[PATCH v3] Documentation: provide documentation for RNG interfaces.

[Oleksij Rempel]

Signed-off-by: Oleksij Rempel <o.rempel at pengutronix.de>
---
 Documentation/user/random.rst      | 63 ++++++++++++++++++++++++++++++++++++++
 Documentation/user/user-manual.rst |  1 +
 2 files changed, 64 insertions(+)
 create mode 100644 Documentation/user/random.rst

diff --git a/Documentation/user/random.rst b/Documentation/user/random.rst
new file mode 100644
index 0000000000..95dad72e55
--- /dev/null
+++ b/Documentation/user/random.rst
@@ -0,0 +1,63 @@
+Random Number Generator support
+===============================
+
+Barebox provides two types of RNG sources - PRNG and HWRNG:
+
+- "A pseudorandom number generator (PRNG), also known as a deterministic random
+  bit generator (DRBG),[1] is an algorithm for generating a sequence of numbers
+  whose properties approximate the properties of sequences of random numbers.
+  The PRNG-generated sequence is not truly random, because it is completely
+  determined by a relatively small set of initial values, called the PRNG's seed
+  (which may include truly random values). Although sequences that are closer to
+  truly random can be generated using hardware random number generators."
+  Pseudorandom number generator. https://en.wikipedia.org/wiki/Pseudorandom_number_generator (2017.05.08).
+  The PRNG used by Barebox is LCG (linear congruential generator) non cryptographically
+  secure, so please use with caution.
+
+- The HWRNG framework is software that makes use of a special hardware feature on
+  your CPU, SoC or motherboard. It can‘t provide any guarantee about cryptographic
+  security of used HW. Please refer to vendor documentation and/or RNG certification.
+
+API
+^^^
+
+.. code-block:: c
+
+        /* seed the PRNG. */
+        void srand(unsigned int seed);
+
+        /* Fill the buffer with PRNG bits. */
+        void get_random_bytes(void *buf, int len);
+
+        /* Fill the buffer with bits provided by HWRNG.
+         * This function may fail with a message “error: no HWRNG available!”
+         * in case HWRNG is not available or HW got some runtime error.
+         * If barebox is compiled with CONFIG_ALLOW_PRNG_FALLBACK,
+         * then get_crypto_bytes() will print “warning: falling back to Pseudo RNG source!”
+         * and use PRNG instead of returning error.
+         */
+        int get_crypto_bytes(void *buf, int len);
+
+User interface
+^^^^^^^^^^^^^^
+
+- /dev/hwrng0
+  provides access to first available HWRNG. To examine this source you can use:
+
+.. code-block:: sh
+
+  md -s /dev/hwrng0
+
+- /dev/prng
+  provides access to PRNG. To examine this source you can use:
+
+.. code-block:: sh
+
+  md -s /dev/prng
+
+To seed PRNG from user space the :ref:`command_seed` is provided. For example:
+
+.. code-block:: sh
+
+  seed 12345
+  md -s /dev/prng
diff --git a/Documentation/user/user-manual.rst b/Documentation/user/user-manual.rst
index 435649f353..791c8e0979 100644
--- a/Documentation/user/user-manual.rst
+++ b/Documentation/user/user-manual.rst
@@ -33,6 +33,7 @@ Contents:
    reset-reason
    system-reset
    state
+   random
 
 * :ref:`search`
 * :ref:`genindex`
-- 
2.11.0