Contributors: 1
Author Tokens Token Proportion Commits Commit Proportion
Benjamin Tissoires 1385 100.00% 1 100.00%
Total 1385 1 


// SPDX-License-Identifier: GPL-2.0-only
/* Copyright (c) 2025 Red Hat
 */

#include "vmlinux.h"
#include "hid_bpf.h"
#include "hid_bpf_helpers.h"
#include <bpf/bpf_tracing.h>

#define VID_WALTOP 0x172F
#define PID_BATTERYLESS_TABLET 0x0505

HID_BPF_CONFIG(
	HID_DEVICE(BUS_USB, HID_GROUP_ANY, VID_WALTOP, PID_BATTERYLESS_TABLET)
);

#define EXPECTED_RDESC_SIZE 335
#define PEN_REPORT_ID 16

#define TIP_SWITCH BIT(0)
#define BARREL_SWITCH BIT(1)
#define SECONDARY_BARREL_SWITCH BIT(5)

static __u8 last_button_state;

static const __u8 fixed_rdesc[] = {
	0x05, 0x01,                    // Usage Page (Generic Desktop)
	0x09, 0x02,                    // Usage (Mouse)
	0xa1, 0x01,                    // Collection (Application)
	0x85, 0x01,                    //   Report ID (1)
	0x09, 0x01,                    //   Usage (Pointer)
	0xa1, 0x00,                    //   Collection (Physical)
	0x05, 0x09,                    //     Usage Page (Button)
	0x19, 0x01,                    //     Usage Minimum (1)
	0x29, 0x05,                    //     Usage Maximum (5)
	0x15, 0x00,                    //     Logical Minimum (0)
	0x25, 0x01,                    //     Logical Maximum (1)
	0x75, 0x01,                    //     Report Size (1)
	0x95, 0x05,                    //     Report Count (5)
	0x81, 0x02,                    //     Input (Data,Var,Abs)
	0x75, 0x03,                    //     Report Size (3)
	0x95, 0x01,                    //     Report Count (1)
	0x81, 0x03,                    //     Input (Cnst,Var,Abs)
	0x05, 0x01,                    //     Usage Page (Generic Desktop)
	0x09, 0x30,                    //     Usage (X)
	0x09, 0x31,                    //     Usage (Y)
	0x09, 0x38,                    //     Usage (Wheel)
	0x15, 0x81,                    //     Logical Minimum (-127)
	0x25, 0x7f,                    //     Logical Maximum (127)
	0x75, 0x08,                    //     Report Size (8)
	0x95, 0x03,                    //     Report Count (3)
	0x81, 0x06,                    //     Input (Data,Var,Rel)
	0x05, 0x0c,                    //     Usage Page (Consumer)
	0x15, 0x81,                    //     Logical Minimum (-127)
	0x25, 0x7f,                    //     Logical Maximum (127)
	0x75, 0x08,                    //     Report Size (8)
	0x95, 0x01,                    //     Report Count (1)
	0x0a, 0x38, 0x02,              //     Usage (AC Pan)
	0x81, 0x06,                    //     Input (Data,Var,Rel)
	0xc0,                          //   End Collection
	0xc0,                          // End Collection
	0x05, 0x0d,                    // Usage Page (Digitizers)
	0x09, 0x02,                    // Usage (Pen)
	0xa1, 0x01,                    // Collection (Application)
	0x85, 0x02,                    //   Report ID (2)
	0x09, 0x20,                    //   Usage (Stylus)
	0xa1, 0x00,                    //   Collection (Physical)
	0x09, 0x00,                    //     Usage (0x0000)
	0x15, 0x00,                    //     Logical Minimum (0)
	0x26, 0xff, 0x00,              //     Logical Maximum (255)
	0x75, 0x08,                    //     Report Size (8)
	0x95, 0x09,                    //     Report Count (9)
	0x81, 0x02,                    //     Input (Data,Var,Abs)
	0x09, 0x3f,                    //     Usage (Azimuth)
	0x09, 0x40,                    //     Usage (Altitude)
	0x15, 0x00,                    //     Logical Minimum (0)
	0x26, 0xff, 0x00,              //     Logical Maximum (255)
	0x75, 0x08,                    //     Report Size (8)
