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UM0844

Working at application level (userland)

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Working at application level (userland)

4.1

Workflow models

When working at application level (the so-called "userland") developers are only concerned

with programs and libraries stored in a root filesystem. Instead, boot loaders and the kernel

are assumed to be stable and stored in Flash memory.

There are many approaches (workflows) to modify/extend the root file system for specific

application scenarios; the main ones are described in the following subsections.

4.1.1

Remote mounting of the root filesystem (NFS)

If the board can be connected to the development PC through a Ethernet LAN, the most

practical solution is to leave the root filesystem stored on the PC and remotely mount it on

the target embedded Linux OS through the NFS protocol.

The advantages of this approach are the following:

●

The root filesystem has no global size constraints. Developers can keep hundreds or

thousands of packages (programs and libraries) in a directory of their PC disk. All file

access from the Linux OS running on the board is performed over the network in a

transparent way. Files are not copied to Flash memory, but loaded to DDR RAM strictly

on demand.

●

A program or library can be simply built (compiled and linked) with the output file on the

PC disk; the new version is then available for execution on the board without any need

for manual transfer or board reboot

The drawbacks are:

●

File access by NFS over LAN can be slower than direct Flash memory access

●

There is no early assessment of which files are actually used and of the overall

required size for future migration to Flash memory

To remotely mount the root filesystem, you have to configure and start the NFS server on

Linux PC.

Assuming the NFS server functionality is already provided by your host, the only

configuration is an entry for your target root directory to your /etc/exports file, for example:

/opt/STM/STLinux-2.3/devkit/arm/target 192.168.0.0/24 (rw,no_root_squash,sync)

This line exports the /opt/STM/STLinux-2.3/devkit/arm/target directory with read and writes

permissions to all hosts on the 192.168.0.0 subnet.

To check NFS availability and start the services, use the following commands (from user

root account):

# rpm -q nfs-utils

After modifying the /etc/exports file, you must make sure the NFS system is notified about

the change, for example by running the command:

# service portmap start

Starting portmap: [ OK ]

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