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Added some new points I want to talk about in the DESIGN.md document

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Johannes Leupolz 2025-12-06 19:57:38 +00:00
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@ -447,7 +447,7 @@ Note that evdev_handler is registered as an input_handler during the initializat
2.1.1.2.1.1.1.1) device_create_file() in [core.c](https://github.com/torvalds/linux/blob/master/drivers/base/core.c): creates the sysfs attribute file for the device.
2.1.1.2.1.1.1.2) create diverse symlinks and data for the sysfs entry
2.1.1.2.1.1.1.3) create node via devtmpfs
2.1.1.2.1.1.1.3.1) devtmpfs_create_node() in [devtmpfs.c](https://github.com/torvalds/linux/blob/master/drivers/base/devtmpfs.c) creates `/dev/input/eventY`. name is determined by input_devnode in [input.c](https://github.com/torvalds/linux/blob/master/drivers/input/input.c) which was set via dev_set_name in [evdev.c](https://github.com/torvalds/linux/blob/master/drivers/input/evdev.c)
2.1.1.2.1.1.1.3.1) devtmpfs_create_node() in [devtmpfs.c](https://github.com/torvalds/linux/blob/master/drivers/base/devtmpfs.c) creates `/dev/input/eventY`. name is determined by input_devnode() in [input.c](https://github.com/torvalds/linux/blob/master/drivers/input/input.c) which was set via dev_set_name in [evdev.c](https://github.com/torvalds/linux/blob/master/drivers/input/evdev.c)
2.1.1.2.1.1.1.4) kobject_uevent() in [kobject_uevent.c](https://github.com/torvalds/linux/blob/master/lib/kobject_uevent.c) notify user space via netlink with `DEVPATH` set to the sysfs entry under `/sys`, e.g., `/devices/virtual/input/input155/event12`
3) udev in userspace (TODO)
- udev_event_execute_rules() in [udev-event.c](https://github.com/systemd/systemd/blob/main/src/udev/udev-event.c#)
@ -533,6 +533,39 @@ TAGS=:seat_vuinput:
CURRENT_TAGS=:seat_vuinput:
```
From my investigation, the ioctl thread immediately sees /dev/input/eventX, because:
- UI_DEV_CREATE runs device_add()
- device_add() calls devtmpfs_create_node()
- devtmpfs_create_node() directly creates the node in VFS and waits via a wait_for_completion().
- The kernel returns from those calls only after the node is fully present in the dcache and directory.
### 5.2 CUSE
### 5.3 uinput users
#### 5.3.1 inputtino
https://github.com/games-on-whales/inputtino
Library used by wolf and sunshine
#### 5.3.2 Steam
https://github.com/games-on-whales/inputtino
Library used by wolf and sunshine
### 5.4 Applications that use the created devices
#### 5.4.1 SDL
https://github.com/libsdl-org/SDL/blob/main/src/joystick/linux/SDL_sysjoystick.c
#### 5.4.2 libudev and netlink
#### 5.4.3 libinput
#### 5.4.4. Proton
## 6. Alternative Approaches