Dynamic Searching for MMC in U-Boot

Summary

A common issue is that after hardware changes the MMC device number can change (e.g. mmc0 → mmc1), so U-Boot scripts or operations that rely on a fixed MMC number fail. Instead of changing the device tree or rebuilding for every change, modify disk/part.c so U-Boot tries multiple MMC numbers dynamically.

Problem

When MMC numbering changes, operations like env import, sysboot, file read/write, or any MMC-based actions may fail because the code expects a fixed device number.

Simple solution

Edit disk/part.c (example: around line 536, v2023.10) to attempt one MMC number first and fall back to another if it fails. The example below tries mmc 1 first, then mmc 0. Add more attempts ("2", "3", …) as needed for your board.

/* Look up the device */
/* Try mmc 1 first, then fallback to mmc 0 */
dev = blk_get_device_by_str(ifname, "1", dev_desc);
printf("\n*** Trying to read mmc 1 ***");
if (dev < 0) {
    printf(" -> Failed. Trying mmc 0 ...");
    dev = blk_get_device_by_str(ifname, "0", dev_desc);
    if (dev < 0) {
        printf(" ** Bad device specification %s %s **\n", ifname, dev_str);
        ret = dev;
        goto cleanup;
    }
}

Practical steps

1. Edit disk/part.c and add the fallback logic shown above.
2. Keep printf messages concise for visibility in serial output.
3. Rebuild U-Boot using your cross-toolchain:
   make CROSS_COMPILE=arm-linux-gnueabihf-
4. Flash the new U-Boot or test with QEMU/hardware.
5. Observe serial logs to ensure it dynamically detects the correct MMC.
6. If necessary, extend the fallback to include additional MMC numbers.

Result

This simple modification allows U-Boot to dynamically find the correct MMC device,
eliminating the need to repeatedly edit the device tree or rebuild for each configuration change.