Import FIELD_PREP_CONST macro from Linux Kernel to permit usage of
FIELD_PREP with scenario where a constant value is needed.
Refer to commit e2192de59e45 ("bitfield: add FIELD_PREP_CONST()") in
Linux kernel for extensive explaination of why this is useful.
This is also to better align with the Linux Kernel for easier porting of
driver.
Signed-off-by: Christian Marangi <ansuels...@gmail.com>
---
include/linux/bitfield.h | 26 ++++++++++++++++++++++++++
1 file changed, 26 insertions(+)
diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index 7ad02f8cbb9..1b73ba6f9fa 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -90,6 +90,32 @@
((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask); \
})
+#define __BF_CHECK_POW2(n) BUILD_BUG_ON_ZERO(((n) & ((n) - 1)) != 0)
+
+/**
+ * FIELD_PREP_CONST() - prepare a constant bitfield element
+ * @_mask: shifted mask defining the field's length and position
+ * @_val: value to put in the field
+ *
+ * FIELD_PREP_CONST() masks and shifts up the value. The result should
+ * be combined with other fields of the bitfield using logical OR.
+ *
+ * Unlike FIELD_PREP() this is a constant expression and can therefore
+ * be used in initializers. Error checking is less comfortable for this
+ * version, and non-constant masks cannot be used.
+ */
+#define FIELD_PREP_CONST(_mask, _val) \
+ ( \
+ /* mask must be non-zero */ \
+ BUILD_BUG_ON_ZERO((_mask) == 0) + \
+ /* check if value fits */ \
+ BUILD_BUG_ON_ZERO(~((_mask) >> __bf_shf(_mask)) & (_val)) + \
+ /* check if mask is contiguous */ \
+ __BF_CHECK_POW2((_mask) + (1ULL << __bf_shf(_mask))) + \
+ /* and create the value */ \
+ (((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask)) \
+ )
+
/**
* FIELD_GET() - extract a bitfield element
* @_mask: shifted mask defining the field's length and position
--
2.48.1
