Added clarification on buffer alignment.

In v2, the lookahead_buffer was changed from requiring 4-byte alignment
to requiring 8-byte alignment. This was not documented as well as it
could be, and as FabianInostroza noted, this also implies that
lfs_malloc must provide 8-byte alignment.

To protect against this, I've also added an assert on the alignment of
both the lookahead_size and lookahead_buffer.

found by FabianInostroza and amitv87

lfs.c

@@ -3228,8 +3228,9 @@ static int lfs_init(lfs_t *lfs, const struct lfs_config *cfg) {
     lfs_cache_zero(lfs, &lfs->pcache);
 
     // setup lookahead, must be multiple of 64-bits
-    LFS_ASSERT(lfs->cfg->lookahead_size % 8 == 0);
     LFS_ASSERT(lfs->cfg->lookahead_size > 0);
+    LFS_ASSERT(lfs->cfg->lookahead_size % 8 == 0 &&
+            (uintptr_t)lfs->cfg->lookahead_buffer % 8 == 0);
     if (lfs->cfg->lookahead_buffer) {
         lfs->free.buffer = lfs->cfg->lookahead_buffer;
     } else {

lfs.h

@@ -215,8 +215,9 @@ struct lfs_config {
     // By default lfs_malloc is used to allocate this buffer.
     void *prog_buffer;
 
-    // Optional statically allocated program buffer. Must be lookahead_size.
-    // By default lfs_malloc is used to allocate this buffer.
+    // Optional statically allocated lookahead buffer. Must be lookahead_size
+    // and aligned to a 64-bit boundary. By default lfs_malloc is used to
+    // allocate this buffer.
     void *lookahead_buffer;
 
     // Optional upper limit on length of file names in bytes. No downside for

lfs_util.h

@@ -192,6 +192,7 @@ static inline uint32_t lfs_tobe32(uint32_t a) {
 uint32_t lfs_crc(uint32_t crc, const void *buffer, size_t size);
 
 // Allocate memory, only used if buffers are not provided to littlefs
+// Note, memory must be 64-bit aligned
 static inline void *lfs_malloc(size_t size) {
 #ifndef LFS_NO_MALLOC
     return malloc(size);