diff --git a/include/mbedtls/hmac_drbg.h b/include/mbedtls/hmac_drbg.h
index f38337a..b5dc829 100644
--- a/include/mbedtls/hmac_drbg.h
+++ b/include/mbedtls/hmac_drbg.h
@@ -1,10 +1,14 @@
 /**
  * \file hmac_drbg.h
  *
- * \brief HMAC_DRBG (NIST SP 800-90A)
+ * \brief The HMAC_DRBG pseudorandom generator.
+ *
+ * This module implements the HMAC_DRBG pseudorandom generator described
+ * in <em>NIST SP 800-90A: Recommendation for Random Number Generation Using
+ * Deterministic Random Bit Generators</em>.
  */
 /*
- *  Copyright (C) 2006-2015, ARM Limited, All Rights Reserved
+ *  Copyright (C) 2006-2019, ARM Limited, All Rights Reserved
  *  SPDX-License-Identifier: Apache-2.0
  *
  *  Licensed under the Apache License, Version 2.0 (the "License"); you may
@@ -104,38 +108,50 @@
 } mbedtls_hmac_drbg_context;
 
 /**
- * \brief               HMAC_DRBG context initialization
- *                      Makes the context ready for mbedtls_hmac_drbg_seed(),
- *                      mbedtls_hmac_drbg_seed_buf() or
- *                      mbedtls_hmac_drbg_free().
+ * \brief               HMAC_DRBG context initialization.
  *
- * \param ctx           HMAC_DRBG context to be initialized
+ * This function makes the context ready for mbedtls_hmac_drbg_seed(),
+ * mbedtls_hmac_drbg_seed_buf() or mbedtls_hmac_drbg_free().
+ *
+ * \param ctx           HMAC_DRBG context to be initialized.
  */
 void mbedtls_hmac_drbg_init( mbedtls_hmac_drbg_context *ctx );
 
 /**
- * \brief               HMAC_DRBG initial seeding
- *                      Seed and setup entropy source for future reseeds.
+ * \brief               HMAC_DRBG initial seeding.
  *
- * \param ctx           HMAC_DRBG context to be seeded
- * \param md_info       MD algorithm to use for HMAC_DRBG
- * \param f_entropy     Entropy callback (p_entropy, buffer to fill, buffer
- *                      length)
- * \param p_entropy     Entropy context
- * \param custom        Personalization data (Device specific identifiers)
- *                      (Can be NULL)
- * \param len           Length of personalization data
+ * Set the initial seed and set up the entropy source for future reseeds.
+ *
+ * \param ctx           HMAC_DRBG context to be seeded.
+ * \param md_info       MD algorithm to use for HMAC_DRBG.
+ * \param f_entropy     The entropy callback, taking as arguments the
+ *                      \p p_entropy context, the buffer to fill, and the
+ *                      length of the buffer.
+ * \param p_entropy     The entropy context to pass to \p f_entropy.
+ * \param custom        Personalization data, that is device-specific
+ *                      identifiers. This can be NULL, in which case the
+ *                      personalization data is empty regardless of the value
+ *                      of \p len.
+ * \param len           The length of the personalization data.
+ *                      This must be at most #MBEDTLS_HMAC_DRBG_MAX_INPUT
+ *                      and also at most
+ *                      #MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT - \p entropy_len * 3 / 2
+ *                      where \p entropy_len is the entropy length
+ *                      (see mbedtls_hmac_drbg_set_entropy_len()).
  *
  * \note                The "security strength" as defined by NIST is set to:
- *                      128 bits if md_alg is SHA-1,
- *                      192 bits if md_alg is SHA-224,
- *                      256 bits if md_alg is SHA-256 or higher.
+ *                      128 bits if \p md_info is SHA-1,
+ *                      192 bits if \p md_info is SHA-224,
+ *                      256 bits if \p md_info is SHA-256, SHA-384 or SHA-512.
  *                      Note that SHA-256 is just as efficient as SHA-224.
  *
- * \return              0 if successful, or
- *                      MBEDTLS_ERR_MD_BAD_INPUT_DATA, or
- *                      MBEDTLS_ERR_MD_ALLOC_FAILED, or
- *                      MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED.
+ * \return              0 if successful.
+ * \return              #MBEDTLS_ERR_MD_BAD_INPUT_DATA if \p md_info is
+ *                      invalid.
+ * \return              #MBEDTLS_ERR_MD_ALLOC_FAILED if there was not enough
+ *                      memory to allocate context data.
+ * \return              #MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED
+ *                      if the call to \p f_entropy failed.
  */
 int mbedtls_hmac_drbg_seed( mbedtls_hmac_drbg_context *ctx,
                     mbedtls_md_handle_t md_info,
@@ -146,100 +162,134 @@
 
