SD, eMMC and USB

The commands for using SD cards, eMMC flash and USB mass storage devices (hard drives, flash drives, card readers, etc) are all very similar. The biggest difference is that on some hardware we may not be able to run U-Boot out of ROM from the storage device as it is unsupported. Once U-Boot is running however, any of these may be used for the kernel and the root filesystem.

Usually in all the platforms there will be two MMC instances of which one would be SD and the other would be eMMC. The index of them can vary from one class of platforms to the other. For a given platform, the device number (device num) can be found in the following way,

U-Boot# mmc list
sdhci@fa10000: 0 (eMMC)
sdhci@fa00000: 1 (SD) Partitioning eMMC from U-Boot

The eMMC device typically ships without any partition table. We make use of the GPT support in U-Boot to write a GPT partition table to eMMC. In this case we need to use the uuidgen program on the host to create the UUIDs used for the disk and each partition.

$ uuidgen
...first uuid...
$ uuidgen
...second uuid...
U-Boot # printenv partitions
U-Boot # setenv uuid_gpt_disk ...first uuid...
U-Boot # setenv uuid_gpt_rootfs ...second uuid...
U-Boot # gpt write mmc <device num> ${partitions} /* <device num> is device index obtained from mmc list for eMMC */

A reset is required for the partition table to be visible. Updating an SD card from a host PC

This section assume that you have created an SD card following the instructions on Processor SDK Linux Create SD Card Script or have made a compatible layout by hand. In this case, you will need to copy the all the boot images (MLO and u-boot.img for 32-bit platforms, tiboot3.bin, sysfw.itb, tispl.bin, u-boot.img for K3 based platforms, except AM64x and J7200 in which sysfw.itb is already combined with tiboot3.bin) files to the boot partition. At this point, the card is now bootable in the SD card slot. We default to using /boot/${bootfile} on the rootfs partition and the device tree file loaded from /boot with the same name as in the kernel. Updating an SD card or eMMC using DFU

To see the list of available places to write to (in DFU terms, altsettings) use the mmc part command to list the partitions on the MMC device and printenv dfu_alt_settings_mmc or dfu_alt_settings_emmc to see how they are mapped and exposed to dfu-util.

U-Boot# mmc part

Partition Map for MMC device 0  --   Partition Type: DOS

Partition     Start Sector     Num Sectors     Type
    1                   63          144522       c Boot
    2               160650         1847475      83
    3              2024190         1815345      83
U-Boot# printenv dfu_alt_info_mmc
dfu_alt_info=boot part 0 1;rootfs part 0 2;MLO fat 0 1;u-boot.img fat 0 1;uEnv.txt fat 0 1"


The above command mmc part lists the partitions in the current selected device. So, to list the partitions in eMMC or SD one needs to switch to the required device by using the command mmc dev <device num>.

This means that you can tell dfu-util to write anything to any of:

  • boot
  • rootfs
  • MLO
  • u-boot.img
  • uEnv.txt

And that the MLO, u-boot.img and uEnv.txt files are to be written to a FAT filesystem.

To start DFU on the target on the first MMC device:

U-Boot # setenv dfu_alt_info ${dfu_alt_info_mmc}
U-Boot # dfu 0 mmc 0

On boards like AM57x GP EVM or BeagleBoard x15, where the second USB instance is used as USB client, the dfu command becomes:

U-Boot # dfu 1 mmc 0

Then on the host PC to write MLO to an existing boot partition:

$ sudo dfu-util -D MLO -a MLO

On the host PC to overwrite the current boot partition contents with a new created on the host FAT filesystem image:

$ sudo dfu-util -D fat.img -a boot Booting Linux from SD card or eMMC

Within the default environment for each board that supports SD/MMC there is a boot command called mmcboot that will set the boot arguments correctly and start the kernel. In this case however, you must first run loaduimagefat or loaduimage to first load the kernel into memory. For the exact details of each use printenv on the mmcboot, loaduimagefat and loaduimage variables and then in turn printenv other sub-sections of the command. The most important variables here are mmcroot and mmcrootfstype. Booting tiboot3.bin, tispl.bin and u-boot.img from eMMC boot partition (For K3 class of SoCs)

The K3 based processors support booting from the eMMC boot partition. The following commands can be used to download tiboot3.bin, tispl.bin and u-boot.img from an SD card and write them to the eMMC boot0 partition at respective addresses.