	0x95, 0x02,                    //     Report Count (2)
	0xb1, 0x02,                    //     Feature (Data,Var,Abs)
	0xc0,                          //   End Collection
	0x85, 0x05,                    //   Report ID (5)
	0x05, 0x0d,                    //   Usage Page (Digitizers)
	0x09, 0x20,                    //   Usage (Stylus)
	0xa1, 0x00,                    //   Collection (Physical)
	0x09, 0x00,                    //     Usage (0x0000)
	0x15, 0x00,                    //     Logical Minimum (0)
	0x26, 0xff, 0x00,              //     Logical Maximum (255)
	0x75, 0x08,                    //     Report Size (8)
	0x95, 0x07,                    //     Report Count (7)
	0x81, 0x02,                    //     Input (Data,Var,Abs)
	0xc0,                          //   End Collection
	0x85, 0x0a,                    //   Report ID (10)
	0x05, 0x0d,                    //   Usage Page (Digitizers)
	0x09, 0x20,                    //   Usage (Stylus)
	0xa1, 0x00,                    //   Collection (Physical)
	0x09, 0x00,                    //     Usage (0x0000)
	0x15, 0x00,                    //     Logical Minimum (0)
	0x26, 0xff, 0x00,              //     Logical Maximum (255)
	0x75, 0x08,                    //     Report Size (8)
	0x95, 0x07,                    //     Report Count (7)
	0x81, 0x02,                    //     Input (Data,Var,Abs)
	0xc0,                          //   End Collection
	0x85, 0x10,                    //   Report ID (16)
	0x09, 0x20,                    //   Usage (Stylus)
	0xa1, 0x00,                    //   Collection (Physical)
	0x09, 0x42,                    //     Usage (Tip Switch)
	0x09, 0x44,                    //     Usage (Barrel Switch)
	0x09, 0x3c,                    //     Usage (Invert)
	0x09, 0x45,                    //     Usage (Eraser)
	0x09, 0x32,                    //     Usage (In Range)
	0x09, 0x5a,                    //     Usage (Secondary Barrel Switch)  <-- added
	0x15, 0x00,                    //     Logical Minimum (0)
	0x25, 0x01,                    //     Logical Maximum (1)
	0x75, 0x01,                    //     Report Size (1)
	0x95, 0x06,                    //     Report Count (6)                 <--- changed from 5
	0x81, 0x02,                    //     Input (Data,Var,Abs)
	0x95, 0x02,                    //     Report Count (2)                 <--- changed from 3
	0x81, 0x03,                    //     Input (Cnst,Var,Abs)
	0x05, 0x01,                    //     Usage Page (Generic Desktop)
	0x09, 0x30,                    //     Usage (X)
	0x75, 0x10,                    //     Report Size (16)
	0x95, 0x01,                    //     Report Count (1)
	0xa4,                          //     Push
	0x55, 0x0d,                    //       Unit Exponent (-3)
	0x65, 0x33,                    //       Unit (EnglishLinear: in³)
	0x15, 0x00,                    //       Logical Minimum (0)
	0x26, 0x00, 0x7d,              //       Logical Maximum (32000)
	0x35, 0x00,                    //       Physical Minimum (0)
	0x46, 0x00, 0x7d,              //       Physical Maximum (32000)
	0x81, 0x02,                    //       Input (Data,Var,Abs)
	0x09, 0x31,                    //       Usage (Y)
	0x15, 0x00,                    //       Logical Minimum (0)
	0x26, 0x20, 0x4e,              //       Logical Maximum (20000)
	0x35, 0x00,                    //       Physical Minimum (0)
	0x46, 0x20, 0x4e,              //       Physical Maximum (20000)
	0x81, 0x02,                    //       Input (Data,Var,Abs)
	0x05, 0x0d,                    //       Usage Page (Digitizers)
	0x09, 0x30,                    //       Usage (Tip Pressure)
	0x15, 0x00,                    //       Logical Minimum (0)
