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 RT1050, so a three-component design (Cortex-M + external non-volatile memory + external RAM) is the only possibility. However, thanks to the i.MX RT1050 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 RT1050 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, here are some footprint numbers for a sample Linux configuration that Emcraft includes in the software distribution. The project is called hello (for "Hello, world") and can be found in the projects/hello directory, relative to the top of the distribution directory.
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.
file /bin/hello ${INSTALL_ROOT}/projects/${SAMPLE}/hello/hello 755 0 0
slink /init /bin/hello 777 0 0
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:
#include <stdio.h>
#include <unistd.h>
#include "hello.h"
#include <sys/mount.h>
int main(int argc, char **argv)
{
char buff[4096];
int rc;
FILE *fp;
printf("Mounting /proc..\n");
if (mount("proc", "/proc", "proc", MS_MGC_VAL, NULL)) {
perror("Unable to mount /proc");
goto xit;
}
if (!(fp = fopen("/proc/meminfo", "r"))) {
perror("fopen");
goto xit;
}
printf("Reading /proc/meminfo:\n");
while (fgets(buff, sizeof(buff), fp)) {
fputs(buff, stdout);
}
printf("Done\n");
while(1) {
printf(HELLO_STRING);
sleep(3);
}
xit:
return -1;
}
The bootable Linux image ready for installation to non-volatile storage (SD Card) of the i.MX RT1050 (hello.uImage) is ~4.7 MBytes in size. This is a multi-file Image: ~4 MBytes is the kernel Image itself, and ~7.4 KBytes is the Device Tree Blob file.
Here is a snapshot of the boot session for that configuration, as run on the i.MX RT1050 EVK. The kernel runs from SD card and is quite fast (note the 1196 BogoMIPS metrics in the kernel output). Boot time to the application (from the point where U-Boot passes control to the kernel) is less than a second. Notice the 23660KBytes of unused memory:
=> run mmcboot
4836822 bytes read in 860 ms (5.4 MiB/s)
## Booting kernel from Legacy Image at 80007fc0 ...
Image Name: Linux-5.15.71
Image Type: ARM Linux Multi-File Image (uncompressed)
Data Size: 4836758 Bytes = 4.6 MiB
Load Address: 80008000
Entry Point: 80008001
Contents:
Image 0: 4829216 Bytes = 4.6 MiB
Image 1: 7530 Bytes = 7.4 KiB
Verifying Checksum ... OK
## Flattened Device Tree from multi component Image at 80007FC0
Booting using the fdt at 0x804a302c
Loading Multi-File Image
Loading Device Tree to 81e81000, end 81e85d69 ... OK
Starting kernel ...
Booting Linux on physical CPU 0x0
Linux version 5.15.71 (sasha@workbench.emcraft.com) (arm-none-eabi-gcc
(GNU Arm Embedded Toolchain 10.3-2021.10) 10.3.1 20210824 (release),
GNU ld (GNU Arm Embedded Toolchain 10.3-2021.10) 2.36.1.20210621) #6
Tue Sep 26 06:53:24 UTC 2023
CPU: ARMv7-M [411fc271] revision 1 (ARMv7M), cr=00000000
CPU: PIPT / VIPT nonaliasing data cache, PIPT instruction cache
OF: fdt: Machine model: NXP IMXRT1050-evk board
Reserved memory: created DMA memory pool at 0x81f00000, size 1 MiB
OF: reserved mem: initialized node dmapool@81f00000, compatible id
shared-dma-pool
Zone ranges:
Normal [mem 0x0000000080000000-0x0000000081ffffff]
Movable zone start for each node
Early memory node ranges
node 0: [mem 0x0000000080000000-0x0000000081efffff]
node 0: [mem 0x0000000081f00000-0x0000000081ffffff]
Initmem setup node 0 [mem 0x0000000080000000-0x0000000081ffffff]
Built 1 zonelists, mobility grouping off. Total pages: 8128
Kernel command line: ip=192.168.1.86:192.168.1.96:192.168.1.254:255.255.0.0::
eth0:off
Unknown kernel command line parameters "ip=192.168.1.86:192.168.1.96:192.168.1.
