X-LINUX-RT is an STM32 MPU OpenSTLinux Expansion Package that targets real-time (RT) feature in the Linux kernel.
A RT system is a system which responds to a request in a time laps and not as fast as possible:
- determinism
- LATENCY but not SPEED
To have a better understanding about RT Linux, you can refer to the Linux RT foundation website[1].
X-LINUX-RT expansion package is mainly designed to be used with Yocto and OpenSTLinux Distribution Package. However following chapters will show how to use it either OpenSTLinux Distribution Package or the Developer Package.
1. X-LINUX-RT[edit source]
1.1. Main software modification[edit source]
Moving on a Linux-RT impacts kernel configuration:
- Power management is not "compliant" with a real time context as it changes system behavior. Because there is no more frequency scaling, to avoid to reduce the lifetime of these chips, the frequency is fixed to their industrial profile.
- 900MHZ for STM32MP13x lines
- 650MHz for STM32MP15x lines
- 900MHZ for STM32MP13x lines
- High Resolution timers and periodic ticks should be used in order to have a precise tick.
2. Distribution package: re-generate X-LINUX-RT OpenSTLinux distribution[edit source]
With the following procedure, the complete distribution can be re-generated by enabling the X-LINUX-RT expansion package.
2.1. Download the STM32MP1 Distribution Package[edit source]
For ecosystem release v4.1.0 :
Install the STM32MP1 Distribution Package v4.1.0.
2.2. Install X-LINUX-RT environment[edit source]
- Clone the meta-st-x-linux-rt git repositories
cd <Distribution Package installation directory>/layers/meta-st git clone https://github.com/STMicroelectronics/meta-st-x-linux-rt.git cd ../..
- Source the build environment with the correct board and layer.
For STM32MP13x lines :
DISTRO=openstlinux-weston MACHINE=stm32mp13-rt source layers/meta-st/scripts/envsetup.sh
For STM32MP15x lines :
DISTRO=openstlinux-weston MACHINE=stm32mp15-rt source layers/meta-st/scripts/envsetup.sh
2.3. Build the image[edit source]
bitbake st-image-weston
2.4. Program the built image into the Flash memory[edit source]
Follow this link to see how to program the built image.
3. Developer package: build RT BSP with the SDK[edit source]
Linux RT can be built with sources provided in the OpenSTLinux BSP packages in addition with patches and device tree files provided in X-LINUX-RT meta layer.
3.1. Install the OpenSTLinux SDK[edit source]
The OpenSTLinux SDK contains all the basis needed for the X-LINUX-RT Expansion package, so it needs to be downloaded and installed: see STM32MP1_Developer_Package#Installing_the_SDK chapter. Once done, a directory is present on your machine containing the OpenSTLinux SDK.
3.2. Download the Developer package[edit source]
The STM32MP1 Developer Package, containing all the BSP source code, must be downloaded and extracted. To do this, follow only the STM32MP1_Developer_Package#Installing_the_OpenSTLinux_BSP_packages chapter (and not the sub-chapters).
Two BSP components (OP-TEE OS and Linux kernel) must be modified for RT feature. The procedure to modify them is explained in next chapters.
3.3. Download X-LINUX-RT meta layer[edit source]
- Clone the meta-st-x-linux-rt git repository
cd <Developer Package installation directory>/stm32mp1-openstlinux-6.1-yocto-mickledore-mp1-v23.06.21/sources/arm-ostl-linux-gnueabi/ git clone https://github.com/STMicroelectronics/meta-st-x-linux-rt.git
3.4. Modify the BSP[edit source]
As written above, to enable the RT feature, two BSP components must be modified.
3.4.1. Modify the Linux kernel[edit source]
Follow now the sub-chapter STM32MP1_Developer_Package#Building_and_deploying_the_Linux_kernel_for_the_first_time to prepare the Linux kernel.