 /**
  * \brief               Initilisation of simpified HMAC_DRBG (never reseeds).
- *                      (For use with deterministic ECDSA.)
  *
- * \param ctx           HMAC_DRBG context to be initialised
- * \param md_info       MD algorithm to use for HMAC_DRBG
- * \param data          Concatenation of entropy string and additional data
- * \param data_len      Length of data in bytes
+ * This function is meant for use in algorithms that need a pseudorandom
+ * input such as deterministic ECDSA.
  *
- * \return              0 if successful, or
- *                      MBEDTLS_ERR_MD_BAD_INPUT_DATA, or
- *                      MBEDTLS_ERR_MD_ALLOC_FAILED.
+ * \param ctx           HMAC_DRBG context to be initialised.
+ * \param md_info       MD algorithm to use for HMAC_DRBG.
+ * \param data          Concatenation of the initial entropy string and
+ *                      the additional data.
+ * \param data_len      Length of \p data in bytes.
+ *
+ * \return              0 if successful. or
+ * \return              #MBEDTLS_ERR_MD_BAD_INPUT_DATA if \p md_info is
+ *                      invalid.
+ * \return              #MBEDTLS_ERR_MD_ALLOC_FAILED if there was not enough
+ *                      memory to allocate context data.
  */
 int mbedtls_hmac_drbg_seed_buf( mbedtls_hmac_drbg_context *ctx,
                         mbedtls_md_handle_t  md_info,
                         const unsigned char *data, size_t data_len );
 
 /**
- * \brief               Enable / disable prediction resistance (Default: Off)
+ * \brief               This function turns prediction resistance on or off.
+ *                      The default value is off.
  *
- * Note: If enabled, entropy is used for ctx->entropy_len before each call!
- *       Only use this if you have ample supply of good entropy!
+ * \note                If enabled, entropy is gathered at the beginning of
+ *                      every call to mbedtls_hmac_drbg_random_with_add()
+ *                      or mbedtls_hmac_drbg_random().
+ *                      Only use this if your entropy source has sufficient
+ *                      throughput.
  *
- * \param ctx           HMAC_DRBG context
- * \param resistance    MBEDTLS_HMAC_DRBG_PR_ON or MBEDTLS_HMAC_DRBG_PR_OFF
+ * \param ctx           The HMAC_DRBG context.
+ * \param resistance    #MBEDTLS_HMAC_DRBG_PR_ON or #MBEDTLS_HMAC_DRBG_PR_OFF.
  */
 void mbedtls_hmac_drbg_set_prediction_resistance( mbedtls_hmac_drbg_context *ctx,
                                           int resistance );
 
 /**
- * \brief               Set the amount of entropy grabbed on each reseed
- *                      (Default: given by the security strength, which
- *                      depends on the hash used, see \c mbedtls_hmac_drbg_init() )
+ * \brief               This function sets the amount of entropy grabbed on each
+ *                      seed or reseed.
  *
- * \param ctx           HMAC_DRBG context
- * \param len           Amount of entropy to grab, in bytes
+ * The default value is given by the security strength, which depends on the
+ * hash used. See mbedtls_hmac_drbg_init().
+ *
+ * \param ctx           The HMAC_DRBG context.
+ * \param len           The amount of entropy to grab, in bytes.
  */
 void mbedtls_hmac_drbg_set_entropy_len( mbedtls_hmac_drbg_context *ctx,
                                 size_t len );
 
 /**
- * \brief               Set the reseed interval
- *                      (Default: MBEDTLS_HMAC_DRBG_RESEED_INTERVAL)
+ * \brief               Set the reseed interval.
  *
- * \param ctx           HMAC_DRBG context
- * \param interval      Reseed interval
+ * The reseed interval is the number of calls to mbedtls_hmac_drbg_random()
+ * or mbedtls_hmac_drbg_random_with_add() after which the entropy function
+ * is called again.
+ *
+ * The default value is #MBEDTLS_HMAC_DRBG_RESEED_INTERVAL.
+ *
+ * \param ctx           The HMAC_DRBG context.
+ * \param interval      The reseed interval.
  */
 void mbedtls_hmac_drbg_set_reseed_interval( mbedtls_hmac_drbg_context *ctx,
                                     int interval );
 
