NXP i.MXRT10XX: What is the Minimal Footprint of uClinux?

This is perhaps one of the most frequently asked questions about uClinux on Cortex-M.

First thing to say is that external RAM is a must for uClinux. However small a Linux configuration, it still requires at least several MBytes of RAM to run from. All Cortex-M devices Emcraft is aware of limit they internal SRAM to hundreds KBytes at best. There is no way uClinux can be run from a single-chip Cortex-M design, as of this writing.

A two-component design (Cortex-M + external RAM) is a possibility for chips that have on-chip non-volatile (Flash) memory, which can be used as a storage for the bootloader and uClinux images. There is no on-chip Flash in i.MX RT10XX, so a three-component design (Cortex-M + external non-volatile memory + external RAM) is the only possibility. However, thanks to the i.MX RT10XX internal bootloader (located in a small on-chip ROM) you may select from different types of boot devices. You may boot from: SD/MMC card, serial NOR/NAND Flash, etc.

Regarding the minimal size, there are two separate "footprint" metrics that need to be considered:

Size of the bootable Linux image. In the embedded world it is often comfortable to have the root file system to be built-into the bootable image as initramfs (instead of mounting the root file system from some other partitions or other device). More specifically, the bootable image is comprised of two major pieces:

• Linux kernel itself;

• cpio representation of the root file system, which gets expanded into a RAM-based initramfs and mounted as rootfs when Linux is booting up.

The size of such a bootable image starts at maybe 0.8 MBytes for truly minimal configurations and ranges to whatever, depending on what you have put into your root file system and, to a lesser extent, what configuration options you have enabled in your kernel. For instance, enabling the TCP/IP stack in the kernel increases the image size somewhat substantially.

The size of a practical bootable image, with Ethernet, TCP/IP and a reasonable set of user-space tools and applications configured, would be in a 3-5 MBytes ballpark.

Size of external RAM required for run-time Linux operation. The answer we give to our customers when asked how much RAM is needed is the more the better, but no less than 16 MBytes. Admittedly, it may be possible to run some very basic configurations with rootfs mounted from NFS or some external device even out of 2 MBytes but frankly this is more of a joke than a configuration one can build a practical uClinux design on.

As the rule of thumb, consider at least 32 MB RAM if your intention is to use uClinux in a serious product. When you approach deployment and it becomes clear that you can fit into less RAM, you might want to downsize to a compatible 16 MB RAM device, however the safe advice is to start with more RAM rather than less. Requirements for embedded applications grow at an incredibly fast rate and it is a sure bet that in a 1-year's time you will be wanting to add new software features to your design. Seeing that you are already considering uClinux for your microcontroller application, you are probably fed up with the "if only I had another 512 MB of RAM" kind of situation MCU developers are used to. If you want Linux and "features", plan for reasonable amounts of RAM.

On a practical side of things, and given the specific context of SDRAM memory used with the advanced i.MX RT10XX microcontrollers, the BOM differences between compatible 16 MB and 32 MB devices are often times quite tolerable. Again, the advice we can give is to play safely and plan for more RAM rather than less.

With that background in mind, below is some footprint numbers for a sample Linux configuration. This project is called hello (for "Hello, world").

The kernel configuration of this project is minimalistic, for instance, the TCP/IP stack is disabled. At the application level, this is really a single-process configuration. Specifically, instead of the standard Linux init, the project runs a custom application called hello. The following are relevant snippets from the hello.intramfs specification file that illustrate how the custom application is installed on the target instead of init.

The application itself is a simple endless-loop Hello, world C program, except that before entering the loop it prints the content of /proc/meminfo to stdout (which is the Linux serial console in this specific configuration). The meminfo printout gives information on how much memory is available when the application is running.

The application source can be found in projects/hello/hello/hello.c. For the sake of completeness, here is the full source of that program:

The bootable Linux image ready for installation to non-volatile storage (SD Card) of the i.MX RT10XX (hello.uImage) is ~3.3 MBytes in size. This is a multi-file Image: ~3.2 MBytes is the kernel Image itself, and ~11.5 KBytes is the Device Tree Blob file.

Here is a snapshot of the boot session for that configuration, as run on the i.MX RT10XX EVKB. The kernel runs from the network. Boot time to the application (from the point where U-Boot passes control to the kernel) is less than a second. Notice the 23332 KBytes of unused memory: