docs: Add description for Linux getrandom() function

components/esp_hw_support/include/esp_random.h

@@ -29,7 +29,8 @@ extern "C" {
  * Guide "Random Number Generation" section for necessary prerequisites.
  *
  * This function automatically busy-waits to ensure enough external entropy has been
- * introduced into the hardware RNG state, before returning a new random number.
+ * introduced into the hardware RNG state, before returning a new random number. This delay
+ * is very short (always less than 100 CPU cycles).
  *
  * @return Random value between 0 and UINT32_MAX
  */

docs/en/api-reference/system/random.rst

@@ -45,5 +45,22 @@ API Reference
 .. include-build-file:: inc/esp_random.inc
 .. include-build-file:: inc/bootloader_random.inc
 
+getrandom
+---------
+
+A compatible version of the Linux ``getrandom()`` function is also provided for ease of porting:
+
+.. code-block:: c
+
+   #include <sys/random.h>
+
+   ssize_t getrandom(void *buf, size_t buflen, unsigned int flags);
+
+This function is implemented by calling :cpp:func:`esp_fill_random` internally.
+
+The ``flags`` argument is ignored, this function is always non-blocking but the strength of any random numbers is dependent on the same conditions described above.
+
+Return value is -1 (with ``errno`` set to ``EFAULT``) if the ``buf`` argument is NULL, and equal to ``buflen`` otherwise.
+
 .. _Dieharder: https://webhome.phy.duke.edu/~rgb/General/dieharder.php