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$ cat fastboot.env
#=uEnv
bootdelay=0
bootcmd=fastboot usb 0
$ sudo sunxi-fel uboot u-boot-sunxi-with-spl.bin write 0x44000000 fastboot.env
$ fastboot version
version: 0.4
finished. total time: 0.001s

/* private helper function, gets used for "write*" and "multi*" transfers */
static unsigned int file_upload(feldev_handle *dev, size_t count,
				size_t argc, char **argv, progress_cb_t callback)
{
	if (argc < count * 2)
		pr_fatal("error: too few arguments for uploading %zu files\n",
			 count);

	/* get all file sizes, keeping track of total bytes */
	size_t size = 0;
	unsigned int i;
	for (i = 0; i < count; i++)
		size += file_size(argv[i * 2 + 1]);

	progress_start(callback, size); /* set total size and progress callback */

	/* now transfer each file in turn */
	for (i = 0; i < count; i++) {
		void *buf = load_file(argv[i * 2 + 1], &size);
		if (size > 0) {
			uint32_t offset = strtoul(argv[i * 2], NULL, 0);
			aw_write_buffer(dev, buf, offset, size, callback != NULL);

			/* If we transferred a script, try to inform U-Boot about its address. */
			if (get_image_type(buf, size) == IH_TYPE_SCRIPT)
				pass_fel_information(dev, offset, 0);
			if (is_uEnv(buf, size)) /* uEnv-style data */
				pass_fel_information(dev, offset, size);
		}
		free(buf);
	}

	return i; /* return number of files that were processed */
}

/*
 * Check the SPL header for the "sunxi" variant. If found: parse values
 * that might have been passed by the loader ("fel" utility), and update
 * the environment accordingly.
 */
static void parse_spl_header(const uint32_t spl_addr)
{
	struct boot_file_head *spl = get_spl_header(SPL_ENV_HEADER_VERSION);

	if (spl == INVALID_SPL_HEADER)
		return;

	if (!spl->fel_script_address)
		return;

	if (spl->fel_uEnv_length != 0) {
		/*
		 * data is expected in uEnv.txt compatible format, so "env
		 * import -t" the string(s) at fel_script_address right away.
		 */
		himport_r(&env_htab, (char *)(uintptr_t)spl->fel_script_address,
			  spl->fel_uEnv_length, '\n', H_NOCLEAR, 0, 0, NULL);
		return;
	}
	/* otherwise assume .scr format (mkimage-type script) */
	env_set_hex("fel_scriptaddr", spl->fel_script_address);
}