Add bit-field macros

Arm architectural documentation typically defines bit-fields as
`[msb,lsb]` and single-bit fields as `[bit]`. For clarity it would be
helpful if we could define fields in the same way.

Add helpers so that we can do so, along with helper to extract/insert
bit-field values.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>

1 files changed, 66 insertions, 0 deletions

diff --git a/include/bits.h b/include/bits.h
new file mode 100644
index 0000000..3dba897
--- /dev/null
+++ b/include/bits.h
@@ -0,0 +1,66 @@
+/*
+ * include/bits.h - helpers for bit-field definitions.
+ *
+ * Copyright (C) 2021 ARM Limited. All rights reserved.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE.txt file.
+ */
+#ifndef __BITS_H
+#define __BITS_H
+
+#ifdef __ASSEMBLY__
+#define UL(x)	x
+#define ULL(x)	x
+#else
+#define UL(x)	x##UL
+#define ULL(x)	x##ULL
+#endif
+
+/*
+ * Define a contiguous mask of bits with `msb` as the most significant bit and
+ * `lsb` as the least significant bit. The `msb` value must be greater than or
+ * equal to `lsb`.
+ *
+ * For example:
+ * - BITS(63, 63) is 0x8000000000000000
+ * - BITS(63, 0)  is 0xFFFFFFFFFFFFFFFF
+ * - BITS(0, 0)   is 0x0000000000000001
+ * - BITS(49, 17) is 0x0003FFFFFFFE0000
+ */
+#define BITS(msb, lsb) \
+	((~ULL(0) >> (63 - msb)) & (~ULL(0) << lsb))
+
+/*
+ * Define a mask of a single set bit `b`.
+ *
+ * For example:
+ * - BIT(63) is 0x8000000000000000
+ * - BIT(0)  is 0x0000000000000001
+ * - BIT(32) is 0x0000000100000000
+ */
+#define BIT(b)	BITS(b, b)
+
+/*
+ * Find the least significant set bit in the contiguous set of bits in `mask`.
+ *
+ * For example:
+ * - BITS_LSB(0x0000000000000001) is 0
+ * - BITS_LSB(0x000000000000ff00) is 8
+ * - BITS_LSB(0x8000000000000000) is 63
+ */
+#define BITS_LSB(mask)	(__builtin_ffsll(mask) - 1)
+
+/*
+ * Extract a bit-field out of `val` described by the contiguous set of bits in
+ * `mask`.
+ *
+ * For example:
+ * - BITS_EXTRACT(0xABCD, BITS(15, 12)) is 0xA
+ * - BITS_EXTRACT(0xABCD, BITS(11, 8))  is 0xB
+ * - BITS_EXTRACT(0xABCD, BIT(7))       is 0x1
+ */
+#define BITS_EXTRACT(val, mask) \
+	(((val) & (mask)) >> BITS_LSB(mask))
+
+#endif