crypto: doc - document crypto engine API

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Documentation/crypto/crypto_engine.rst

+=============
+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