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Subject[PATCHv5 wl-drv-next 1/2] add basic register-field manipulation macros
Date
Common approach to accessing register fields 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 REG_FIELD 0x000ff000

field = FIELD_GET(REG_FIELD, reg);

reg &= ~REG_FIELD;
reg |= FIELD_PUT(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.
Attempts to use static inlines instead of macros failed due
to false positive triggering of BUILD_BUG_ON()s, especially with
GCC < 6.0.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
---
include/linux/bitfield.h | 109 +++++++++++++++++++++++++++++++++++++++++++++++
include/linux/bug.h | 3 ++
2 files changed, 112 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..ff9fd0af2ac7
--- /dev/null
+++ b/include/linux/bitfield.h
@@ -0,0 +1,109 @@
+/*
+ * Copyright (C) 2014 Felix Fietkau <nbd@nbd.name>
+ * Copyright (C) 2004 - 2009 Ivo van Doorn <IvDoorn@gmail.com>
+ *
+ * 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>
+
+#define _bf_shf(x) (__builtin_ffsll(x) - 1)
+
+#define _BF_FIELD_CHECK(_mask, _val) \
+ ({ \
+ BUILD_BUG_ON(!(_mask)); \
+ BUILD_BUG_ON(__builtin_constant_p(_val) ? \
+ ~((_mask) >> _bf_shf(_mask)) & (_val) : \
+ 0); \
+ __BUILD_BUG_ON_NOT_POWER_OF_2((_mask) + \
+ (1ULL << _bf_shf(_mask))); \
+ })
+
+/*
+ * Bitfield access macros
+ *
+ * This file contains macros which take as input shifted mask
+ * from which they extract the base mask and shift amount at
+ * compilation time. There are two separate sets of the macros
+ * one for 32bit registers and one for 64bit ones.
+ *
+ * Fields can be defined using GENMASK (which is usually
+ * less error-prone and easier to match with datasheets).
+ *
+ * FIELD_{GET,PUT} macros are designed to be used with masks which
+ * are compilation time constants.
+ *
+ * Example:
+ *
+ * #define REG_FIELD_A GENMASK(6, 0)
+ * #define REG_FIELD_B BIT(7)
+ * #define REG_FIELD_C GENMASK(15, 8)
+ * #define REG_FIELD_D GENMASK(31, 16)
+ *
+ * Get:
+ * a = FIELD_GET(REG_FIELD_A, reg);
+ * b = FIELD_GET(REG_FIELD_B, reg);
+ *
+ * Set:
+ * reg = FIELD_PUT(REG_FIELD_A, 1) |
+ * FIELD_PUT(REG_FIELD_B, 0) |
+ * FIELD_PUT(REG_FIELD_C, c) |
+ * FIELD_PUT(REG_FIELD_D, 0x40);
+ *
+ * Modify:
+ * reg &= ~REG_FIELD_C;
+ * reg |= FIELD_PUT(REG_FIELD_C, c);
+ */
+
+/**
+ * FIELD_PUT() - construct a bitfield element
+ * @_mask: shifted mask defining the field's length and position
+ * @_val: value to put in the field
+ *
+ * FIELD_PUT() masks and shifts up the value. The result should
+ * be combined with other fields of the bitfield using logical OR.
+ */
+#define FIELD_PUT(_mask, _val) \
+ ({ \
+ _BF_FIELD_CHECK(_mask, _val); \
+ ((u32)(_val) << _bf_shf(_mask)) & (_mask); \
+ })
+
+/**
+ * FIELD_GET() - extract a bitfield element
+ * @_mask: shifted mask defining the field's length and position
+ * @_val: 32bit value of entire bitfield
+ *
+ * FIELD_GET() extracts the field specified by @_mask from the
+ * bitfield passed in as @_val.
+ */
+#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/bug.h b/include/linux/bug.h
index e51b0709e78d..bba5bdae1681 100644
--- a/include/linux/bug.h
+++ b/include/linux/bug.h
@@ -13,6 +13,7 @@ enum bug_trap_type {
struct pt_regs;

#ifdef __CHECKER__
+#define __BUILD_BUG_ON_NOT_POWER_OF_2(n) (0)
#define BUILD_BUG_ON_NOT_POWER_OF_2(n) (0)
#define BUILD_BUG_ON_ZERO(e) (0)
#define BUILD_BUG_ON_NULL(e) ((void*)0)
@@ -24,6 +25,8 @@ struct pt_regs;
#else /* __CHECKER__ */

/* Force a compilation error if a constant expression is not a power of 2 */
+#define __BUILD_BUG_ON_NOT_POWER_OF_2(n) \
+ BUILD_BUG_ON(((n) & ((n) - 1)) != 0)
#define BUILD_BUG_ON_NOT_POWER_OF_2(n) \
BUILD_BUG_ON((n) == 0 || (((n) & ((n) - 1)) != 0))

--
1.9.1
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