254:255.255.0.0::eth0:off", will be passed to user space.
Dentry cache hash table entries: 4096 (order: 2, 16384 bytes, linear)
Inode-cache hash table entries: 2048 (order: 1, 8192 bytes, linear)
mem auto-init: stack:off, heap alloc:off, heap free:off
Memory: 26584K/32768K available (3282K kernel code, 180K rwdata, 868K rodata,
184K init, 97K bss, 6184K reserved, 0K cma-reserved)
NR_IRQS: 16, nr_irqs: 16, preallocated irqs: 16
Switching to timer-based delay loop, resolution 333ns
sched_clock: 32 bits at 3000kHz, resolution 333ns, wraps every 715827882841ns
clocksource: mxc_timer1: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns:
637086815595 ns
Calibrating delay loop (skipped), value calculated using timer frequency.. 6.00
BogoMIPS (lpj=30000)
pid_max: default: 4096 minimum: 301
Mount-cache hash table entries: 1024 (order: 0, 4096 bytes, linear)
Mountpoint-cache hash table entries: 1024 (order: 0, 4096 bytes, linear)
devtmpfs: initialized
DMA: default coherent area is set
clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns:
19112604462750000 ns
futex hash table entries: 16 (order: -5, 192 bytes, linear)
pinctrl core: initialized pinctrl subsystem
NET: Registered PF_NETLINK/PF_ROUTE protocol family
imxrt1050-pinctrl 401f8000.pinctrl: initialized IMX pinctrl driver
imxrt1050-pinctrl 400a8000.pinctrl-snvs: initialized IMX pinctrl driver
SCSI subsystem initialized
usbcore: registered new interface driver usbfs
usbcore: registered new interface driver hub
usbcore: registered new device driver usb
pps_core: LinuxPPS API ver. 1 registered
pps_core: Software ver. 5.3.6 - Copyright 2005-2007 Rodolfo Giometti
<
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>
PTP clock support registered
Bluetooth: Core ver 2.22
NET: Registered PF_BLUETOOTH protocol family
Bluetooth: HCI device and connection manager initialized
Bluetooth: HCI socket layer initialized
Bluetooth: L2CAP socket layer initialized
Bluetooth: SCO socket layer initialized
clocksource: Switched to clocksource mxc_timer1
NET: Registered PF_INET protocol family
IP idents hash table entries: 2048 (order: 2, 16384 bytes, linear)
tcp_listen_portaddr_hash hash table entries: 512 (order: 0, 4096 bytes, linear)
Table-perturb hash table entries: 65536 (order: 6, 262144 bytes, linear)
TCP established hash table entries: 1024 (order: 0, 4096 bytes, linear)
TCP bind hash table entries: 1024 (order: 0, 4096 bytes, linear)
TCP: Hash tables configured (established 1024 bind 1024)
UDP hash table entries: 256 (order: 0, 4096 bytes, linear)
UDP-Lite hash table entries: 256 (order: 0, 4096 bytes, linear)
NET: Registered PF_UNIX/PF_LOCAL protocol family
RPC: Registered named UNIX socket transport module.
RPC: Registered udp transport module.
RPC: Registered tcp transport module.
RPC: Registered tcp NFSv4.1 backchannel transport module.
