- Last edited 2 weeks ago ago
This article introduces how NVMEM Linux® framework manages BSEC OTP data and how to read/write from/to it.
- 1 Framework purpose
- 2 System overview
- 3 Configuration
- 4 How to use the framework
- 5 How to trace and debug the framework
- 6 References
1 Framework purpose
The NVMEM Linux® framework provides a generic interface for the device non-volatile memory data such as:
- OTP (one-time programmable) fuses
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
2.1 Component description
- 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 STM32 ROMEM Linux® driver that exposes BSEC 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)
- NVMEM hardware
NVMEM controller(s) such as the BSEC internal peripheral
2.2 API description
The NVMEM kernel documentation describes:
- Kernel space API for NVMEM providers and NVMEM consumers.
- Userspace binary interface (sysfs).
See also sysfs-bus-nvmem ABI documentation.
3.1 Kernel configuration
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
The NVMEM data device tree bindings describe:
4 How to use the framework
4.1 How to use NVMEM with sysfs interface
4.1.1 How to list NVMEM devices
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
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
The BSEC OTP can be written by 32-bit word starting at OTP N as follows:
# write OTP N word by word dd if=/tmp/file of=/sys/bus/nvmem/devices/stm32-romem0/nvmem bs=4 seek=N
# write OTP N, all the file in one request dd if=/tmp/file of=/sys/bus/nvmem/devices/stm32-romem0/nvmem seek=4*N oflag=seek_bytes
With a file /tmp/file containing the OTP data to write, its size is 32-bit word aligned; for example:
# Create a 4 bytes length file filled with ones, e.g. 0xffffffff) dd if=/dev/zero count=1 bs=4 | tr '\000' '\377' > file # Create a 4 bytes length file, here 0x00000001 to update one OTP echo -n -e '\x01\x00\x00\x00' > /tmp/file # Create a 8 bytes length file, here 0x67452301 0xEFCDAB89 to update two OTPs echo -n -e '\x01\x23\x45\x67\x89\xAB\xCD\xEF' > /tmp/file
A lower OTP can be written several time for a bit per bit update if it is not locked.
An upper OTP data can be written only if it is allowed in secure world device tree and only one time; when the upper OTP is written, it is permanent locked at the end of the NVMEM request to avoid ECC issue on second update. For the first example with bs=4, this lock is performed after each OTP update and for the second example with oflag=seek_bytes the lock is done when all the OTPs in input file are updated.
The full example to write the upper OTP 60 is:
echo -n -e '\x01\x23\x45\x67' > /tmp/file hexdump -C /tmp/file 00000000 01 23 45 67 |.#Eg| 00000004 dd if=/tmp/file of=/sys/bus/nvmem/devices/stm32-romem0/nvmem bs=4 seek=60 reboot << >> hexdump -C -v /sys/bus/nvmem/devices/stm32-romem0/nvmem .... 000000f0 01 23 45 67 00 00 00 00 00 00 00 00 00 00 00 00 |.#Eg............| ....
The associated output in STM32CubeProgrammer is:
OTP REGISTERS: --------------------------------------------------------------------------- ID | value | status --------------------------------------------------------------------------- ... 060 | 0x67452301 | 0x40000000 |_ Permanent write lock
or in U-Boot
> fuse read 0 0 96 ... Word 0x0000003c: 67452301 00000000 00000000 00000000 ...
5 How to trace and debug the framework
5.1 How to trace
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  .... 423.502278: bin_attr_nvmem_read <-sysfs_kf_bin_read hexdump-478  .... 423.502290: nvmem_reg_read <-bin_attr_nvmem_read hexdump-478  .... 423.515804: bin_attr_nvmem_read <-sysfs_kf_bin_read
- 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