File Download via TFTP in U-Boot

Summary

If your saved environment (and scripts) are unavailable after boot, you can embed a small U-Boot command in C that downloads a file via TFTP and writes it to MMC. Save the C file in cmd/, add it to the Makefile, rebuild U-Boot and you’ll have a getfile command available at the prompt.

What it does

• Downloads <filename> from TFTP into kernel_addr_r.
• Writes the downloaded data to the specified MMC device and part using fatwrite.
• Uses the existing kernel_addr_r environment variable (must be set).

How to add

1. Put the C file (example below) into u-boot/cmd/ (or the appropriate cmd directory).
2. Add the new source file to the cmd Makefile (obj-y += yourfile.o or similar).
3. Build U-Boot (make CROSS_COMPILE=...) and flash/test on the board.
4. Usage in U-Boot prompt: getfile

Example: getfile uImage mmc 0:1

Example C command (save as cmd_getfile.c in cmd/)

#include <common.h>
#include <command.h>
#include <env.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

static int do_getfile(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
{
    char cmd[256];
    const char *filename;
    const char *device;
    const char *part;
    const char *kernel_addr_r;

    if (argc != 4) {
        printf("Usage: getfile <filename> <device> <part>\n");
        return CMD_RET_USAGE;
    }

    filename = argv[1];
    device = argv[2];
    part = argv[3];

    kernel_addr_r = env_get("kernel_addr_r");
    if (!kernel_addr_r) {
        printf("Error: kernel_addr_r environment variable not set.\n");
        return CMD_RET_FAILURE;
    }

    snprintf(cmd, sizeof(cmd), "tftp %s %s && fatwrite %s %s %s %s ${filesize}",
             kernel_addr_r, filename, device, part, kernel_addr_r, filename);

    printf("Executing command: %s\n", cmd);

    if (run_command(cmd, 0) != 0) {
        printf("Error executing command.\n");
        return CMD_RET_FAILURE;
    }

    return CMD_RET_SUCCESS;
}

U_BOOT_CMD(
    getfile,
    4,
    0,
    do_getfile,
    "Download file via TFTP and write to MMC",
    "<filename> <device> <part>"
)

Notes

• Make sure the environment variable kernel_addr_r is defined before using the command, for example:
  setenv kernel_addr_r 0x82000000

• The command assumes you are writing to a FAT partition.
  If your target uses another filesystem (like ext4), replace fatwrite with the proper write command (e.g. ext4write).

• The combined TFTP and write operation is handled as a single chained command with &&,
  so the write step only runs if the TFTP download succeeds.

• You can change the behavior easily by modifying the snprintf() command format —
  for example, to store the file in memory only, remove the fatwrite part.

• Ensure that CONFIG_CMD_TFTP and CONFIG_CMD_FAT (or equivalent) are enabled in your U-Boot configuration.

• This approach helps when you don’t have access to environment variables or saved scripts after boot,
  as the functionality is built directly into the U-Boot binary.