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 Signed-off-by: Yury Norov --- include/linux/bitfield.h | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h index 54aeeef1f0ec..cd44013281c7 100644 --- a/include/linux/bitfield.h +++ b/include/linux/bitfield.h @@ -178,6 +178,22 @@ __FIELD_GET(_mask, _reg, "FIELD_GET: "); \ }) +/** + * FIELD_GET_SIGNED() - extract a signed bitfield element + * @mask: shifted mask defining the field's length and position + * @reg: value of entire bitfield + * + * Returns the sign-extended field specified by @_mask from the + * bitfield passed in as @_reg by masking and shifting it down. + */ +#define FIELD_GET_SIGNED(mask, reg) \ + ({ \ + __BF_FIELD_CHECK(mask, reg, 0U, "FIELD_GET_SIGNED: "); \ + ((__signed_scalar_typeof(mask)) \ + (((long long)(reg) << __builtin_clzll(mask)) >> \ + (__builtin_clzll(mask) + __builtin_ctzll(mask)))); \ + }) + /** * FIELD_MODIFY() - modify a bitfield element * @_mask: shifted mask defining the field's length and position -- 2.51.0