fs/shm: Add a flag FS_SHMFS_NO_ALIGN to remove SHM alignment

This can be used on small systems to save RAM if the SHM object
doesn't need to be cache-aligned.

Signed-off-by: Jukka Laitinen <jukka.laitinen@tii.ae>

fs/shm/Kconfig

@@ -19,4 +19,14 @@ config FS_SHMFS_VFS_PATH
 		The path to where shared memory objects will exist in the VFS
 		namespace.
 
+config FS_SHMFS_NO_ALIGN
+	bool "Skip cache-line alignment/padding of shm objects"
+	default n
+	---help---
+		Skip cache-line alignment of the shm object metadata header and
+		the object length. Saves RAM in FLAT builds where shm objects
+		are never used as DMA targets and no cache maintenance is
+		performed on their contents. Callers that need DMA-safe shm
+		buffers must not enable this.
+
 endif # FS_SHMFS

fs/shm/shmfs_alloc.c

@@ -53,7 +53,11 @@ FAR struct shmfs_object_s *shmfs_alloc_object(size_t length)
 
   size_t hdr_size = sizeof(struct shmfs_object_s);
   size_t alloc_size = length;
+#ifdef CONFIG_FS_SHMFS_NO_ALIGN
+  size_t cachesize = 0;
+#else
   size_t cachesize = up_get_dcache_linesize();
+#endif
 
   if (cachesize > 0)
     {
@@ -83,7 +87,11 @@ FAR struct shmfs_object_s *shmfs_alloc_object(size_t length)
   object = fs_heap_zalloc(sizeof(struct shmfs_object_s));
   if (object)
     {
+#ifdef CONFIG_FS_SHMFS_NO_ALIGN
+      size_t cachesize = 0;
+#else
       size_t cachesize = up_get_dcache_linesize();
+#endif
 
       if (cachesize > 0)
         {