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

Gwan-gyeong Mun
2022-10-28T06:49:21Z

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