lib/crc32: Remove unused combination support

Remove crc32_le_combine() and crc32_le_shift(), since they are no longer
used.

Although combination is an interesting thing that can be done with CRCs,
it turned out that none of the users of it in the kernel were even close
to being worthwhile.  All were much better off simply chaining the CRCs
or processing zeroes.

Let's remove the CRC32 combination code for now.  It can come back
(potentially optimized with carryless multiplication instructions) if
there is ever a case where it would actually be worthwhile.

Link: https://lore.kernel.org/r/20250607032228.27868-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

1 files changed, 1 insertions, 38 deletions

diff --git a/lib/tests/crc_kunit.c b/lib/tests/crc_kunit.c
index 064c2d581557..f08d985d8860 100644
--- a/lib/tests/crc_kunit.c
+++ b/lib/tests/crc_kunit.c
@@ -36,14 +36,12 @@ static size_t test_buflen;
  *	  can fit any CRC up to CRC-64.  The CRC is passed in, and is expected
  *	  to be returned in, the least significant bits of the u64.  The
  *	  function is expected to *not* invert the CRC at the beginning and end.
- * @combine_func: Optional function to combine two CRCs.
  */
 struct crc_variant {
 	int bits;
 	bool le;
 	u64 poly;
 	u64 (*func)(u64 crc, const u8 *p, size_t len);
-	u64 (*combine_func)(u64 crc1, u64 crc2, size_t len2);
 };
 
 static u32 rand32(void)
@@ -144,7 +142,7 @@ static size_t generate_random_length(size_t max_length)
 }
 
 /* Test that v->func gives the same CRCs as a reference implementation. */
-static void crc_main_test(struct kunit *test, const struct crc_variant *v)
+static void crc_test(struct kunit *test, const struct crc_variant *v)
 {
 	size_t i;
 
@@ -188,35 +186,6 @@ static void crc_main_test(struct kunit *test, const struct crc_variant *v)
 	}
 }
 
-/* Test that CRC(concat(A, B)) == combine_CRCs(CRC(A), CRC(B), len(B)). */
-static void crc_combine_test(struct kunit *test, const struct crc_variant *v)
-{
-	int i;
-
-	for (i = 0; i < 100; i++) {
-		u64 init_crc = generate_random_initial_crc(v);
-		size_t len1 = generate_random_length(CRC_KUNIT_MAX_LEN);
-		size_t len2 = generate_random_length(CRC_KUNIT_MAX_LEN - len1);
-		u64 crc1, crc2, expected_crc, actual_crc;
-
-		prandom_bytes_state(&rng, test_buffer, len1 + len2);
-		crc1 = v->func(init_crc, test_buffer, len1);
-		crc2 = v->func(0, &test_buffer[len1], len2);
-		expected_crc = v->func(init_crc, test_buffer, len1 + len2);
-		actual_crc = v->combine_func(crc1, crc2, len2);
-		KUNIT_EXPECT_EQ_MSG(test, expected_crc, actual_crc,
-				    "CRC combination gave wrong result with len1=%zu len2=%zu\n",
-				    len1, len2);
-	}
-}
-
-static void crc_test(struct kunit *test, const struct crc_variant *v)
-{
-	crc_main_test(test, v);
-	if (v->combine_func)
-		crc_combine_test(test, v);
-}
-
 static __always_inline void
 crc_benchmark(struct kunit *test,
 	      u64 (*crc_func)(u64 crc, const u8 *p, size_t len))
@@ -337,17 +306,11 @@ static u64 crc32_le_wrapper(u64 crc, const u8 *p, size_t len)
 	return crc32_le(crc, p, len);
 }
 
-static u64 crc32_le_combine_wrapper(u64 crc1, u64 crc2, size_t len2)
-{
-	return crc32_le_combine(crc1, crc2, len2);
-}
-
 static const struct crc_variant crc_variant_crc32_le = {
 	.bits = 32,
 	.le = true,
 	.poly = 0xedb88320,
 	.func = crc32_le_wrapper,
-	.combine_func = crc32_le_combine_wrapper,
 };
 
 static void crc32_le_test(struct kunit *test)