Summary

• Add readonly parameter to InputDevice.__init__() (readonly=False by default, no behavior change)
• Add writable parameter to list_devices() and is_device() (writable=True by default, no behavior change)

Motivation

Currently InputDevice.__init__() always attempts O_RDWR first and falls back to O_RDONLY on failure. This has two practical issues:

1. Permission-restricted environments: When a process only has read access to /dev/input/eventX (common without udev rules or root), the O_RDWR attempt always fails before falling back. The readonly=True parameter lets callers skip the unnecessary failed syscall.

2. API correctness: Callers that only need to read device state (e.g. querying leds(), capabilities()) have no way to express "I only need read access" — the constructor always attempts write access regardless of intent.

The writable parameter on list_devices()/is_device() complements this by allowing enumeration of devices that are readable but not writable.

The existing need_write decorator on EventIO.write() already guards against writes on read-only fds, so no additional protection is needed.

Note on LED pulsing issue

A real-world report described touchpad LED pulsing when polling InputDevice.leds() at 1–2 Hz. After investigating the Linux kernel source (drivers/input/evdev.c, drivers/input/input.c), the root cause is not O_RDWR vs O_RDONLY — the kernel's evdev_open() does not distinguish between open modes at all. Both go through the same path: evdev_open() → input_open_device() → dev->open(dev).

The actual cause is the repeated open/close cycle itself. Each time the fd is closed, dev->users drops to 0; the next open triggers the hardware driver's open() callback again (e.g. hidinput_open() → hid_hw_open() → USB/I2C transport layer reinitialization), which on certain hardware (Tuxedo Stellaris 15 Gen3) causes the EC firmware to re-assert LED state.

The correct fix for the LED pulsing issue is to keep the InputDevice fd open across queries rather than creating/destroying it each time. The EVIOCGLED ioctl used by leds() is a pure kernel memory read (bitmap_copy) and does not touch hardware.

Changes

Usage

# Read-only device open — no O_RDWR attempted
device = InputDevice('/dev/input/event0', readonly=True)
print(device.leds())  # works
device.set_led(LED_NUML, 1)  # raises EvdevError (existing need_write guard)

# List readable-only devices
paths = list_devices(writable=False)

Relates to #35