This article introduces how NVMEM Linux® framework manages BSEC OTP data and how to read/write from/to it.
1 Framework purpose[edit source]
The NVMEM Linux® framework provides a generic interface for the device non-volatile memory data such as:
- OTP (one-time programmable) fuses
- EEPROM
It offers kernel space and user space interfaces to read and/or write data such as analog calibration data or MAC address.
2 System overview[edit source]
2.1 Component description[edit source]
- NVMEM user (user space)
The user can use the NVMEM sysfs interface, from a user terminal or a custom application, to read/write data from/to NVMEM device(s) from user space.
- NVMEM user (kernel space)
User drivers can use the NVMEM API to read/write data from/to NVMEM device(s) from kernel space (such as the analog calibration data used by an ADC driver).
- NVMEM framework (kernel space)
The NVMEM core provides sysfs interface and NVMEM API. They can be used to implement NVMEM user and NVMEM controller drivers.
- NVMEM drivers (kernel space)
Provider drivers such as BSEC Linux® driver that exposes OTP data to the core.
- TEE framework (kernel space)
The TEE framework provides TEE client API to communicate with secure services, as the services provided by the OP-TEE Linux® driver.
- OP-TEE (Secure)
The OP-TEE secure OS is running on the Cortex-A in secure mode and exposes secure service with Trusted Applications (TA), as BSEC PTA.
- NVMEM hardware
NVMEM controller(s) such as the BSEC internal peripheral[1]
2.2 API description[edit source]
The NVMEM kernel documentation[2] describes:
- Kernel space API for NVMEM providers and NVMEM consumers.
- Userspace binary interface (sysfs).
See also sysfs-bus-nvmem[3] ABI documentation.
3 Configuration[edit source]
3.1 Kernel configuration[edit source]
Activate NVMEM framework in the kernel configuration through the Linux® menuconfig tool, Menuconfig or how to configure kernel (CONFIG_NVMEM=y):
Device Drivers --->
[*] NVMEM Support --->
<*> STMicroelectronics STM32 factory-programmed memory support
3.2 Device tree configuration[edit source]
The NVMEM data device tree bindings describe:
The BSEC internal peripheral[1] device tree bindings are explained in BSEC device tree configuration article.
4 How to use the framework[edit source]
4.1 How to use NVMEM with sysfs interface[edit source]
4.1.1 How to list NVMEM devices[edit source]
The available NVMEM devices can be listed in sysfs directory /sys/bus/nvmem/devices
Example to list nvmem devices: BSEC is stm32-romem0
ls /sys/bus/nvmem/devices/ stm32-romem0
4.1.2 How to read OTPs using NVMEM[edit source]
Userspace can read/write the raw NVMEM file located at: /sys/bus/nvmem/devices/*/nvmem
For BSEC, the NVEM stm32-romem0 device, the content of non-secure OTPs can be read but the secured OTPs are masked, theirs values are replaced by 0.
Normally only the 32 lower OTPs can be accessed and the upper OTPS is restricted to security. If user needs more than the 32 lower OTPs, there is an exception management explained in BSEC device tree configuration.
- Example to read all nvmem data content on stm32-romem0 devices
dd if=/sys/bus/nvmem/devices/stm32-romem0/nvmem of=/tmp/file
- Example to display nvmem data content
hexdump -C -v /sys/bus/nvmem/devices/stm32-romem0/nvmem
4.1.3 How to write BSEC OTPs using NVMEM[edit source]
OTP can be updated by using STM32CubeProgrammer.
5 How to trace and debug the framework[edit source]
5.1 How to trace[edit source]
Ftrace can be used to trace the NVMEM framework:
cd /sys/kernel/debug/tracing cat available_filter_functions | grep nvmem # Show available filter functions rtc_nvmem_register rtc_nvmem_unregister nvmem_reg_read bin_attr_nvmem_read ...
Enable the kernel function tracer, then start using nvmem and display the result:
echo function > current_tracer echo "*nvmem*" > set_ftrace_filter # Trace all nvmem filter functions echo 1 > tracing_on # start ftrace hexdump -C -v /sys/bus/nvmem/devices/stm32-romem0/nvmem # dump nvmem 00000000 17 00 00 00 01 80 00 00 00 00 00 00 00 00 00 00 |................| ... echo 0 > tracing_on # stop ftrace cat trace # tracer: function # # _-----=> irqs-off # / _----=> need-resched # | / _---=> hardirq/softirq # || / _--=> preempt-depth # ||| / delay # TASK-PID CPU# |||| TIMESTAMP FUNCTION # | | | |||| | | hexdump-478 [000] .... 423.502278: bin_attr_nvmem_read <-sysfs_kf_bin_read hexdump-478 [000] .... 423.502290: nvmem_reg_read <-bin_attr_nvmem_read hexdump-478 [000] .... 423.515804: bin_attr_nvmem_read <-sysfs_kf_bin_read
6 References[edit source]
- ↑ 1.0 1.1 BSEC internal peripheral
- ↑ Documentation/driver-api/nvmem.rst , NVMEM subsytem kernel documentation
- ↑ Documentation/ABI/stable/sysfs-bus-nvmem , NVMEM ABI documentation
- ↑ Documentation/devicetree/bindings/nvmem/nvmem.yaml , NVMEM device tree bindings
- ↑ Documentation/devicetree/bindings/nvmem/nvmem-consumer.yaml , NVMEM consumer device tree bindings