Example SOM eMMC


The goal of this example is to demonstrate how to use the SOM eMMC storage. Note: not all SOMs include eMMC. Check the model number and datasheet to confirm that eMMC is available on your device.

This storage can be used for:

  • General purpose storage.
  • As a boot device

General Purpose Storage


  • The Device needs to be divided into one or more partitions.
  • To use as a Linux file system, a file system must be created in the partition.


Creating a partition

The eMMC device should appear as /dev/mmcblk0. In this example, we will create 1 large partition using the total available space on the eMMC and format as an ext3 journaled filesystem. From the command shell, run the following command to print the current partition table:

root@mitysom-am62x:~# fdisk -l /dev/mmcblk0
Disk /dev/mmcblk0: 59.29 GiB, 63652757504 bytes, 124321792 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: F097536D-8E87-46CF-B35A-5A176F2AE3DB

Device         Start       End   Sectors  Size Type
/dev/mmcblk0p1    34 124321758 124321725 59.3G Microsoft basic data

In this example, there is already a partition setup on the device. If there is not, you can create a partition by following the example below:

root@mitysom-am62x:~# fdisk /dev/mmcblk0
Device contains neither a valid DOS partition table, nor Sun, SGI, OSF or GPT disklabel
Building a new DOS disklabel. Changes will remain in memory only,
until you decide to write them. After that the previous content
won't be recoverable.

The number of cylinders for this disk is set to 1942528.
There is nothing wrong with that, but this is larger than 1024,
and could in certain setups cause problems with:
1) software that runs at boot time (e.g., old versions of LILO)
2) booting and partitioning software from other OSs
   (e.g., DOS FDISK, OS/2 FDISK)

Command (m for help): n
Partition type
   p   primary partition (1-4)
   e   extended
Partition number (1-4): 1
First sector (16-124321791, default 16): 1024
Last sector or +size{,K,M,G,T} (1024-124321791, default 124321791):.
Using default value 124321791

Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table

Formatting a partition

To format the partition with an ext3 filesystem (linux compatible with a journal), run the following command.

root@mitysom-am62x:~# mke2fs -j /dev/mmcblk0p1

Mounting / locating the filesystem

If you just created / formatted the partition, reboot the unit. The base filesystem will auto mount a detected mmcblk0 partition on boot up.

To locate the mount point of the filesystem, run the command shown below:

root@mitysom-am62x:~# lsblk
mtdblock0     31:0    0   512K  0 disk
mtdblock1     31:1    0     2M  0 disk
mtdblock2     31:2    0     4M  0 disk
mtdblock3     31:3    0   256K  0 disk
mtdblock4     31:4    0   256K  0 disk
mtdblock5     31:5    0 247.8M  0 disk
mtdblock6     31:6    0   256K  0 disk
mmcblk0      179:0    0  59.3G  0 disk
`-mmcblk0p1  179:1    0  59.3G  0 part /run/media/mmcblk0p1
mmcblk0boot0 179:32   0  31.5M  1 disk
mmcblk0boot1 179:64   0  31.5M  1 disk
mmcblk1      179:96   0  14.9G  0 disk
|-mmcblk1p1  179:97   0 132.6M  0 part /media/mmcblk1p1
`-mmcblk1p2  179:98   0   3.3G  0 part /

In the above listing, the mmcblk0p1 (first partition of mmcblk0, the eMMC device) is mounted on /run/media/mmcblk0p1.

Test Accessing the Device

To run a simple write and read test, run the following commands:

root@mitysom-am62x:~# dd if=/dev/urandom of=/run/media/mmcblk0p1/test.dat bs=1M count=100
100+0 records in
100+0 records out
104857600 bytes (105 MB, 100 MiB) copied, 1.54506 s, 67.9 MB/s
root@mitysom-am62x:~# dd if=/run/media/mmcblk0p1/test.dat of=/dev/null
204800+0 records in
204800+0 records out
104857600 bytes (105 MB, 100 MiB) copied, 0.517582 s, 203 MB/s
root@mitysom-am62x:~# rm /run/media/mmcblk0p1/test.dat

Boot Device



This example has demonstrated how to use the eMMC as a regular filesystem and how it could be used as a boot device.

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