new file mode 100644
@@ -0,0 +1,93 @@
+/*
+ * 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 <linux/bug.h>
+
+/*
+ * Bitfield access macros
+ *
+ * FIELD_{GET,PREP} macros take as first parameter shifted mask
+ * from which they extract the base mask and shift amount.
+ * Mask must be a compilation time constant.
+ *
+ * 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_PREP(REG_FIELD_A, 1) |
+ * FIELD_PREP(REG_FIELD_B, 0) |
+ * FIELD_PREP(REG_FIELD_C, c) |
+ * FIELD_PREP(REG_FIELD_D, 0x40);
+ *
+ * Modify:
+ * reg &= ~REG_FIELD_C;
+ * reg |= FIELD_PREP(REG_FIELD_C, c);
+ */
+
+#define __bf_shf(x) (__builtin_ffsll(x) - 1)
+
+#define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx) \
+ ({ \
+ BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask), \
+ _pfx "mask is not constant"); \
+ BUILD_BUG_ON_MSG(!(_mask), _pfx "mask is zero"); \
+ BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ? \
+ ~((_mask) >> __bf_shf(_mask)) & (_val) : 0, \
+ _pfx "value too large for the field"); \
+ BUILD_BUG_ON_MSG((_mask) > (typeof(_reg))~0ull, \
+ _pfx "type of reg too small for mask"); \
+ __BUILD_BUG_ON_NOT_POWER_OF_2((_mask) + \
+ (1ULL << __bf_shf(_mask))); \
+ })
+
+/**
+ * FIELD_PREP() - prepare a bitfield element
+ * @_mask: shifted mask defining the field's length and position
+ * @_val: value to put in the field
+ *
+ * FIELD_PREP() masks and shifts up the value. The result should
+ * be combined with other fields of the bitfield using logical OR.
+ */
+#define FIELD_PREP(_mask, _val) \
+ ({ \
+ __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: "); \
+ ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask); \
+ })
+
+/**
+ * FIELD_GET() - extract a bitfield element
+ * @_mask: shifted mask defining the field's length and position
+ * @_reg: 32bit value of entire bitfield
+ *
+ * FIELD_GET() extracts the field specified by @_mask from the
+ * bitfield passed in as @_reg by masking and shifting it down.
+ */
+#define FIELD_GET(_mask, _reg) \
+ ({ \
+ __BF_FIELD_CHECK(_mask, _reg, 0U, "FIELD_GET: "); \
+ (typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask)); \
+ })
+
+#endif
@@ -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))
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_PREP(REG_FIELD, field); FIELD_{GET,PREP} 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> Reviewed-by: Dinan Gunawardena <dinan.gunawardena@netronome.com> --- v8: - use two underscores as prefix for __bf_shf() and __BF_FIELD_CHECK() include/linux/bitfield.h | 93 ++++++++++++++++++++++++++++++++++++++++++++++++ include/linux/bug.h | 3 ++ 2 files changed, 96 insertions(+) create mode 100644 include/linux/bitfield.h