linux: add notes about input char devices

4 files changed, 154 insertions, 0 deletions

diff --git a/src/linux/README.md b/src/linux/README.md
index 00e7e68..0cd68a5 100644
--- a/src/linux/README.md
+++ b/src/linux/README.md
@@ -5,3 +5,4 @@
 - [ptrace_scope](./ptrace_scope.md)
 - [cryptsetup](./cryptsetup.md)
 - [swap](./swap.md)
+- [input](./input.md)
diff --git a/src/linux/input.md b/src/linux/input.md
new file mode 100644
index 0000000..8a9771e
--- /dev/null
+++ b/src/linux/input.md
@@ -0,0 +1,68 @@
+# 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
diff --git a/src/linux/src/Makefile b/src/linux/src/Makefile
new file mode 100644
index 0000000..9b6e47a
--- /dev/null
+++ b/src/linux/src/Makefile
@@ -0,0 +1,10 @@
+SRC = event.c
+BIN = $(SRC:.c=)
+
+all: $(BIN)
+
+%: %.c
+	bash $<
+
+clean:
+	$(RM) $(BIN)
diff --git a/src/linux/src/event.c b/src/linux/src/event.c
new file mode 100644
index 0000000..d047416
--- /dev/null
+++ b/src/linux/src/event.c
@@ -0,0 +1,75 @@
+#if NODEF
+gcc -o event event.c -Wall -Wextra -Werror -g
+exit 0
+#endif
+// Copyright (C) 2022 johannst
+
+#include <stdio.h>
+#include <fcntl.h>
+#include <assert.h>
+#include <unistd.h>
+#include <time.h>
+
+#include <sys/time.h>
+#include <linux/input-event-codes.h>
+
+struct input_event {
+    struct timeval time;
+    unsigned short type;
+    unsigned short code;
+    unsigned int value;
+};
+
+const char* type(unsigned short t) {
+    static char buf[32];
+    const char* fmt = "0x%x";
+    switch (t) {
+#define FMT(TYPE) case TYPE: fmt = #TYPE"(0x%x)"; break
+        FMT(EV_SYN);
+        FMT(EV_KEY);
+        FMT(EV_REL);
+        FMT(EV_ABS);
+#undef FMT
+    }
+    snprintf(buf, sizeof(buf), fmt, t);
+    return buf;
+}
+
+const char* code(unsigned short c) {
+    static char buf[32];
+    const char* fmt = "0x%x";
+    switch (c) {
+#define FMT(CODE) case CODE: fmt = #CODE"(0x%x)"; break
+        FMT(BTN_LEFT);
+        FMT(BTN_RIGHT);
+        FMT(BTN_MIDDLE);
+        FMT(REL_X);
+        FMT(REL_Y);
+#undef FMT
+    }
+    snprintf(buf, sizeof(buf), fmt, c);
+    return buf;
+}
+
+const char* timefmt(const struct timeval* t) {
+    assert(t);
+    struct tm* lt = localtime(&t->tv_sec); // Returns pointer to static tm object.
+    static char buf[64];
+    strftime(buf, sizeof(buf), "%H:%M:%S", lt);
+    return buf;
+}
+
+int main(int argc, char* argv[]) {
+    assert(argc == 2);
+
+    int fd = open(argv[1], O_RDONLY);
+    assert(fd != -1);
+
+    struct input_event inp;
+    while (1) {
+        int ret = read(fd, &inp, sizeof(inp));
+        assert(ret == sizeof(inp));
+        printf("time: %s type: %s code: %s value: 0x%x\n",
+               timefmt(&inp.time), type(inp.type), code(inp.code), inp.value);
+    }
+}