[PATCHv2 1/2] add basic register-field manipulation macros

[Jakub Kicinski]

C bitfields are problematic and best avoided.  Developers
interacting with hardware registers find themselves searching
for easy-to-use alternatives.  Common approach is to define
structures or sets of macros containing mask and shift pair.
Operations on the register are then performed as follows:

 field = (reg >> shift) & mask;

 reg &= ~(mask << shift);
 reg |= (field & mask) << shift;

Defining shift and mask separately is tedious.  Ivo van Doorn
came up with an idea of computing them at compilation time
based on a single shifted mask (later refined by Felix) which
can be used like this:

 #define X_REG_FIELD 0x000ff000

 field = FIELD_GET(X_REG_FIELD, reg);

 reg &= ~X_REG_FIELD;
 reg |= FIELD_PUT(X_REG_FIELD, field);

FIELD_{GET,PUT} macros take care of finding out what the
appropriate shift is based on compilation time ffs operation.

GENMASK can be used to define registers (which is usually
less error-prone and easier to match with datasheets).

This approach is the most convenient I've seen so to limit code
multiplication let's move the macros to a global header file.

Compared to Felix Fietkau's implementation from mt76 this one
uses standard Linux and GCC functions such as is_power_of_2()
and __builtin_ffsll().

Signed-off-by: Jakub Kicinski <jakub.kicinski@xxxxxxxxxxxxx>
---
v2:
 - change Felix's email address.

 include/linux/bitfield.h | 58 ++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/log2.h     |  6 +++++
 2 files changed, 64 insertions(+)
 create mode 100644 include/linux/bitfield.h

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
new file mode 100644
index 000000000000..9560d1877cbc
--- /dev/null
+++ b/include/linux/bitfield.h
@@ -0,0 +1,58 @@
+/*
+ * Copyright (C) 2014 Felix Fietkau <nbd@xxxxxxxx>
+ * Copyright (C) 2004 - 2009 Ivo van Doorn <IvDoorn@xxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2
+ * as published by the Free Software Foundation
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#ifndef _LINUX_BITFIELD_H
+#define _LINUX_BITFIELD_H
+
+#include <asm/types.h>
+#include <linux/bug.h>
+#include <linux/log2.h>
+
+#define _bf_shf(x) (__builtin_ffsll(x) - 1)
+
+#define _BF_FIELD_CHECK(_mask, _val)					\
+	({								\
+		const u64 hi = (_mask) + (1ULL << _bf_shf(_mask));	\
+									\
+		BUILD_BUG_ON(!(_mask) || (hi && !is_power_of_2_u64(hi))); \
+		BUILD_BUG_ON(__builtin_constant_p(_val) ?		\
+			     ~((_mask) >> _bf_shf(_mask)) & (_val) :	\
+			     0);					\
+	})
+
+#define FIELD_PUT(_mask, _val)					\
+	({							\
+		_BF_FIELD_CHECK(_mask, _val);			\
+		((u32)(_val) << _bf_shf(_mask)) & (_mask);	\
+	})
+
+#define FIELD_GET(_mask, _val)					\
+	({							\
+		_BF_FIELD_CHECK(_mask, 0);			\
+		(u32)(((_val) & (_mask)) >> _bf_shf(_mask));	\
+	})
+
+#define FIELD_PUT64(_mask, _val)				\
+	({							\
+		_BF_FIELD_CHECK(_mask, _val);			\
+		((u64)(_val) << _bf_shf(_mask)) & (_mask);	\
+	})
+
+#define FIELD_GET64(_mask, _val)				\
+	({							\
+		_BF_FIELD_CHECK(_mask, 0);			\
+		(u64)(((_val) & (_mask)) >> _bf_shf(_mask));	\
+	})
+
+#endif
diff --git a/include/linux/log2.h b/include/linux/log2.h
index fd7ff3d91e6a..1b5e1f4da043 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -54,6 +54,12 @@ bool is_power_of_2(unsigned long n)
 	return (n != 0 && ((n & (n - 1)) == 0));
 }
 
+static inline __attribute__((const))
+bool is_power_of_2_u64(u64 n)
+{
+	return (n != 0 && ((n & (n - 1)) == 0));
+}
+
 /*
  * round up to nearest power of two
  */
-- 
1.9.1