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Started to document the flow of original uinput and netlink

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Johannes Leupolz 2025-12-02 23:05:51 +00:00
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docs/DESIGN.md
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@ -57,6 +57,116 @@ libinput/game->>eventX: open /dev/input/eventX
`vuinputd` forwards udev events into the container via netlink, because otherwise the game in the container would not recognize when its net namespace is different from the one of udevd.
The external visible state is determined by actions originating from the following locations (WIP):
1) userspace
1.1) ioctl UI_DEV_CREATE to open file handle of `/dev/uinput`
2) the linux kernel (device creation)
2.1) uinput_create_device [uinput.c](https://github.com/torvalds/linux/blob/master/drivers/input/misc/uinput.c)
2.2) input_register_device [input.c](https://github.com/torvalds/linux/blob/master/drivers/input/input.c)
2.3) device_add in [core.c](https://github.com/torvalds/linux/blob/master/drivers/base/core.c)
2.3.1) device_create_file in [core.c](https://github.com/torvalds/linux/blob/master/drivers/base/core.c): create sysfs attribute file for device.
2.3.2) create diverse symlinks and data for the sysfs entry
2.3.3) kobject_uevent in [kobject_uevent](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/input97`
3) udev in userspace
- udev_event_execute_rules in [udev-event.c](https://github.com/systemd/systemd/blob/main/src/udev/udev-event.c#)
Example of kernel messages that were send via netlink
```
KERNEL[1674737.476205] add /devices/virtual/input/input155 (input)
ACTION=add
DEVPATH=/devices/virtual/input/input155
SUBSYSTEM=input
PRODUCT=3/beef/dead/0
NAME="Example device"
PROP=0
EV=7
KEY=10000 0 0 0 0
REL=3
MODALIAS=input:b0003vBEEFpDEADe0000-e0,1,2,k110,r0,1,amlsfw
SEQNUM=18608
KERNEL[1674737.476270] add /devices/virtual/input/input155/mouse2 (input)
ACTION=add
DEVPATH=/devices/virtual/input/input155/mouse2
SUBSYSTEM=input
DEVNAME=/dev/input/mouse2
SEQNUM=18609
MAJOR=13
MINOR=34
KERNEL[1674737.476328] add /devices/virtual/input/input155/event12 (input)
ACTION=add
DEVPATH=/devices/virtual/input/input155/event12
SUBSYSTEM=input
DEVNAME=/dev/input/event12
SEQNUM=18610
MAJOR=13
MINOR=76
```
Example of udev (after adding the hwdb and rules entries)
```
UDEV [1674737.478882] add /devices/virtual/input/input155 (input)
ACTION=add
DEVPATH=/devices/virtual/input/input155
SUBSYSTEM=input
PRODUCT=3/beef/dead/0
NAME="Example device"
PROP=0
EV=7
KEY=10000 0 0 0 0
REL=3
MODALIAS=input:b0003vBEEFpDEADe0000-e0,1,2,k110,r0,1,amlsfw
SEQNUM=18608
USEC_INITIALIZED=1674737476194
ID_VUINPUT=1
ID_INPUT=1
.INPUT_CLASS=mouse
ID_SERIAL=noserial
ID_VUINPUT_MOUSE=1
ID_SEAT=seat_vuinput
TAGS=:seat:
CURRENT_TAGS=:seat:
UDEV [1674737.480016] add /devices/virtual/input/input155/mouse2 (input)
ACTION=add
DEVPATH=/devices/virtual/input/input155/mouse2
SUBSYSTEM=input
DEVNAME=/dev/input/mouse2
SEQNUM=18609
USEC_INITIALIZED=1674737477357
ID_INPUT=1
ID_INPUT_MOUSE=1
.INPUT_CLASS=mouse
ID_SERIAL=noserial
ID_SEAT=seat_vuinput
MAJOR=13
MINOR=34
TAGS=:seat_vuinput:
CURRENT_TAGS=:seat_vuinput:
UDEV [1674737.498627] add /devices/virtual/input/input155/event12 (input)
ACTION=add
DEVPATH=/devices/virtual/input/input155/event12
SUBSYSTEM=input
DEVNAME=/dev/input/event12
SEQNUM=18610
USEC_INITIALIZED=1674737477373
ID_VUINPUT=1
.HAVE_HWDB_PROPERTIES=1
ID_INPUT=1
.INPUT_CLASS=mouse
ID_SERIAL=noserial
ID_SEAT=seat_vuinput
ID_VUINPUT_MOUSE=1
MAJOR=13
MINOR=76
TAGS=:seat_vuinput:
CURRENT_TAGS=:seat_vuinput:
```
---
## 3. Design Decisions