NVMEM overview

Applicable for STM32MP13x lines, STM32MP15x lines

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

1 Framework purpose[edit]

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]

NVMEM sysfs interfaceNVMEM consumers interfaceBSEC internal peripheralBSEC PTAOP-TEEOP-TEE linux driverTEE Client APINVMEM overview.png

2.1 Component description[edit]

  • 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)

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]

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]

3.1 Kernel configuration[edit]

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]

The NVMEM data device tree bindings describe:

  • The location of non-volatile memory data
  • The NVMEM data providers[4]
  • The NVMEM data consumers[5]

The BSEC internal peripheral[1] device tree bindings are explained in BSEC device tree configuration article.

4 How to use the framework[edit]

4.1 How to use NVMEM with sysfs interface[edit]

4.1.1 How to list NVMEM devices[edit]

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]

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
Info white.png Information
A dedicated page describe the OTP mapping for STM32MP13 and STM32MP15.

4.1.3 How to write BSEC OTPs using NVMEM[edit]

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)
Info white.png Information
Note that lower BSEC OTPs are using 2:1 redundancy, so they can be written bit per bit, whereas upper BSEC OTPs only support one time 32-bit programming, they are automatically locked by driver.

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

or

# 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.

Info white.png Information
When a new OTP value has been written using this SYSFS interface, it may be necessary to reboot the board before reading it back. The OTP value can't be read directly after a write because the OTP value is read in a shadow area not directly in the OTP area.

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
                                 |_[30] 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[edit]

5.1 How to trace[edit]

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]