LittleFS: correct CRC calculation

When using MbedCRC, init value must be non-reversed, regardless of
`reflect_data` or `reflect_out` settings. This means we need to reflect
the intermediate output before passing using it as the next init value.

(In GCC this ends up putting in two `RBIT` instructions back-to-back,
because it's implemented as assembler, so it doesn't know how to
optimise. In ARMC6, `__RBIT` is implemented as an intrinsic, so adding
this reflection cancels the existing reflection and makes the code
smaller).

Author: kjbracey

features/storage/filesystem/littlefs/LittleFileSystem.cpp

@@ -24,7 +24,7 @@ namespace mbed {
 
 extern "C" void lfs_crc(uint32_t *crc, const void *buffer, size_t size)
 {
-    uint32_t initial_xor = *crc;
+    uint32_t initial_xor = lfs_rbit(*crc);
     // lfs_cache_crc calls lfs_crc for every byte individually, so can't afford
     // start-up overhead for hardware acceleration. Limit to table-based.
     MbedCRC<POLY_32BIT_ANSI, 32, CrcMode::TABLE> ct(initial_xor, 0x0, true, true);

features/storage/filesystem/littlefs/littlefs/lfs_util.h

@@ -47,6 +47,7 @@ extern "C"
 #ifdef __MBED__
 #include "mbed_debug.h"
 #include "mbed_assert.h"
+#include "cmsis_compiler.h"
 #else
 #define MBED_LFS_ENABLE_INFO   false
 #define MBED_LFS_ENABLE_DEBUG  true
@@ -183,6 +184,20 @@ static inline uint32_t lfs_tole32(uint32_t a) {
     return lfs_fromle32(a);
 }
 
+// Reverse the bits in a
+static inline uint32_t lfs_rbit(uint32_t a) {
+#if !defined(LFS_NO_INTRINSICS) && MBED_LFS_INTRINSICS
+    return __RBIT(a);
+#else
+    a = ((a & 0xaaaaaaaa) >> 1) | ((a & 0x55555555) << 1);
+    a = ((a & 0xcccccccc) >> 2) | ((a & 0x33333333) << 2);
+    a = ((a & 0xf0f0f0f0) >> 4) | ((a & 0x0f0f0f0f) << 4);
+    a = ((a & 0xff00ff00) >> 8) | ((a & 0x00ff00ff) << 8);
+    a = (a >> 16) | (a << 16);
+    return a;
+#endif
+}
+
 // Calculate CRC-32 with polynomial = 0x04c11db7
 void lfs_crc(uint32_t *crc, const void *buffer, size_t size);