=============
  CRYPTO ENGINE
  =============
  
  Overview
  --------
  The crypto engine API (CE), is a crypto queue manager.
  
  Requirement
  -----------

  You have to put at start of your tfm_ctx the struct crypto_engine_ctx::
  
    struct your_tfm_ctx {

          struct crypto_engine_ctx enginectx;
          ...

    };

  Why: Since CE manage only crypto_async_request, it cannot know the underlying
  request_type and so have access only on the TFM.
  So using container_of for accessing __ctx is impossible.
  Furthermore, the crypto engine cannot know the "struct your_tfm_ctx",
  so it must assume that crypto_engine_ctx is at start of it.
  
  Order of operations
  -------------------
  You have to obtain a struct crypto_engine via crypto_engine_alloc_init().
  And start it via crypto_engine_start().
  
  Before transferring any request, you have to fill the enginectx.
  - prepare_request: (taking a function pointer) If you need to do some processing before doing the request
  - unprepare_request: (taking a function pointer) Undoing what's done in prepare_request
  - do_one_request: (taking a function pointer) Do encryption for current request
  
  Note: that those three functions get the crypto_async_request associated with the received request.
  So your need to get the original request via container_of(areq, struct yourrequesttype_request, base);
  
  When your driver receive a crypto_request, you have to transfer it to
  the cryptoengine via one of:
  - crypto_transfer_ablkcipher_request_to_engine()
  - crypto_transfer_aead_request_to_engine()
  - crypto_transfer_akcipher_request_to_engine()
  - crypto_transfer_hash_request_to_engine()
  - crypto_transfer_skcipher_request_to_engine()
  
  At the end of the request process, a call to one of the following function is needed:
  - crypto_finalize_ablkcipher_request
  - crypto_finalize_aead_request
  - crypto_finalize_akcipher_request
  - crypto_finalize_hash_request
  - crypto_finalize_skcipher_request