[PATCH] bitfield.h: add FIELD_MAX_CONST

[Michal Schmidt <mschmidt@redhat.com>]

FIELD_MAX_CONST is like FIELD_MAX, but it can be used where statement
expressions are forbidden. For example, using FIELD_MAX in a
static_assert gives:
  error: braced-group within expression allowed only inside a function

It can be used also in array declarations, where using FIELD_MAX would
trigger a warning :
  warning: ISO C90 forbids variable length array ‘buf’ [-Wvla]
(It's a bit surprising, because despite the warning, gcc calculated
the array size correctly at compile time.)

A simplified example of what I actually want to use in a driver:
  #define DATA_SIZE_M GENMASK(3, 0)
  #define MAX_DATA_SIZE FIELD_MAX_CONST(DATA_SIZE_M)
  static void f(void) {
  	char buf[MAX_DATA_SIZE];
  	/* ... */
  }

In the implementation, reuse the existing compile-time checks from
FIELD_PREP_CONST.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
---
 include/linux/bitfield.h | 34 +++++++++++++++++++++++++++-------
 1 file changed, 27 insertions(+), 7 deletions(-)

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index 63928f173223..50bbab317319 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -76,6 +76,16 @@
 					      (1ULL << __bf_shf(_mask))); \
 	})
 
+#define __BF_FIELD_CHECK_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)))	\
+	)
+
 /**
  * FIELD_MAX() - produce the maximum value representable by a field
  * @_mask: shifted mask defining the field's length and position
@@ -89,6 +99,22 @@
 		(typeof(_mask))((_mask) >> __bf_shf(_mask));		\
 	})
 
+/**
+ * FIELD_MAX_CONST() - produce the maximum value representable by a field
+ * @_mask: shifted mask defining the field's length and position
+ *
+ * FIELD_MAX_CONST() returns the maximum value that can be held in
+ * the field specified by @_mask.
+ *
+ * Unlike FIELD_MAX(), it can be used where statement expressions can't.
+ * Error checking is less comfortable for this version.
+ */
+#define FIELD_MAX_CONST(_mask)						\
+	(								\
+		__BF_FIELD_CHECK_CONST(_mask, 0ULL) +			\
+		(typeof(_mask))((_mask) >> __bf_shf(_mask))		\
+	)
+
 /**
  * FIELD_FIT() - check if value fits in the field
  * @_mask: shifted mask defining the field's length and position
@@ -132,13 +158,7 @@
  */
 #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 */				\
+		__BF_FIELD_CHECK_CONST(_mask, _val) +			\
 		(((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask))	\
 	)
 
-- 
2.44.0