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

[Yury Norov]

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@xxxxxxxxxx>
---
 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