linux-kernel - [PATCH v2] bitfield.h: add FIELD_MAX() and field

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Message-Id: <20200306042302.17602-1-elder@linaro.org>
Date:   Thu,  5 Mar 2020 22:23:02 -0600
From:   Alex Elder <elder@...aro.org>
To:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     Johannes Berg <johannes@...solutions.net>,
        Arnd Bergmann <arnd@...db.de>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: [PATCH v2] bitfield.h: add FIELD_MAX() and field_max()

Define FIELD_MAX(), which supplies the maximum value that can be
represented by a bitfield.  Define field_max() as well, to go
along with the lower-case forms of the field mask functions.

Signed-off-by: Alex Elder <elder@...aro.org>
Acked-by: Jakub Kicinski <kuba@...nel.org>
---
v2: Added Acked-by from Jakub.

David, I added you to this because it's probably easiest to take
this change in through your tree with the rest of the IPA code
(which uses field_max(), defined here).

					-Alex

 include/linux/bitfield.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index 4bbb5f1c8b5b..48ea093ff04c 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -55,6 +55,19 @@
 					      (1ULL << __bf_shf(_mask))); \
 	})
 
+/**
+ * FIELD_MAX() - produce the maximum value representable by a field
+ * @_mask: shifted mask defining the field's length and position
+ *
+ * FIELD_MAX() returns the maximum value that can be held in the field
+ * specified by @_mask.
+ */
+#define FIELD_MAX(_mask)						\
+	({								\
+		__BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_MAX: ");	\
+		(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
@@ -110,6 +123,7 @@ static __always_inline u64 field_mask(u64 field)
 {
 	return field / field_multiplier(field);
 }
+#define field_max(field)	((typeof(field))field_mask(field))
 #define ____MAKE_OP(type,base,to,from)					\
 static __always_inline __##type type##_encode_bits(base v, base field)	\
 {									\
-- 
2.20.1