Building Littlefs with NO_OPTIMIZATIONS fails unless DISK_ACCESS is explicitly set

[jonppe]

Currently the littlefs subsystem build fails if NO_OPTIMIZATIONS is set but DISK_ACCESS is not.

[102/106] Linking C executable zephyr/zephyr_pre0.elf FAILED: zephyr/zephyr_pre0.elf zephyr/zephyr_pre0.map /home/johannes/zephyr/build_fs_sample/zephyr/zephyr_pre0.map : && ccache /usr/bin/gcc -gdwarf-4 zephyr/CMakeFiles/zephyr_pre0.dir/misc/empty_file.c.obj -o zephyr/zephyr_pre0.elf zephyr/CMakeFiles/offsets.dir/./arch/posix/core/offsets/offsets.c.obj -Wl,-T zephyr/linker_zephyr_pre0.cmd -Wl,-Map=/home/johannes/zephyr/build_fs_sample/zephyr/zephyr_pre0.map -Wl,--whole-archive app/libapp.a zephyr/libzephyr.a zephyr/arch/arch/posix/core/libarch__posix__core.a zephyr/soc/posix/inf_clock/libsoc__posix__inf_clock.a zephyr/boards/boards/posix/native_posix_64/libboards__posix__native_posix.a zephyr/subsys/fs/libsubsys__fs.a zephyr/drivers/console/libdrivers__console.a zephyr/drivers/flash/libdrivers__flash.a zephyr/drivers/timer/libdrivers__timer.a modules/nimbus-global-id-list/libnimbusglobalidlist.a modules/littlefs/libmodules__littlefs.a -Wl,--no-whole-archive zephyr/kernel/libkernel.a -L/home/johannes/zephyr/build_fs_sample/zephyr -no-pie -Wl,--gc-sections -Wl,--build-id=none -Wl,--sort-common=descending -Wl,--sort-section=alignment -Wl,-u,_OffsetAbsSyms -Wl,-u,_ConfigAbsSyms -m64 -ldl -pthread -lm && cd /home/johannes/zephyr/build_fs_sample/zephyr && /usr/bin/cmake -E echo /usr/bin/ld: zephyr/subsys/fs/libsubsys__fs.a(littlefs_fs.c.obj): in function littlefs_init_backend':
/home/johannes/zephyr/zephyr/subsys/fs/littlefs_fs.c:656: undefined reference to disk_access_init' /usr/bin/ld: zephyr/subsys/fs/libsubsys__fs.a(littlefs_fs.c.obj): in function littlefs_init_cfg':
/home/johannes/zephyr/zephyr/subsys/fs/littlefs_fs.c:703: undefined reference to disk_access_ioctl' /usr/bin/ld: /home/johannes/zephyr/zephyr/subsys/fs/littlefs_fs.c:746: undefined reference to disk_access_ioctl'
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.`

This clearly because of code like (in littlefs_fs.c):

static int littlefs_init_backend(struct fs_littlefs *fs, uintptr_t dev_id, int flags)
{
int ret = 0;

if (littlefs_on_blkdev(flags)) {
	fs->backend = (void *) dev_id;
	ret = disk_access_init((char *) fs->backend);
	if (ret < 0) {
		LOG_ERR("Storage init ERROR!");
		return ret;
	}
} else {

where the littlefs_on_blkdev() definition is:

static inline bool littlefs_on_blkdev(int flags) { return IS_ENABLED(CONFIG_FS_LITTLEFS_BLK_DEV) && flags & FS_MOUNT_FLAG_USE_DISK_ACCESS; }

In other words, with optimizations enabled (but not CONFIG_FS_LITTLEFS_BLK_DEV), the compiler is able drop the call to disk_access_init(). When optimizations are disabled, the build fails unless CONFIG_DISK_ACCESS=y.

Some other sections of code in the same file are behind
#ifdef CONFIG_FS_LITTLEFS_BLK_DEV
checks.

The code could be refactored to include these checks for all code that uses disk_access_x() but I guess there could be other less obvious fixes. What do you think?