foo_filter.c

#include <linux/module.h>
#include <linux/serio.h>
#include <linux/vmalloc.h>
#include <linux/i8042.h>

// Simplified i8042 key filter for example purposes

struct key_data {
	bool is_pressed;
	unsigned long updated_at;
};

#define NUM_KEYS 128
#define SIZEOF_KEYS (sizeof(struct key_data) * NUM_KEYS)
static struct key_data *keys;

#define KEYUP_MASK 0x80 // High bit: 1 (keyup), 0 (keydown)
#define KEYID_MASK 0x7f // Remaining 7 bits: which key

static bool foo_filter(unsigned char data, unsigned char str, struct serio *serio) {
	struct key_data *key;

	key = keys + (data & KEYID_MASK);

	key->is_pressed = !(data & KEYUP_MASK);
	key->updated_at = jiffies;

	return false; // let event flow through
}

static int __init foo_init(void) {
	int ret, err;

	ret = i8042_install_filter(foo_filter);
	if (ret) {
		pr_err("Unable to install key filter\n");
		err = -EBUSY;
		goto err_filter;
	}

	keys = vmalloc(SIZEOF_KEYS);
	if (!keys) {
		pr_err("Unable to allocate memory\n");
		err = -ENOMEM;
		goto err_mem;
	}

	memset(keys, 0x00, SIZEOF_KEYS);

	pr_info("foo init\n");
	return 0;

err_mem:
	i8042_remove_filter(foo_filter);
err_filter:
	return err;
}

static void __exit foo_exit(void) {
	i8042_remove_filter(foo_filter);
	vfree(keys);
	pr_info("foo exit\n");
}

module_init(i8042_debounce_init);
module_exit(i8042_debounce_exit);