 /**
- * \brief               HMAC_DRBG update state
+ * \brief               This function updates the state of the HMAC_DRBG context.
  *
- * \param ctx           HMAC_DRBG context
- * \param additional    Additional data to update state with, or NULL
- * \param add_len       Length of additional data, or 0
+ * \param ctx           The HMAC_DRBG context.
+ * \param additional    The data to update the state with.
+ *                      If this is \p NULL, there is no additional data.
+ * \param add_len       Length of \p additional in bytes.
+ *                      Unused if \p additional is null.
  *
  * \return              \c 0 on success, or an error from the underlying
  *                      hash calculation or
  *                      MBEDTLS_ERR_PLATFORM_FAULT_DETECTED.
- *
- * \note                Additional data is optional, pass NULL and 0 as second
- *                      third argument if no additional data is being used.
  */
 int mbedtls_hmac_drbg_update_ret( mbedtls_hmac_drbg_context *ctx,
                        const unsigned char *additional, size_t add_len );
 
 /**
- * \brief               HMAC_DRBG reseeding (extracts data from entropy source)
+ * \brief               This function reseeds the HMAC_DRBG context, that is
+ *                      extracts data from the entropy source.
  *
- * \param ctx           HMAC_DRBG context
- * \param additional    Additional data to add to state (Can be NULL)
- * \param len           Length of additional data
+ * \param ctx           The HMAC_DRBG context.
+ * \param additional    Additional data to add to the state.
+ *                      If this is \c NULL, there is no additional data
+ *                      and \p len should be \c 0.
+ * \param len           The length of the additional data.
+ *                      This must be at most #MBEDTLS_HMAC_DRBG_MAX_INPUT
+ *                      and also at most
+ *                      #MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT - \p entropy_len
+ *                      where \p entropy_len is the entropy length
+ *                      (see mbedtls_hmac_drbg_set_entropy_len()).
  *
- * \return              0 if successful, or
- *                      MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED
+ * \return              0 if successful.
+ * \return              #MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED
+ *                      if a call to the entropy function failed.
  */
 int mbedtls_hmac_drbg_reseed( mbedtls_hmac_drbg_context *ctx,
                       const unsigned char *additional, size_t len );
 
 /**
- * \brief               HMAC_DRBG generate random with additional update input
+ * \brief   This function updates an HMAC_DRBG instance with additional
+ *          data and uses it to generate random data.
  *
- * Note: Automatically reseeds if reseed_counter is reached or PR is enabled.
+ * \note    The function automatically reseeds if the reseed counter is exceeded.
  *
- * \param p_rng         HMAC_DRBG context
- * \param output        Buffer to fill
- * \param output_len    Length of the buffer
- * \param additional    Additional data to update with (can be NULL)
- * \param add_len       Length of additional data (can be 0)
+ * \param p_rng         The HMAC_DRBG context. This must be a pointer to a
+ *                      #mbedtls_hmac_drbg_context structure.
+ * \param output        The buffer to fill.
+ * \param output_len    The length of the buffer in bytes.
+ *                      This must be at most #MBEDTLS_HMAC_DRBG_MAX_REQUEST.
+ * \param additional    Additional data to update with.
+ *                      If this is \p NULL, there is no additional data
+ *                      and \p add_len should be \c 0.
+ * \param add_len       The length of the additional data.
+ *                      This must be at most #MBEDTLS_HMAC_DRBG_MAX_INPUT.
  *
- * \return              0 if successful, or
- *                      MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED, or
- *                      MBEDTLS_ERR_HMAC_DRBG_REQUEST_TOO_BIG, or
- *                      MBEDTLS_ERR_HMAC_DRBG_INPUT_TOO_BIG, or
- *                      MBEDTLS_ERR_PLATFORM_FAULT_DETECTED.
+ * \return              \c 0 if successful.
+ * \return              #MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED
+ *                      if a call to the entropy source failed.
+ * \return              #MBEDTLS_ERR_HMAC_DRBG_REQUEST_TOO_BIG if
+ *                      \p output_len > #MBEDTLS_HMAC_DRBG_MAX_REQUEST.
+ * \return              #MBEDTLS_ERR_HMAC_DRBG_INPUT_TOO_BIG if
+ *                      \p add_len > #MBEDTLS_HMAC_DRBG_MAX_INPUT.
+ * \return              #MBEDTLS_ERR_PLATFORM_FAULT_DETECTED if
+ *                      a logical fault is detected.
  */
 int mbedtls_hmac_drbg_random_with_add( void *p_rng,
                                unsigned char *output, size_t output_len,
@@ -247,26 +297,30 @@
                                size_t add_len );
 
