lib/crc: crc32: Document crc32_le(), crc32_be(), and crc32c()

Document these widely used functions.

Update kernel-api.rst to point to the correct place, instead of to
crc32-main.c which no longer contains kerneldoc comments.

Simplify the documentation in crc32poly.h to just point to the
corresponding functions, now that they are properly documented.  Change
the value of CRC32C_POLY_LE to lower case, for consistency.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250619183414.100082-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

Author: Eric Biggers

Documentation/core-api/kernel-api.rst

@@ -148,14 +148,14 @@ CRC Functions
 .. kernel-doc:: lib/crc/crc16.c
    :export:
 
-.. kernel-doc:: lib/crc/crc32-main.c
-
 .. kernel-doc:: lib/crc/crc-ccitt.c
    :export:
 
 .. kernel-doc:: lib/crc/crc-itu-t.c
    :export:
 
+.. kernel-doc:: include/linux/crc32.h
+
 Base 2 log and power Functions
 ------------------------------
 

include/linux/crc32.h

@@ -5,8 +5,74 @@
 #include <linux/types.h>
 #include <linux/bitrev.h>
 
+/**
+ * crc32_le() - Compute least-significant-bit-first IEEE CRC-32
+ * @crc: Initial CRC value.  ~0 (recommended) or 0 for a new CRC computation, or
+ *	 the previous CRC value if computing incrementally.
+ * @p: Pointer to the data buffer
+ * @len: Length of data in bytes
+ *
+ * This implements the CRC variant that is often known as the IEEE CRC-32, or
+ * simply CRC-32, and is widely used in Ethernet and other applications:
+ *
+ * - Polynomial: x^32 + x^26 + x^23 + x^22 + x^16 + x^12 + x^11 + x^10 + x^8 +
+ *		 x^7 + x^5 + x^4 + x^2 + x^1 + x^0
+ * - Bit order: Least-significant-bit-first
+ * - Polynomial in integer form: 0xedb88320
+ *
+ * This does *not* invert the CRC at the beginning or end.  The caller is
+ * expected to do that if it needs to.  Inverting at both ends is recommended.
+ *
+ * For new applications, prefer to use CRC-32C instead.  See crc32c().
+ *
+ * Context: Any context
+ * Return: The new CRC value
+ */
 u32 crc32_le(u32 crc, const void *p, size_t len);
+
+/**
+ * crc32_be() - Compute most-significant-bit-first IEEE CRC-32
+ * @crc: Initial CRC value.  ~0 (recommended) or 0 for a new CRC computation, or
+ *	 the previous CRC value if computing incrementally.
+ * @p: Pointer to the data buffer
+ * @len: Length of data in bytes
+ *
+ * crc32_be() is the same as crc32_le() except that crc32_be() computes the
+ * *most-significant-bit-first* variant of the CRC.  I.e., within each byte, the
+ * most significant bit is processed first (treated as highest order polynomial
+ * coefficient).  The same bit order is also used for the CRC value itself:
+ *
+ * - Polynomial: x^32 + x^26 + x^23 + x^22 + x^16 + x^12 + x^11 + x^10 + x^8 +
+ *		 x^7 + x^5 + x^4 + x^2 + x^1 + x^0
+ * - Bit order: Most-significant-bit-first
+ * - Polynomial in integer form: 0x04c11db7
+ *
+ * Context: Any context
+ * Return: The new CRC value
+ */
 u32 crc32_be(u32 crc, const void *p, size_t len);
+
+/**
+ * crc32c() - Compute CRC-32C
+ * @crc: Initial CRC value.  ~0 (recommended) or 0 for a new CRC computation, or
+ *	 the previous CRC value if computing incrementally.
+ * @p: Pointer to the data buffer
+ * @len: Length of data in bytes
+ *
+ * This implements CRC-32C, i.e. the Castagnoli CRC.  This is the recommended
+ * CRC variant to use in new applications that want a 32-bit CRC.
+ *
+ * - Polynomial: x^32 + x^28 + x^27 + x^26 + x^25 + x^23 + x^22 + x^20 + x^19 +
+ *		 x^18 + x^14 + x^13 + x^11 + x^10 + x^9 + x^8 + x^6 + x^0
+ * - Bit order: Least-significant-bit-first
+ * - Polynomial in integer form: 0x82f63b78
+ *
+ * This does *not* invert the CRC at the beginning or end.  The caller is
+ * expected to do that if it needs to.  Inverting at both ends is recommended.
+ *
+ * Context: Any context
+ * Return: The new CRC value
+ */
 u32 crc32c(u32 crc, const void *p, size_t len);
 
 /*

include/linux/crc32poly.h

@@ -2,19 +2,13 @@
 #ifndef _LINUX_CRC32_POLY_H
 #define _LINUX_CRC32_POLY_H
 
-/*
- * There are multiple 16-bit CRC polynomials in common use, but this is
- * *the* standard CRC-32 polynomial, first popularized by Ethernet.
- * x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x^1+x^0
- */
+/* The polynomial used by crc32_le(), in integer form.  See crc32_le(). */
 #define CRC32_POLY_LE 0xedb88320
+
+/* The polynomial used by crc32_be(), in integer form.  See crc32_be(). */
 #define CRC32_POLY_BE 0x04c11db7
 
-/*
- * This is the CRC32c polynomial, as outlined by Castagnoli.
- * x^32+x^28+x^27+x^26+x^25+x^23+x^22+x^20+x^19+x^18+x^14+x^13+x^11+x^10+x^9+
- * x^8+x^6+x^0
- */
-#define CRC32C_POLY_LE 0x82F63B78
+/* The polynomial used by crc32c(), in integer form.  See crc32c(). */
+#define CRC32C_POLY_LE 0x82f63b78
 
 #endif /* _LINUX_CRC32_POLY_H */