OpenBMC’s kernel configuration disables /dev/mem by default. In the early days of the project some functionality was implemented using simple userspace programs that read and wrote straight to peripherals, instead of kernel drivers. Now that it has kernel drivers userspace access to memory is restricted.

Sometimes you need this access, and to cover that use case there’s a kernel option that can be set to re-enable /dev/mem. To enable this from a shell on the bmc, first take note of the currently set command line:

# fw_printenv  bootargs
bootargs=console=ttyS4,115200n8

And then add the devmem parameter:

fw_setenv bootargs console=ttyS4,115200n8 mem.devmem=1

After rebooting the /dev/mem character device will be back. This will persist until the command line is changed back to the default.

References

https://github.com/openbmc/skeleton/issues/20 https://github.com/openbmc/openbmc/issues/762 https://github.com/openbmc/openbmc/issues/3252