 /**
- * \brief               HMAC_DRBG generate random
+ * \brief   This function uses HMAC_DRBG to generate random data.
  *
- * Note: Automatically reseeds if reseed_counter is reached or PR is enabled.
+ * \note    The function automatically reseeds if the reseed counter is exceeded.
  *
- * \param p_rng         HMAC_DRBG context
- * \param output        Buffer to fill
- * \param out_len       Length of the buffer
+ * \param p_rng         The HMAC_DRBG context. This must be a pointer to a
+ *                      #mbedtls_hmac_drbg_context structure.
+ * \param output        The buffer to fill.
+ * \param out_len       The length of the buffer in bytes.
+ *                      This must be at most #MBEDTLS_HMAC_DRBG_MAX_REQUEST.
  *
- * \return              0 if successful, or
- *                      MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED, or
- *                      MBEDTLS_ERR_HMAC_DRBG_REQUEST_TOO_BIG,
- *                      MBEDTLS_ERR_HMAC_DRBG_INPUT_TOO_BIG, or
- *                      MBEDTLS_ERR_PLATFORM_FAULT_DETECTED.
+ * \return              \c 0 if successful.
+ * \return              #MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED
+ *                      if a call to the entropy source failed.
+ * \return              #MBEDTLS_ERR_HMAC_DRBG_REQUEST_TOO_BIG if
+ *                      \p out_len > #MBEDTLS_HMAC_DRBG_MAX_REQUEST.
+ * \return              #MBEDTLS_ERR_PLATFORM_FAULT_DETECTED if
+ *                      a logical fault is detected.
  */
 int mbedtls_hmac_drbg_random( void *p_rng, unsigned char *output, size_t out_len );
 
 /**
  * \brief               Free an HMAC_DRBG context
  *
- * \param ctx           HMAC_DRBG context to free.
+ * \param ctx           The HMAC_DRBG context to free.
  */
 void mbedtls_hmac_drbg_free( mbedtls_hmac_drbg_context *ctx );
 
@@ -277,17 +331,16 @@
 #define MBEDTLS_DEPRECATED
 #endif
 /**
- * \brief               HMAC_DRBG update state
+ * \brief               This function updates the state of the HMAC_DRBG context.
  *
  * \deprecated          Superseded by mbedtls_hmac_drbg_update_ret()
  *                      in 2.16.0.
  *
- * \param ctx           HMAC_DRBG context
- * \param additional    Additional data to update state with, or NULL
- * \param add_len       Length of additional data, or 0
- *
- * \note                Additional data is optional, pass NULL and 0 as second
- *                      third argument if no additional data is being used.
+ * \param ctx           The HMAC_DRBG context.
+ * \param additional    The data to update the state with.
+ *                      If this is \p NULL, there is no additional data.
+ * \param add_len       Length of \p additional in bytes.
+ *                      Unused if \p additional is null.
  */
 MBEDTLS_DEPRECATED void mbedtls_hmac_drbg_update(
     mbedtls_hmac_drbg_context *ctx,
@@ -297,26 +350,31 @@
 
 #if defined(MBEDTLS_FS_IO)
 /**
- * \brief               Write a seed file
+ * \brief               This function writes a seed file.
  *
- * \param ctx           HMAC_DRBG context
- * \param path          Name of the file
+ * \param ctx           The HMAC_DRBG context.
+ * \param path          The name of the file.
  *
- * \return              0 if successful, 1 on file error, or
- *                      MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED
+ * \return              \c 0 on success.
+ * \return              #MBEDTLS_ERR_HMAC_DRBG_FILE_IO_ERROR on file error.
+ * \return              #MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED on reseed
+ *                      failure.
  */
 int mbedtls_hmac_drbg_write_seed_file( mbedtls_hmac_drbg_context *ctx, const char *path );
 
 /**
- * \brief               Read and update a seed file. Seed is added to this
- *                      instance
+ * \brief               This function reads and updates a seed file. The seed
+ *                      is added to this instance.
  *
- * \param ctx           HMAC_DRBG context
- * \param path          Name of the file
+ * \param ctx           The HMAC_DRBG context.
+ * \param path          The name of the file.
  *
- * \return              0 if successful, 1 on file error,
- *                      MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED or
- *                      MBEDTLS_ERR_HMAC_DRBG_INPUT_TOO_BIG
+ * \return              \c 0 on success.
+ * \return              #MBEDTLS_ERR_HMAC_DRBG_FILE_IO_ERROR on file error.
+ * \return              #MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED on
+ *                      reseed failure.
+ * \return              #MBEDTLS_ERR_HMAC_DRBG_INPUT_TOO_BIG if the existing
+ *                      seed file is too large.
  */
 int mbedtls_hmac_drbg_update_seed_file( mbedtls_hmac_drbg_context *ctx, const char *path );
 #endif /* MBEDTLS_FS_IO */
@@ -324,9 +382,10 @@
 
 #if defined(MBEDTLS_SELF_TEST)
 /**
- * \brief               Checkup routine
+ * \brief               The HMAC_DRBG Checkup routine.
  *
- * \return              0 if successful, or 1 if the test failed
+ * \return              \c 0 if successful.
+ * \return              \c 1 if the test failed.
  */
 int mbedtls_hmac_drbg_self_test( int verbose );
 #endif