View source for How to support EXT4 through MMC

==Purpose==
The purpose of this article is to introduce EXT4 filesystem:
* General information
* Main components
* How to use EXT4

==Overview==
EXT4 (fourth extended file system)<ref>[https://ext4.wiki.kernel.org/index.php/Main_Page Official ext4 wiki]</ref><ref>{{DocSource | domain=Linux kernel | path=filesystems/ext4/ | text=Kernel.org Documentation}}</ref> is an advanced level of the EXT3 filesystem which incorporates scalability and reliability enhancements for supporting large filesystems (64 bit) in keeping with increasing disk capacities and state-of-the-art feature requirements.

*EXT4 is backward-compatible with EXT3 and EXT2. It is possible to mount both EXT3 and EXT2 filesystems directly using the EXT4 filesystem driver.
*EXT4 can support volumes with sizes up to 1 exbibyte (EiB) and files with sizes up to 16 tebibytes (TiB).

This file system may be used on emmc/sd-card (please refer to the [[MMC overview|MMC framework]]). It does not work for raw Flash memory like NOR/NAND.

==Kernel configuration==
EXT4 support is activated by default in ST deliveries. Nevertheless, if a specific configuration is needed, this section indicates how EXT4 can be activated/deactivated in the kernel.

Activate EXT4 in the kernel configuration with the Linux Menuconfig tool: [[Menuconfig or how to configure kernel]].

<pre>
File systems  --->
    <*> The Extended 4 (ext4) filesystem
    [*]   Use ext4 for ext2 file systems 
</pre>

==Using a EXT4 partition as root file system==
Assuming a rootfs EXT4 image is already flashed to the memory device, the user has to provide: 
*The partition that has to be mounted, using root=<partition_device_path> or root=PARTUUID=XXXX where X represents the unique id of a partition.
*The file system type (rootfstype=ext4 in that case). Optional, by default the kernel find the file system type of partition.


Please refer to the [[STM32MP15_Flash_mapping#SD_card_memory_mapping|SD card memory mapping]] to check the "rootfs" location in ST deliveries.
<br>In this case, the kernel command-line parameters <ref>{{CodeSource | Linux kernel | Documentation/admin-guide/kernel-parameters.rst | The kernel's command-line parameters}} </ref> that have to be added are:

*In case PARTUUID is used.
  root=PARTUUID=45e5fc02-d536-43a4-a941-94a8329afeaf
*In case the partition device path is used.
  root=/dev/mmcblk0p6

==Mounting an EXT4 partition==
Assuming that the "userfs" partition has been flashed on partition 7, the below steps show how to mount this partition.
<br>Please refer to the [[STM32MP15_Flash_mapping#SD_card_memory_mapping|SD card memory mapping]] to check the "userfs" location in ST deliveries.

* Mount "userfs".

  {{Board$}} mount /dev/mmcblk0p7 /media/

* Check that "userfs" partition is mounted.

  {{Board$}} mount | grep "/media"
  /dev/mmcblk0p7 on /media type ext4 (rw,sync,relatime)

==Create a default EXT4 filesystem on a MMC partition==

* Format a MMC partition (mmcblk0p7 will be used in this example).
  {{Board$}} mke2fs -t ext4 -L "testfs" /dev/mmcblk0p7
  mke2fs 1.43.5 (04-Aug-2017)
  /dev/mmcblk0p7 contains a ext4 file system
          created on Tue Aug  7 08:28:50 2018
  Proceed anyway? (y,N) y
  Creating filesystem with 163595 4k blocks and 40960 inodes
  Filesystem UUID: b7c6e8f5-373c-4c91-aace-0c8f69649165
  Superblock backups stored on blocks:
          32768, 98304
  
  Allocating group tables: done
  Writing inode tables: done
  Creating journal (4096 blocks): done
  Writing superblocks and filesystem accounting information: done

* Mount "testfs" with device partition path or with label.
  {{Board$}} mount /dev/mmcblk0p7 /media

  {{Board$}} mount /dev/disk/by-label/testfs /media

* Check that the file system is empty.
  {{Board$}} ls -la /media
  total 21
  drwxr-xr-x 3 root root  4096 Aug  7 08:34 .
  drwxr-xr-x 3 root root  1024 Aug  7 08:38 ..
  drwx------ 2 root root 16384 Aug  7 08:34 lost+found

* Create a random data file.
  {{Board$}} dd if=/dev/urandom of=/tmp/random.hex bs=1M count=100 conv=fsync
  100+0 records in
  100+0 records out
  104857600 bytes (105 MB, 100 MiB) copied, 6.49739 s, 16.1 MB/s

* Copy the random data file in /media.
  {{Board$}} cp /tmp/random.hex /media/

* Un-mount /media.
  {{Board$}} umount /media

* Mount "testfs".
  {{Board$}} mount /dev/disk/by-label/testfs /media

* Check that the random data file created is identical in /tmp and /media.
  {{Board$}} md5sum /tmp/random.hex /media/random.hex
  6ab2f920c81bba53b01f9e758116a172  /tmp/random.hex
  6ab2f920c81bba53b01f9e758116a172  /media/random.hex

* Un-mount /media.
  {{Board$}} umount /media

==References==
Please refer to the following links for full description:
<references />

<noinclude>
[[Category:How to run use cases]]
[[Category:Mass storage]]
</noinclude>