[PATCH v2 0/9] bitfield: add FIELD_GET_SIGNED()

Yury Norov
2026-04-27T21:41:38Z

The bitfields are designed in assumption that fields contain unsigned
integer values, thus extracting the values from the field implies
zero-extending.

Some drivers need to sign-extend their fields, and currently do it like:

dc_re += sign_extend32(FIELD_GET(0xfff000, tmp), 11);
dc_im += sign_extend32(FIELD_GET(0xfff, tmp), 11);

It's error-prone because it relies on user to provide the correct
index of the most significant bit and proper 32 vs 64 function flavor.

Thus, introduce a FIELD_GET_SIGNED(). With the new API, the above
snippet turns into the more convenient:

dc_re += FIELD_GET_SIGNED(0xfff000, tmp);
dc_im += FIELD_GET_SIGNED(0xfff, tmp);

It compiles (on x86_64) into just a couple instructions: shl and sar.
When the mask includes MSB, the '<< __builtin_clzll(mask)' part becomes
a NOP, and the compiler only emits a single sar:

long long foo(long long reg)
{
10: f3 0f 1e fa endbr64
return FIELD_GET_SIGNED(GENMASK_ULL(63, 60), reg);
14: 48 89 f8 mov %rdi,%rax
17: 48 c1 f8 3c sar $0x3c,%rax
}

32-bit code generation is equally well. On arm32:

long long foo(long long reg)
{
return FIELD_GET_SIGNED(0x00f00000ULL, reg);
}

generates:

foo(long long):
lsls r1, r0, #8
asrs r0, r1, #28
asrs r1, r1, #31
bx lr

Immutable branch:

https://github.com/norov/linux/pull/new/fgsv2

v1: https://lore.kernel.org/all/20260417173621.368914-1-ynorov@nvidia.com/
v2:
- more examples of the new API and code generation (Andy, David);
- fix #7 FIELD_GET() / FIELD_GET_SIGNED() typo (Ping-Ke);
- re-indent the macro (Andy, Peter);

Yury Norov (9):
bitfield: add FIELD_GET_SIGNED()
x86/extable: switch to using FIELD_GET_SIGNED()
iio: intel_dc_ti_adc: switch to using FIELD_GET_SIGNED()
iio: magnetometer: yas530: switch to using FIELD_GET_SIGNED()
iio: pressure: bmp280: switch to using FIELD_GET_SIGNED()
iio: mcp9600: switch to using FIELD_GET_SIGNED()
wifi: rtw89: switch to using FIELD_GET_SIGNED()
rtc: rv3032: switch to using FIELD_GET_SIGNED()
ptp: switch to using FIELD_GET_SIGNED()

arch/x86/include/asm/extable_fixup_types.h | 13 ++++---------
arch/x86/mm/extable.c | 2 +-
drivers/iio/adc/intel_dc_ti_adc.c | 4 ++--
drivers/iio/magnetometer/yamaha-yas530.c | 12 ++++++------
drivers/iio/pressure/bmp280-core.c | 2 +-
drivers/iio/temperature/mcp9600.c | 2 +-
.../net/wireless/realtek/rtw89/rtw8852a_rfk.c | 4 ++--
.../net/wireless/realtek/rtw89/rtw8852b_common.c | 4 ++--
.../net/wireless/realtek/rtw89/rtw8852b_rfk.c | 4 ++--
drivers/net/wireless/realtek/rtw89/rtw8852c.c | 4 ++--
drivers/ptp/ptp_fc3.c | 4 ++--
drivers/rtc/rtc-rv3032.c | 2 +-
include/linux/bitfield.h | 16 ++++++++++++++++
13 files changed, 42 insertions(+), 31 deletions(-)

--
2.51.0