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Message-ID: <20260119195817.GA1035354@ax162>
Date: Mon, 19 Jan 2026 12:58:17 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: James Calligeros <jcalligeros99@...il.com>,
Sven Peter <sven@...nel.org>, Janne Grunau <j@...nau.net>,
Guenter Roeck <linux@...ck-us.net>
Cc: asahi@...ts.linux.dev, linux-hwmon@...r.kernel.org,
linux-kernel@...r.kernel.org, Justin Stitt <justinstitt@...gle.com>,
Neal Gompa <neal@...pa.dev>
Subject: FIELD_PREP failure in drivers/hwmon/macsmc-hwmon.c
Hi folks,
There is a build error from a FIELD_PREP in drivers/hwmon/macsmc-hwmon.c
when building with clang-17 and older:
drivers/hwmon/macsmc-hwmon.c:249:10: error: call to '__compiletime_assert_814' declared with 'error' attribute: FIELD_PREP: value too large for the field
249 | FIELD_PREP(FLT_MANT_MASK, val);
| ^
include/linux/bitfield.h:137:3: note: expanded from macro 'FIELD_PREP'
137 | __FIELD_PREP(_mask, _val, "FIELD_PREP: "); \
| ^
include/linux/bitfield.h:90:3: note: expanded from macro '__FIELD_PREP'
90 | __BF_FIELD_CHECK_MASK(mask, val, pfx); \
| ^
include/linux/bitfield.h:69:3: note: expanded from macro '__BF_FIELD_CHECK_MASK'
69 | BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ? \
| ^
note: (skipping 2 expansions in backtrace; use -fmacro-backtrace-limit=0 to see all)
include/linux/compiler_types.h:619:2: note: expanded from macro '_compiletime_assert'
619 | __compiletime_assert(condition, msg, prefix, suffix)
| ^
include/linux/compiler_types.h:612:4: note: expanded from macro '__compiletime_assert'
612 | prefix ## suffix(); \
| ^
<scratch space>:18:1: note: expanded from here
18 | __compiletime_assert_814
| ^
This does not appear to be strictly clang related, as this can be
reproduced with GCC if macsmc_hwmon_write_f32() and
macsmc_hwmon_write_key() are marked as __always_inline (after an
internal change in LLVM, these functions are not inlined anymore):
===
diff --git a/drivers/hwmon/macsmc-hwmon.c b/drivers/hwmon/macsmc-hwmon.c
index 1c0bbec7e8eb..e66d6ee30d7f 100644
--- a/drivers/hwmon/macsmc-hwmon.c
+++ b/drivers/hwmon/macsmc-hwmon.c
@@ -224,7 +224,7 @@ static int macsmc_hwmon_read_key(struct apple_smc *smc,
return 0;
}
-static int macsmc_hwmon_write_f32(struct apple_smc *smc, smc_key key, int value)
+static __always_inline int macsmc_hwmon_write_f32(struct apple_smc *smc, smc_key key, int value)
{
u64 val;
u32 fval = 0;
@@ -252,7 +252,7 @@ static int macsmc_hwmon_write_f32(struct apple_smc *smc, smc_key key, int value)
return apple_smc_write_u32(smc, key, fval);
}
-static int macsmc_hwmon_write_key(struct apple_smc *smc,
+static __always_inline int macsmc_hwmon_write_key(struct apple_smc *smc,
struct macsmc_hwmon_sensor *sensor, long val)
{
switch (sensor->info.type_code) {
===
In file included from <command-line>:
In function 'macsmc_hwmon_write_f32',
inlined from 'macsmc_hwmon_write_key' at drivers/hwmon/macsmc-hwmon.c:261:10,
inlined from 'macsmc_hwmon_write_fan.isra' at drivers/hwmon/macsmc-hwmon.c:317:10:
include/linux/compiler_types.h:631:45: error: call to '__compiletime_assert_814' declared with attribute error: FIELD_PREP: value too large for the field
631 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
| ^
include/linux/compiler_types.h:612:25: note: in definition of macro '__compiletime_assert'
612 | prefix ## suffix(); \
| ^~~~~~
include/linux/compiler_types.h:631:9: note: in expansion of macro '_compiletime_assert'
631 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
| ^~~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
| ^~~~~~~~~~~~~~~~~~
include/linux/bitfield.h:69:17: note: in expansion of macro 'BUILD_BUG_ON_MSG'
69 | BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ? \
| ^~~~~~~~~~~~~~~~
include/linux/bitfield.h:90:17: note: in expansion of macro '__BF_FIELD_CHECK_MASK'
90 | __BF_FIELD_CHECK_MASK(mask, val, pfx); \
| ^~~~~~~~~~~~~~~~~~~~~
include/linux/bitfield.h:137:17: note: in expansion of macro '__FIELD_PREP'
137 | __FIELD_PREP(_mask, _val, "FIELD_PREP: "); \
| ^~~~~~~~~~~~
drivers/hwmon/macsmc-hwmon.c:249:24: note: in expansion of macro 'FIELD_PREP'
249 | FIELD_PREP(FLT_MANT_MASK, val);
| ^~~~~~~~~~
Justin did a little digging and saw that through
macsmc_hwmon_write_fan()
-> macsmc_hwmon_write_key()
-> macsmc_hwmon_write_f32()
value is 1 so:
static __always_inline int macsmc_hwmon_write_f32(struct apple_smc *smc, smc_key key, int value)
{
u64 val;
u32 fval = 0;
int exp = 0, neg;
val = abs(value); // val == 1
neg = val != value;
if (val) {
int msb = __fls(val) - exp; // __fls(1) == 0, thus msb == 0
if (msb > 23) {
val >>= msb - FLT_MANT_BIAS;
exp -= msb - FLT_MANT_BIAS;
} else if (msb < 23) {
val <<= FLT_MANT_BIAS - msb; // val = (1 << 23) == 0x800000
exp += msb;
}
fval = FIELD_PREP(FLT_SIGN_MASK, neg) |
FIELD_PREP(FLT_EXP_MASK, exp + FLT_EXP_BIAS) |
// Fails __BF_FIELD_CHECK_MASK as
//
// __builtin_ffsll(0x7FFFFF) - 1) == 0
//
// ~(0x7FFFFF >> 0) & (0x800000) == 0x800000
FIELD_PREP(FLT_MANT_MASK, val);
}
return apple_smc_write_u32(smc, key, fval);
}
It seems like msb of zero is unexpected in this path but I am not sure
what the fix is, hence the report.
Cheers,
Nathan
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