NVMEM overview

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SUMMARY

This article introduces how NVMEM Linux® framework manages BSEC OTP data and how to read/write from/to it.

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
  • 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

NVMEM sysfs interfaceNVMEM consumers interfaceBSEC internal peripheral
NVMEM system overview

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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 BSEC Linux® driver that exposes OTP data to the core.

  • NVMEM hardware

NVMEM controller(s) such as the BSEC internal peripheral[1]

2.2 API description

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

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  --->

3.2 Device tree configuration

The NVMEM data device tree bindings[4] describe:

  • The location of non-volatile memory data
  • The NVMEM data providers
  • The NVMEM data consumers

4 How to use the framework

4.1 How to use NVMEM with sysfs interface

The available NVMEM devices can be listed in sysfs:

Board $> ls /sys/bus/nvmem/devices/                                            # Example to list nvmem devices
stm32-romem0

The data content of an NVMEM device can be dumped to a binary file:

Board $> cat /sys/bus/nvmem/devices/stm32-romem0/nvmem > file                  # Example to read nvmem data content

The data content of an NVMEM device can be displayed:

Board $> hexdump -C -v /sys/bus/nvmem/devices/stm32-romem0/nvmem
Warning white.png Warning
The below examples show how to write data to an NVMEM device. This may cause unrecoverable damage to the STM32 device (for example when writing to an OTP area)

The full data content of an NVMEM device can be written as follows:

Board $> cat file > /sys/bus/nvmem/devices/stm32-romem0/nvmem                  # Example to write nvmem data content

Example of 32-bit data word writing (filling it with ones) in OTP n°95:

Board $> dd if=/dev/zero count=1 bs=4 | tr '\000' '\377' > file                # Create a 4 bytes length file filled with ones, e.g. 0xffffffff)
Board $> dd if=file bs=4 seek=95 of=/sys/bus/nvmem/devices/stm32-romem0/nvmem  # Write it (32-bits, e.g. 4bytes) to OTP data 95

5 How to trace and debug the framework

5.1 How to trace

Ftrace can be used to trace the NVMEM framework:

Board $> cd /sys/kernel/debug/tracing
Board $> 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:

Board $> echo function > current_tracer
Board $> echo "*nvmem*" > set_ftrace_filter                                    # Trace all nvmem filter functions
Board $> echo 1 > tracing_on                                                   # start ftrace
Board $> 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  |................|
...
Board $> echo 0 > tracing_on                                                   # stop ftrace
Board $> 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