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

[Joonas Lahtinen <joonas.lahtinen@linux.intel.com>]

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