[PATCH 1/9] bitfield: add FIELD_GET_SIGNED()

Yury Norov
2026-04-17T17:36:33Z

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() macro, which is the more
convenient and compiles (on x86_64) to just a couple instructions:
shl and sar.

Signed-off-by: Yury Norov <ynorov@nvidia.com>
---
include/linux/bitfield.h | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index 54aeeef1f0ec..35ef63972810 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