FAT_FS SD Card "file open error -2" when file name is longer than 14 characters

[mbustin-miru]

nrf52840 DK, nRF Connect SDK 2.3.0 using vsCode, FAT Filesystem Sample Application

Working with the FAT_FS sample code, I am able to communicate with the SD card perfectly fine, as far as I can tell. The call to lsdir() functions as expected. However, when calling fs_open() to open a file to write to on an external SD card, it is returning error code -2 IF the filepath (mount point + filename) is longer than 14 characters. If the filepath is less than 14 characters then it will successfully write data. I thought it might be resolved by enabling CONFIG_FS_FATFS_LFN, however it did not work. See the code below for my example.

What does error code -2 mean? I wasn't able to find a clear answer anywhere. Is it the same as ENOENT, found in /zephyr/lib/libc/minimal/include/errno.h?

If I get a little deeper with the error, it fails in fs.c at line 163, in fs_open.

fs.c (in the code snippet below the failure is at line 33)

/* File operations */
int fs_open(struct fs_file_t *zfp, const char *file_name, fs_mode_t flags)
{
	struct fs_mount_t *mp;
	int rc = -EINVAL;

	if ((file_name == NULL) ||
			(strlen(file_name) <= 1) || (file_name[0] != '/')) {
		LOG_ERR("invalid file name!!");
		return -EINVAL;
	}

	if (zfp->mp != NULL) {
		return -EBUSY;
	}

	rc = fs_get_mnt_point(&mp, file_name, NULL);
	if (rc < 0) {
		LOG_ERR("mount point not found!!");
		return rc;
	}

	if (((mp->flags & FS_MOUNT_FLAG_READ_ONLY) != 0) &&
	    (flags & FS_O_CREATE || flags & FS_O_WRITE)) {
		return -EROFS;
	}

	CHECKIF(mp->fs->open == NULL) {
		return -ENOTSUP;
	}

	zfp->mp = mp;
	rc = mp->fs->open(zfp, file_name, flags);
	if (rc < 0) {
		LOG_ERR("file open error (%d)", rc);
		zfp->mp = NULL;
		return rc;
	}

	/* Copy flags to zfp for use with other fs_ API calls */
	zfp->flags = flags;

	return rc;
}

main.c

#include <zephyr/kernel.h>
#include <zephyr/device.h>
#include <zephyr/storage/disk_access.h>
#include <zephyr/logging/log.h>
#include <zephyr/fs/fs.h>
#include <ff.h>

static FATFS fat_fs;
/* mounting info */
static struct fs_mount_t mp = {
	.type = FS_FATFS,
	.fs_data = &fat_fs,
};

static const char *disk_mount_pt = "/SD:";

void main(void)
{
	mp.mnt_point = disk_mount_pt;

	int res = fs_mount(&mp);

	if (res == FR_OK) {
		printk("Disk mounted.\n");
		lsdir(disk_mount_pt);
	} else {
		printk("Error mounting disk.\n");
	}

	char datatest[] = "1,2,3,4,5,6\n";
	size_t datatest_size = strlen(datatest);

	struct fs_file_t dfile;
	size_t written_size = 0;

	fs_file_t_init(&dfile);

    // This is where the code fails.
	res = fs_open(&dfile, "/SD:/DKlog.txt", FS_O_RDWR | FS_O_APPEND);

	if(res < 0)
		return false;

	written_size = fs_write(&dfile, datatest, datatest_size);

	res = fs_close(&dfile);

	while (1) {
		ret = toggle_leds();
		k_msleep(SLEEP_TIME_MS);
	}
}

Relevant prj.conf settings

CONFIG_DISK_ACCESS=y
CONFIG_FILE_SYSTEM=y
CONFIG_FAT_FILESYSTEM_ELM=y
CONFIG_DISK_DRIVER_SDMMC=y
CONFIG_FS_FATFS_LFN=y
CONFIG_MAIN_STACK_SIZE=4096
CONFIG_SPI=y