U-Boot UMS

USB Mass Storage (UMS) mode is a feature that allows a device to function like an external USB storage device when connected to a computer.

In UMS mode, the device’s internal storage, such as eMMC (embedded MultiMediaCard), is made accessible to the computer, similar to how you would connect a USB drive or SD card to your computer.

This mode is useful for tasks like transferring files, flashing firmware, or writing disk images to the device’s storage.

Now, here are the steps for booting into UMS mode from either internal eMMC storage or SD card:

Booting into UMS Mode from Internal eMMC Storage:

  • Connect the device to a PC using a USB cable.

  • Connect the Serial to UART converter (FTDI) to your board and to the Host computer.

  • Open the serial monitor application on your Host computer.


More more information on Serial monitor application check out

Serial Console Programs

  • Power on the device.

  • Quickly press Ctrl + c to enter the U-Boot console.

  • From U-Boot console type ums 1 mmc <dev number>, where is the device number for MMC card.

Click here to check ums help command
=> help ums (U-Boot cmdline)
ums - Use the UMS [USB Mass Storage]

ums <USB_controller> [<devtype>] <dev[:part]> e.g. ums 0 mmc 0
devtype defaults to mmc


MMC device number is pre-configured from kernel device tree.

  • mmc0 is the device number for SD-Card.

  • mmc1 is the device number for eMMC.

  • The device will automatically boot into UMS mode, allowing you to share its internal storage like a USB drive.

Click here to check success logs from UMS
=> ums 1 mmc 1
UMS: LUN 0, dev 1, hwpart 0, sector 0x0, count 0x3a3e000
  • Check kernel logs on your Host computer, and you should be able to see a new Storage device added.

Click here to see successful logs from host kernel
[20104.826747] usb 7-1.4.3: new high-speed USB device number 41 using xhci_hcd
[20104.909195] usb 7-1.4.3: New USB device found, idVendor=2207, idProduct=0010, bcdDevice= 2.23
[20104.909211] usb 7-1.4.3: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[20104.909214] usb 7-1.4.3: Product: USB download gadget
[20104.909216] usb 7-1.4.3: Manufacturer: Rockchip
[20104.909221] usb 7-1.4.3: SerialNumber: ac4e87197c0c095
[20104.914797] usb-storage 7-1.4.3:1.0: USB Mass Storage device detected
[20104.914916] scsi host7: usb-storage 7-1.4.3:1.0
[20105.940727] scsi 7:0:0:0: Direct-Access     Linux    UMS disk 0       ffff PQ: 0 ANSI: 2
[20105.941285] sd 7:0:0:0: Attached scsi generic sg2 type 0
[20105.941594] sd 7:0:0:0: [sdc] 61071360 512-byte logical blocks: (31.3 GB/29.1 GiB)
[20105.941748] sd 7:0:0:0: [sdc] Write Protect is off
[20105.941752] sd 7:0:0:0: [sdc] Mode Sense: 0f 00 00 00
[20105.941880] sd 7:0:0:0: [sdc] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[20105.947385]  sdc: sdc1 sdc2 sdc3 sdc4 sdc5 sdc6 sdc7 sdc8
[20105.947726] sd 7:0:0:0: [sdc] Attached SCSI removable disk


With UMS mode, you can easily access and manage the device’s storage from your computer, making it convenient for tasks that involve transferring or writing data to the device.

After successfully entering UMS mode, you should be able to access your eMMC from your host computer. You are now ready to flash or copy different things to your Vicharak board’s eMMC using your PC.

Check using lsblk command to find the eMMC storage; it should appear as sdX where X is an alphabetic letter (e.g., sdc).

Check using parted /dev/sdc to list the various partitions available on the eMMC.


You can directly mount the eMMC boot and root partition on your host computer.

  1. Find the partition that you want to mount. (assuming /dev/sdc4 is the boot partition).

  2. Use the linux mount command to access your partition.

    mkdir -p mountpoint
    mount /dev/sdb1 mountpoint
  3. You can change the kernel images directly just by copying them to mountpoint folder.


Do not insert SD-Card when after you previously entered UMS mode from eMMC. U-Boot might crash in such case.

Always reset the board before switching the storage for UMS mode.