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+# Custom Operators
+
+This document explains about custom operator and how custom op is represented in nnpackage.
+
+## What is custom operator?
+
+Custom operator(hereafter custom op) is used to provide a new operator implementation.
+It can be anything that does not exist in current runtime implementation.
+
+You can use custom operator for several use cases, possible use cases are:
+
+- when an operator in tensorflow is not supported in nnfw runtime
+- when an operator is supported, however, you would like to use your own implementation
+  - it may be for optimization, by grouping several operators into one super operator.
+
+## Custom op in model
+
+nnpackage will support several kinds of models.
+Currently the only type is tflite.
+
+### tflite
+
+If you're using `tflite` format, it is same format to tensorflow lite.
+
+You can generate `tflite` model with custom op using `tflite_convert`.
+Please find the documentation in tensorflow official site.
+
+## Custom op kernel implementation
+
+You need to provide the kernel of custom op in the following form:
+
+```
+/*
+ * Custom kernel evaluation function
+ *
+ * param[in] params custom operation parameters
+ * param[in] userdata pointer to user-specified buffer( kernel instance specific )
+ */
+typedef void (*nnfw_custom_eval)(nnfw_custom_kernel_params *params, char *userdata,
+                                 size_t userdata_size);
+
+```
+
+The structures and relevant APIs are defined in nnfw APIs.
+Please see `nnfw_dev.h` for detail.
+
+You can find example in `nnfw` repository.
+
+Custom op kernel implementation is stored in nnpackage in form of prebuilt library.
+
+It is example nnpackage structure for `FillFrom`:
+
+```
+FillFrom
+├── FillFrom.tflite
+├── custom_op
+│   ├── libFillFrom.armv7l-linux.debug.a
+│   └── libFillFrom.armv7l-linux.release.a
+└── metadata
+    └── MANIFEST
+```
+
+All custom operator libraries are put under `{nnpackage_root}/custom_op/lib{customop_name}.{arch}-{os}-{buildtype}.a`.
+
+## How to use custom op in app
+
+To use custom op, the app has to register the operators with `nnfw_register_custom_op_info`.
+
+
+```
+/*
+ * custom operation registration info
+ */
+typedef struct
+{
+  nnfw_custom_eval eval_function;
+} custom_kernel_registration_info;
+
+NNFW_STATUS nnfw_register_custom_op_info(nnfw_session *session, const char *id,
+                                         custom_kernel_registration_info *info)
+```
+
+Please find sample app in `nnfw` repository
+
+The `id` should be unique in an app.
+