Felix Fietkau
8 years ago
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From: Jakub Kicinski <jakub.kicinski@netronome.com>
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Date: Wed, 31 Aug 2016 12:46:44 +0100
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Subject: [PATCH] add basic register-field manipulation macros
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Common approach to accessing register fields is to define
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structures or sets of macros containing mask and shift pair.
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Operations on the register are then performed as follows:
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field = (reg >> shift) & mask;
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reg &= ~(mask << shift);
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reg |= (field & mask) << shift;
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Defining shift and mask separately is tedious. Ivo van Doorn
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came up with an idea of computing them at compilation time
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based on a single shifted mask (later refined by Felix) which
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can be used like this:
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#define REG_FIELD 0x000ff000
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field = FIELD_GET(REG_FIELD, reg);
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reg &= ~REG_FIELD;
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reg |= FIELD_PREP(REG_FIELD, field);
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FIELD_{GET,PREP} macros take care of finding out what the
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appropriate shift is based on compilation time ffs operation.
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GENMASK can be used to define registers (which is usually
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less error-prone and easier to match with datasheets).
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This approach is the most convenient I've seen so to limit code
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multiplication let's move the macros to a global header file.
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Attempts to use static inlines instead of macros failed due
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to false positive triggering of BUILD_BUG_ON()s, especially with
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GCC < 6.0.
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Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
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Reviewed-by: Dinan Gunawardena <dinan.gunawardena@netronome.com>
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---
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create mode 100644 include/linux/bitfield.h
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--- /dev/null
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+++ b/include/linux/bitfield.h
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@@ -0,0 +1,100 @@
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+/*
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+ * Copyright (C) 2014 Felix Fietkau <nbd@nbd.name>
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+ * Copyright (C) 2004 - 2009 Ivo van Doorn <IvDoorn@gmail.com>
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+ *
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+ * This program is free software; you can redistribute it and/or modify
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+ * it under the terms of the GNU General Public License version 2
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+ * as published by the Free Software Foundation
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+ *
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+ * This program is distributed in the hope that it will be useful,
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+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
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+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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+ * GNU General Public License for more details.
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+ */
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+
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+#ifndef _LINUX_BITFIELD_H
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+#define _LINUX_BITFIELD_H
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+
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+#include <linux/bug.h>
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+
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+#ifdef __CHECKER__
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+#define __BUILD_BUG_ON_NOT_POWER_OF_2(n) (0)
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+#else
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+#define __BUILD_BUG_ON_NOT_POWER_OF_2(n) \
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+ BUILD_BUG_ON(((n) & ((n) - 1)) != 0)
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+#endif
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+
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+/*
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+ * Bitfield access macros
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+ *
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+ * FIELD_{GET,PREP} macros take as first parameter shifted mask
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+ * from which they extract the base mask and shift amount.
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+ * Mask must be a compilation time constant.
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+ *
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+ * Example:
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+ *
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+ * #define REG_FIELD_A GENMASK(6, 0)
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+ * #define REG_FIELD_B BIT(7)
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+ * #define REG_FIELD_C GENMASK(15, 8)
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+ * #define REG_FIELD_D GENMASK(31, 16)
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+ *
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+ * Get:
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+ * a = FIELD_GET(REG_FIELD_A, reg);
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+ * b = FIELD_GET(REG_FIELD_B, reg);
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+ *
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+ * Set:
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+ * reg = FIELD_PREP(REG_FIELD_A, 1) |
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+ * FIELD_PREP(REG_FIELD_B, 0) |
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+ * FIELD_PREP(REG_FIELD_C, c) |
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+ * FIELD_PREP(REG_FIELD_D, 0x40);
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+ *
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+ * Modify:
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+ * reg &= ~REG_FIELD_C;
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+ * reg |= FIELD_PREP(REG_FIELD_C, c);
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+ */
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+
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+#define __bf_shf(x) (__builtin_ffsll(x) - 1)
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+
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+#define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx) \
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+ ({ \
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+ BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask), \
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+ _pfx "mask is not constant"); \
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+ BUILD_BUG_ON_MSG(!(_mask), _pfx "mask is zero"); \
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+ BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ? \
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+ ~((_mask) >> __bf_shf(_mask)) & (_val) : 0, \
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+ _pfx "value too large for the field"); \
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+ BUILD_BUG_ON_MSG((_mask) > (typeof(_reg))~0ull, \
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+ _pfx "type of reg too small for mask"); \
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+ __BUILD_BUG_ON_NOT_POWER_OF_2((_mask) + \
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+ (1ULL << __bf_shf(_mask))); \
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+ })
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+
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+/**
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+ * FIELD_PREP() - prepare a bitfield element
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+ * @_mask: shifted mask defining the field's length and position
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+ * @_val: value to put in the field
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+ *
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+ * FIELD_PREP() masks and shifts up the value. The result should
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+ * be combined with other fields of the bitfield using logical OR.
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+ */
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+#define FIELD_PREP(_mask, _val) \
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+ ({ \
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+ __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: "); \
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+ ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask); \
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+ })
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+
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+/**
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+ * FIELD_GET() - extract a bitfield element
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+ * @_mask: shifted mask defining the field's length and position
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+ * @_reg: 32bit value of entire bitfield
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+ *
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+ * FIELD_GET() extracts the field specified by @_mask from the
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+ * bitfield passed in as @_reg by masking and shifting it down.
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+ */
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+#define FIELD_GET(_mask, _reg) \
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+ ({ \
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+ __BF_FIELD_CHECK(_mask, _reg, 0U, "FIELD_GET: "); \
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+ (typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask)); \
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+ })
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+
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+#endif
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