From de17d29ae0f3e4bbc1e1ff61071472f051cdce75 Mon Sep 17 00:00:00 2001
From: Johannes Stoelp <johannes.stoelp@gmail.com>
Date: Sat, 29 Oct 2022 21:49:26 +0200
Subject: linux: add notes about input char devices

---
 src/linux/input.md | 68 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)
 create mode 100644 src/linux/input.md

(limited to 'src/linux/input.md')

diff --git a/src/linux/input.md b/src/linux/input.md
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+# Linux input
+
+Some notes on using `/dev/input/*` device driver files.
+
+## mouseX / mice
+
+These device files are created by the [mousedev] driver.
+
+- `/dev/input/mouseX` represents the input stream for a *SINGLE* mouse device.
+- `/dev/input/mice` represents the merged input stream for *ALL* mouse devices.
+
+The data stream consists of `3 bytes` per `event`. An event is encoded as `(BTN, X, Y)`.
+- `BTN` button pressed
+- `X` movement in x-direction `-1 -> left` and `1 -> right`
+- `Y` movement in y-direction `-1 -> down` and `1 -> up`
+
+The raw data stream can be inspected as follows.
+```bash
+sudo cat /dev/input/mice | od -tx1 -w3 -v
+```
+
+## eventX
+
+These device files are created by the [evdev] driver.
+
+- `/dev/input/eventX` represents the generic input event interface a *SINGLE* input deivece.
+
+Input events are encoded as given by the `input_event` struct below. Reading
+from the `eventX` device file will always yield whole number of input events.
+```c
+struct input_event {
+    struct timeval time;
+    unsigned short type;
+    unsigned short code;
+    unsigned int value;
+};
+```
+
+On most 64bit machines the raw data stream can be inspected as follows.
+```bash
+sudo cat /dev/input/event4 | od -tx1 -w24 -v
+```
+
+## Identifyin device files.
+
+To find out which device file is assigned to which input device the following
+file `/proc/bus/input/devices` in the proc filesystem can be consulted.
+
+This yields entries as follows and shows which `Handlers` are assigned to which
+`Name`.
+```
+I: Bus=0018 Vendor=04f3 Product=0033 Version=0000
+N: Name="Elan Touchpad"
+...
+H: Handlers=event15 mouse0
+...
+```
+
+## Example: Toying with `/dev/input/eventX`
+
+Once compiled, the example should be run as `sudo ./event /dev/input/eventX`.
+```c
+{{#include src/event.c:7: }}
+```
+
+[linux-input]: https://www.kernel.org/doc/Documentation/input/input.txt
+[mousedev]: TODO /home/johannst/dev/linux/drivers/input/mousedev.c
+[evdev]: TODO /home/johannst/dev/linux/drivers/input/evdev.c
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