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The HW RNG core allows for manual selection of which RNG device to use, but does not allow for no device to be enabled. It may be desirable to do this on systems with only a single suitable hardware RNG, where we need exclusive access to other functionality on this device. In particular when performing TPM firmware upgrades this lets us ensure the kernel does not try to access the device. Before: root@debian-qemu-efi:~# grep "" /sys/devices/virtual/misc/hw_random/rng_* /sys/devices/virtual/misc/hw_random/rng_available:tpm-rng-0 /sys/devices/virtual/misc/hw_random/rng_current:tpm-rng-0 /sys/devices/virtual/misc/hw_random/rng_quality:1024 /sys/devices/virtual/misc/hw_random/rng_selected:0 After: root@debian-qemu-efi:~# grep "" /sys/devices/virtual/misc/hw_random/rng_* /sys/devices/virtual/misc/hw_random/rng_available:tpm-rng-0 none /sys/devices/virtual/misc/hw_random/rng_current:tpm-rng-0 /sys/devices/virtual/misc/hw_random/rng_quality:1024 /sys/devices/virtual/misc/hw_random/rng_selected:0 root@debian-qemu-efi:~# echo none > /sys/devices/virtual/misc/hw_random/rng_current root@debian-qemu-efi:~# grep "" /sys/devices/virtual/misc/hw_random/rng_* /sys/devices/virtual/misc/hw_random/rng_available:tpm-rng-0 none /sys/devices/virtual/misc/hw_random/rng_current:none grep: /sys/devices/virtual/misc/hw_random/rng_quality: No such device /sys/devices/virtual/misc/hw_random/rng_selected:1 (Observe using bpftrace no calls to TPM being made) root@debian-qemu-efi:~# echo "" > /sys/devices/virtual/misc/hw_random/rng_current root@debian-qemu-efi:~# grep "" /sys/devices/virtual/misc/hw_random/rng_* /sys/devices/virtual/misc/hw_random/rng_available:tpm-rng-0 none /sys/devices/virtual/misc/hw_random/rng_current:tpm-rng-0 /sys/devices/virtual/misc/hw_random/rng_quality:1024 /sys/devices/virtual/misc/hw_random/rng_selected:0 (Observe using bpftrace that calls to the TPM resume) Signed-off-by: Jonathan McDowell <noodles@meta.com> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.