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+# Usb device hooking and emulation
+
+This document gives you an overview of how pumptools handles hooking of usb devices. To be more specific, it does that
+by hooking the API calls of the libusb 0.1 library. Why 0.1 and not 1.0? Because that's the version all pump games have
+used so far. However, pumptools is using the libusb 1.0 API for any communication with real usb devices because 0.1 is
+considered deprecated for quite a while now.
+
+## Architecture: PIUIO example
+
+The following ASCII diagram depicts the various modules and how they are involved in the flow for handling the USB PIUIO.
+
+```text
+                                                                        +--------------+  +--------------+
+                                                                        | usb-init-fix |  | usb-hook-mon |
+                                                                        | module       |  | module       |
+              One update cycle:                                         +--------------+  +--------------+
+                4x ctrl out                                                     ^                ^
+                4x ctrl in                                                      |                |                                                                                                                +----------+
+                interleaved      +-----------+             +---------+          |                |                                   +------------+                                                               | ptapi    |
++----------+    per sensor       |           |  forward    |         |          |                |       forward                     |            |                                                          +--->| piuio    |
+| Game PIU +-------------------->|           +------------>|         +----------+----------------+---------------------------------->|            |                                                          |    | keyboard |
++----------+                     |           |             |         |          |                |                                   |            |                                                          |    +----------+
+                                 |           |             |         +----------+                |                                   |            |     +----------+      +--------+      +---------------+  |
+              One update cycle:  | libusb0.1 |             | usbhook |                           |                     4x ctrl out   | piuio-exit |     | usb-emu  |      | piuio  |      | api pumptools +--+
+               1x ctrl out       |    API    |             | module  +---------------------------+                     4x ctrl in    | module     +---->| module   +----->| module +----->| piuio         +--+
+               (in data on call  |           |             |         |                              +-------------+    interleaved   |            |     +----------+      +--------+      +---------------+  |
++----------+   return in buffer) |           |  forward    |         |           forward            | piuio-khack |    per sensor    |            |                                                          |     +-------+
+| Game PRO +-------------------->|           +------------>|         +----------------------------> | module      +----------------->|            |                                                          |     | ptapi |    +-----------+
++----------+                     |           |             |         |                              +-------------+                  |            |                                                          +---->| piuio +--->| libusb1.0 |
+                                 +-----------+             +---------+                                                               +------------+                                                                | real  |    +-----------+
+                                                                                                                                                                                                                   +-------+
+```
+
+Rough idea of how this works:
+
+* Classic PIU games issue 8 calls via the libusb 0.1 API
+* PRO instead issues only a single call to reduce overhead
+    * On a stock PRO OS and setup, this is intercepted by a kernel module
+* All calls that go to libusb0.1 are intercepted by pumptools's `usbhook` module
+    * A separate `usb-init-fix` module takes care of some quirks when calling init more than once
+    * `usb-hook-mon` is used for logging all intercepted calls for tracing and debugging purpose
+* In general, any calls are forwaded by an internal API exposed by `usbhook` to any hooks registering for them
+* `piuio-khack` intercepts the single ctrl out call issued on PRO by the PIUIO device. It converts it to the call pattern
+equivalent to the classic PIU games. This allows us to transparently handle them further downstream
+* Next comes the `piuio-exit` module which checks whether the the service and test buttons are pressed on a ctrl-transfer
+level (allowing this to be used without any emulation layer by real devices) to exit the process
+* The `usb-emu` module creates an abstraction layer for the libusb 0.1 API calls. Furthermore, it allows for injecting
+fake stub devices if the application requires IO hardware to be available to work
+* The `piuio` module dispatches the in and out ctrl calls, extracts the piuio data from the buffers and hooks it up
+to pumptools's piuio API layer
+* For the API layer, we can have different implementations, e.g. keyboard or a PIUIO driver talking to a real IO using
+libusb 1.0
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