bitfield: add FIELD_GET_SIGNED()

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 <ynorov@nvidia.com>

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index 54aeeef1f0ec7d14386cd8a69100b90797c21c8f..989c214ec8a57a4eb97adf73da2974184e1e2ed7 100644 (file)
--- 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