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+/*
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+ * crc4.c - simple crc-4 calculations.
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+ *
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+ * This source code is licensed under the GNU General Public License, Version
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+ * 2. See the file COPYING for more details.
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+ */
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+
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+#include <linux/crc4.h>
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+#include <linux/module.h>
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+
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+static const uint8_t crc4_tab[] = {
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+ 0x0, 0x7, 0xe, 0x9, 0xb, 0xc, 0x5, 0x2,
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+ 0x1, 0x6, 0xf, 0x8, 0xa, 0xd, 0x4, 0x3,
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+};
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+
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+/**
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+ * crc4 - calculate the 4-bit crc of a value.
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+ * @crc: starting crc4
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+ * @x: value to checksum
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+ * @bits: number of bits in @x to checksum
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+ *
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+ * Returns the crc4 value of @x, using polynomial 0b10111.
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+ *
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+ * The @x value is treated as left-aligned, and bits above @bits are ignored
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+ * in the crc calculations.
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+ */
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+uint8_t crc4(uint8_t c, uint64_t x, int bits)
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+{
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+ int i;
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+
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+ /* mask off anything above the top bit */
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+ x &= (1ull << bits) - 1;
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+
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+ /* Align to 4-bits */
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+ bits = (bits + 3) & ~0x3;
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+
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+ /* Calculate crc4 over four-bit nibbles, starting at the MSbit */
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+ for (i = bits - 4; i >= 0; i -= 4)
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+ c = crc4_tab[c ^ ((x >> i) & 0xf)];
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+
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+ return c;
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+}
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+EXPORT_SYMBOL_GPL(crc4);
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+
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+MODULE_DESCRIPTION("CRC4 calculations");
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+MODULE_LICENSE("GPL");
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