Patches found in <Developer Package installation directory>/stm32mp1-openstlinux-6.1-yocto-mickledore-mp1-v23.06.21/sources/arm-ostl-linux-gnueabi/meta-st-x-linux-rt/recipes-kernel/linux/5.15/5.15.67 must also be applied.
Enter in the directory containing Linux kernel sources, then apply patches:
for p in `ls -1 <Developer Package installation directory>/stm32mp1-openstlinux-6.1-yocto-mickledore-mp1-v23.06.21/sources/arm-ostl-linux-gnueabi/meta-st-x-linux-rt/recipes-kernel/linux/5.15/5.15.67/*.patch`; do patch -p1 < $p; done
Once patches are applied, new fragments are present inside the Linux kernel source. Apply fragments as it is explained in the README.HOW_TO.txt helper file, present in the Linux kernel installation directory.
Then add also the following fragments:
For STM32MP13x lines :
* arch/arm/configs/fragment-07-rt.config * arch/arm/configs/fragment-07-rt-sysvinit.config * arch/arm/configs/fragment-08-rt-mp13.config
For STM32MP15x lines :
* arch/arm/configs/fragment-07-rt.config * arch/arm/configs/fragment-07-rt-sysvinit.config
The Linux kernel can now be built and deployed, as explained in STM32MP1_Developer_Package#Building_and_deploying_the_Linux_kernel_for_the_first_time.
3.4.2. Modify the OP-TEE OS[edit source]
Follow now the sub-chapter STM32MP1_Developer_Package#Building_and_deploying_the_OP-TEE_for_the_first_time to prepare the OP-TEE OS.
Patches found in <Developer Package installation directory>/stm32mp1-openstlinux-6.1-yocto-mickledore-mp1-v23.06.21/sources/arm-ostl-linux-gnueabi/meta-st-x-linux-rt/recipes-security/optee/optee-os must also be applied.
Enter in the directory containing OP-TEE OS sources, then apply patches
for p in `ls -1 <Developer Package installation directory>/stm32mp1-openstlinux-6.1-yocto-mickledore-mp1-v23.06.21/sources/arm-ostl-linux-gnueabi/meta-st-x-linux-rt/recipes-security/optee/optee-os/*.patch`; do patch -p1 < $p; done
OP-TEE OS can now be built and deployed, as explained in STM32MP1_Developer_Package#Building_and_deploying_the_OP-TEE_for_the_first_time.
4. To validate RT feature[edit source]
To check that RT feature is enabled and functional, following commands can be executed.
4.1. Check Linux kernel version[edit source]
uname -s -r -v
| Without RT feature | With RT feature |
|---|---|
Linux 5.15.67 #1 SMP PREEMPT Thu Sep 8 10:32:54 UTC 2022 |
Linux 5.15.67-rt49 #1 SMP PREEMPT_RT Thu Sep 8 10:32:54 UTC 2022 |
4.2. Cyclictest[edit source]
Cyclictest accurately and repeatedly measures the difference between a thread's intended wake-up time and the time at which it actually wakes up.
For a better understanding on cyclictest, refer to the cyclictest webpage in Linux RT foundation website[2].
For example, following command can be used:
cyclictest --mlockall -t -a --priority=99 --interval=200 --distance=0 -l 1000
| Without RT feature | With RT feature |
|---|---|
policy: fifo: loadavg: 0.03 0.08 0.08 1/173 8476 T: 0 ( 8475) P:99 I:200 C: 1000 Min: 15 Act: 71 Avg: 32 Max: 100 T: 1 ( 8476) P:99 I:200 C: 853 Min: 15 Act: 48 Avg: 31 Max: 65 |
policy: fifo: loadavg: 0.29 0.32 0.24 1/211 3815 T: 0 ( 3814) P:99 I:200 C: 1000 Min: 15 Act: 21 Avg: 23 Max: 60 T: 1 ( 3815) P:99 I:200 C: 795 Min: 16 Act: 32 Avg: 20 Max: 59 |