Bus freq driver module loaded
Initialise system trusted keyrings
workingset: timestamp_bits=30 max_order=13 bucket_order=0
fuse: init (API version 7.34)
Key type asymmetric registered
Asymmetric key parser 'x509' registered
Block layer SCSI generic (bsg) driver version 0.4 loaded (major 251)
io scheduler mq-deadline registered
io scheduler kyber registered
40184000.serial: ttyLP0 at MMIO 0x40184010 (irq = 17, base_baud = 1250000)
is a FSL_LPUART
fsl-lpuart 40184000.serial: Serial: Console lpuart rounded baud ratefrom 208333
to 115200
printk: console [ttyLP0] enabled
PPP generic driver version 2.4.2
PPP BSD Compression module registered
PPP Deflate Compression module registered
usbcore: registered new interface driver rt2800usb
ehci_hcd: USB 2.0 'Enhanced' Host Controller (EHCI) Driver
usbcore: registered new interface driver cdc_acm
cdc_acm: USB Abstract Control Model driver for USB modems and ISDN adapters
usbcore: registered new interface driver uas
usbcore: registered new interface driver usb-storage
i2c_dev: i2c /dev entries driver
usbcore: registered new interface driver btusb
sdhci: Secure Digital Host Controller Interface driver
sdhci: Copyright(c) Pierre Ossman
sdhci-pltfm: SDHCI platform and OF driver helper
usbcore: registered new interface driver usbhid
usbhid: USB HID core driver
NET: Registered PF_PACKET protocol family
mmc0 bounce up to 128 segments into one, max segment size 65536 bytes
Bluetooth: RFCOMM TTY layer initialized
Bluetooth: RFCOMM socket layer initialized
Bluetooth: RFCOMM ver 1.11
ARMv7-M VFP coprocessor found
VFP: Double precision floating points are supported
Loading compiled-in X.509 certificates
ci_hdrc ci_hdrc.1: EHCI Host Controller
ci_hdrc ci_hdrc.1: new USB bus registered, assigned bus number 1
mmc0: SDHCI controller on 402c0000.mmc [402c0000.mmc] using DMA
ci_hdrc ci_hdrc.1: USB 2.0 started, EHCI 1.00
hub 1-0:1.0: USB hub found
hub 1-0:1.0: 1 port detected
input: gpio-keys as /devices/platform/gpio-keys/input/input0
cfg80211: Loading compiled-in X.509 certificates for regulatory database
cfg80211: Loaded X.509 cert 'sforshee: 00b28ddf47aef9cea7'
Freeing unused kernel image (initmem) memory: 184K
This architecture does not have kernel memory protection.
Run /init as init process
platform regulatory.0: Direct firmware load for regulatory.db failed with error -2
cfg80211: failed to load regulatory.db
Mounting /proc..
Reading /proc/meminfo:
MemTotal: 26768 kB
MemFree: 23660 kB
MemAvailable: 22940 kB
Buffers: 0 kB
Cached: 184 kB
SwapCached: 0 kB
Active: 0 kB
Inactive: 0 kB
Active(anon): 0 kB
Inactive(anon): 0 kB
Active(file): 0 kB
Inactive(file): 0 kB
Unevictable: 180 kB
Mlocked: 0 kB
MmapCopy: 116 kB
SwapTotal: 0 kB
SwapFree: 0 kB
Dirty: 0 kB
Writeback: 0 kB
AnonPages: 0 kB
Mapped: 0 kB
Shmem: 0 kB
KReclaimable: 504 kB
Slab: 2220 kB
SReclaimable: 504 kB
SUnreclaim: 1716 kB
KernelStack: 240 kB
PageTables: 0 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 13384 kB
Committed_AS: 0 kB
VmallocTotal: 0 kB
VmallocUsed: 0 kB
VmallocChunk: 0 kB
Percpu: 64 kB
Done
Hello, IMXRT105X_NXPEVK-Linux!
mmc0: new high speed SDHC card at address 0007
mmcblk0: mmc0:0007 SD8GB 7.42 GiB
mmcblk0: p1
Hello, IMXRT105X_NXPEVK-Linux!
Hello, IMXRT105X_NXPEVK-Linux!
Hello, IMXRT105X_NXPEVK-Linux!
Hello, IMXRT105X_NXPEVK-Linux!
Hello, IMXRT105X_NXPEVK-Linux!
....
