89 100644
>>>>> --- a/include/linux/bitfield.h
>>>>> +++ b/include/linux/bitfield.h
>>>>> @@ -8,6 +8,7 @@
>>>>>    #define _LINUX_BITFIELD_H
>>>>>
>>>>>    #include <linux/build_bug.h>
>>>>> +#include <linux/const.h>
>>>>>    #include <asm/byteorder.h>
>>>>>
>>>>>    /*
>>>>> @@ -62,7 +63,7 @@
>>>>>
>>>>>    #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx)                      \
>>>>>           ({                                                              \
>>>>> -               BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),          \
>>>>> +               BUILD_BUG_ON_MSG(!__is_constexpr(_mask),                \
>>>>>                                    _pfx "mask is not constant");          \
>>>>>                   BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero");    \
>>>>>                   BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?           \
>>>>> ```
>>>>> will produce:
>>>>> error: call to __compiletime_assert_407 declared with 'error'
>>>>> attribute: FIELD_PREP: mask is not constant
>>>>>
>>>>> I haven't tested if that change is also feasible (on top of fixing
>>>>> this specific instance), but I think it might help avoid more of these
>>>>> subtleties wrt. __builtin_constant_p that depende heavily on compiler,
>>>>> compiler version, optimization level.
>>>>
>>>> Not disagreeing, can do something here if needed.
>>>>
>>>> Thanks.
>>>> --
>>>> Ashutosh
>>
>> -- 
>> Jani Nikula, Intel Open Source Graphics Center