=> mmc dev 0 1
=> fatload mmc 1 ${loadaddr} tiboot3.bin
=> mmc write ${loadaddr} 0x0 0x400
=> fatload mmc 1 ${loadaddr} tispl.bin
=> mmc write ${loadaddr} 0x400 0x1000
=> fatload mmc 1 ${loadaddr} u-boot.img
=> mmc write ${loadaddr} 0x1400 0x2000

For loading images from a FAT partition on a different media, replace mmc with the required media. For example, to load images from a FAT partition on a USB Storage device connected to the zeroth instance of usb,

=> fatload usb 0 ${loadaddr} <file name>


There are two instances of USB on AM62 SoC. On the SK board, zeroth instance is brought out through a Type C port and the first instance is brought through a Type A port. By default, USB0 port is configured to be in peripheral mode. Since U-Boot does not support dynamic switching of USB roles, below DT fragment needs to be applied and U-Boot image needs to be rebuilt to make USB0 port to be USB 2.0 host port.

diff --git a/arch/arm/dts/k3-am625-sk-u-boot.dtsi b/arch/arm/dts/k3-am625-sk-u-boot.dtsi
index 20c24d2fa7a4..2b662653023f 100644
--- a/arch/arm/dts/k3-am625-sk-u-boot.dtsi
+++ b/arch/arm/dts/k3-am625-sk-u-boot.dtsi
@@ -117,5 +117,5 @@

 &usb0 {
-       dr_mode = "peripheral";
+       dr_mode = "host";

To give the ROM access to the boot partition, the following commands must be used for the first time:

=> mmc partconf 0 1 1 1
=> mmc bootbus 0 2 0 0
  • eMMC layout:
            boot0 partition (8 MB)                        user partition
   0x0+----------------------------------+      0x0+-------------------------+
      |     tiboot3.bin (1 MB)           |         |                         |
 0x400+----------------------------------+         |                         |
      |       tispl.bin (2 MB)           |         |                         |
0x1400+----------------------------------+         |        rootfs           |
      |       u-boot.img (4 MB)          |         |                         |
0x3400+----------------------------------+         |                         |
      |      environment (128 KB)        |         |                         |
0x3500+----------------------------------+         |                         |
      |   backup environment (128 KB)    |         |                         |
0x3600+----------------------------------+         +-------------------------+


rootfs is written to the user partition. The user partition is first required to be formatted as a ext4 file system and then the rootfs has to be written. It is not possible to format a partition to ext4 in U-Boot. It is required to boot to kernel and write rootfs to user partition after formatting it to ext4. Kernel image and DT are expected to be present in the /boot folder of rootfs.

To boot kernel from eMMC, use the following commands after writing rootfs to user partition:

=> setenv mmcdev 0
=> setenv bootpart 0
=> boot Booting to U-Boot prompt from USB storage


While using usb reset or usb start command in U-Boot, or booting from a USB Mass storage device, some of the USB devices fail to get detected. This issue is seen because these USB devices are failing to follow the spec for power good delay. It can be resolved by overriding the power good delay using the environment variable usb_pgood_delay. Setting it to 2000 should be good enough for all cases. Booting Linux from USB storage

To load the Linux kernel, Device Tree and the Root file system from USB Mass storage device, the following changes are required to be done,

  • U-Boot

    1. In U-Boot the USB controller can be used in either host or peripheral mode. For booting to linux kernel from USB storage device, the USB port is to be set as host.
    2. By default, on AM625-SK board the zero instance of USB connected to the Type C port, is set to peripheral mode and the first instance of USB connected to the Type A port is set to host mode.
    3. Therefore, USB controller needs to be set host mode and custom bootloader images are required to be built, if zeroth instance is used. Please refer to note in section Booting tiboot3.bin, tispl.bin and u-boot.img from eMMC boot partition (For K3 class of SoCs)
  • Kernel

    1. In kernel, by default the USB subsystem is built as modules. For booting from USB mass storage device, USB subsytem is required to be built into the image. This can be done by making the following changes in the configuration used for building kernel,
  • Copying the images to USB storage device

    1. After making the required changes mentioned above, build the kernel, device tree file and modules

    2. The USB Mass storage device should have the rootfs as the second partition with ext4 file system,

      • The following images should be in /boot/ directory
        • Kernel image
        • device tree file
  • During the boot, cancel the autoboot at U-Boot and run the following command on U-Boot prompt

    => run usbboot