	0x26, 0xff, 0x07,              //       Logical Maximum (2047)
	0x35, 0x00,                    //       Physical Minimum (0)
	0x46, 0xff, 0x07,              //       Physical Maximum (2047)
	0x81, 0x02,                    //       Input (Data,Var,Abs)
	0x05, 0x0d,                    //       Usage Page (Digitizers)
	0x09, 0x3d,                    //       Usage (X Tilt)
	0x09, 0x3e,                    //       Usage (Y Tilt)
	0x15, 0xc4,                    //       Logical Minimum (-60)          <- changed from -127
	0x25, 0x3c,                    //       Logical Maximum (60)           <- changed from 127
	0x75, 0x08,                    //       Report Size (8)
	0x95, 0x02,                    //       Report Count (2)
	0x81, 0x02,                    //       Input (Data,Var,Abs)
	0xc0,                          //     End Collection
	0xc0,                          //   End Collection
	0x05, 0x01,                    //   Usage Page (Generic Desktop)
	0x09, 0x06,                    //   Usage (Keyboard)
	0xa1, 0x01,                    //   Collection (Application)
	0x85, 0x0d,                    //     Report ID (13)
	0x05, 0x07,                    //     Usage Page (Keyboard/Keypad)
	0x19, 0xe0,                    //     Usage Minimum (224)
	0x29, 0xe7,                    //     Usage Maximum (231)
	0x15, 0x00,                    //     Logical Minimum (0)
	0x25, 0x01,                    //     Logical Maximum (1)
	0x75, 0x01,                    //     Report Size (1)
	0x95, 0x08,                    //     Report Count (8)
	0x81, 0x02,                    //     Input (Data,Var,Abs)
	0x75, 0x08,                    //     Report Size (8)
	0x95, 0x01,                    //     Report Count (1)
	0x81, 0x01,                    //     Input (Cnst,Arr,Abs)
	0x05, 0x07,                    //     Usage Page (Keyboard/Keypad)
	0x19, 0x00,                    //     Usage Minimum (0)
	0x29, 0x65,                    //     Usage Maximum (101)
	0x15, 0x00,                    //     Logical Minimum (0)
	0x25, 0x65,                    //     Logical Maximum (101)
	0x75, 0x08,                    //     Report Size (8)
	0x95, 0x05,                    //     Report Count (5)
	0x81, 0x00,                    //     Input (Data,Arr,Abs)
	0xc0,                          //   End Collection
	0x05, 0x0c,                    //   Usage Page (Consumer)
	0x09, 0x01,                    //   Usage (Consumer Control)
	0xa1, 0x01,                    //   Collection (Application)
	0x85, 0x0c,                    //     Report ID (12)
	0x09, 0xe9,                    //     Usage (Volume Increment)
	0x09, 0xea,                    //     Usage (Volume Decrement)
	0x09, 0xe2,                    //     Usage (Mute)
	0x15, 0x00,                    //     Logical Minimum (0)
	0x25, 0x01,                    //     Logical Maximum (1)
	0x75, 0x01,                    //     Report Size (1)
	0x95, 0x03,                    //     Report Count (3)
	0x81, 0x06,                    //     Input (Data,Var,Rel)
	0x75, 0x05,                    //     Report Size (5)
	0x95, 0x01,                    //     Report Count (1)
	0x81, 0x07,                    //     Input (Cnst,Var,Rel)
	0xc0,                          //   End Collection
};

static inline unsigned int bitwidth32(__u32 x)
{
	return 32 - __builtin_clzg(x, 32);
}

static inline unsigned int floor_log2_32(__u32 x)
{
	return bitwidth32(x) - 1;
}

/* Maps the interval [0, 2047] to itself using a scaled
 * approximation of the function log2(x+1).
 */
static unsigned int scaled_log2(__u16 v)
{
	const unsigned int XMAX = 2047;
	const unsigned int YMAX = 11; /* log2(2048) = 11 */

	unsigned int x = v + 1;
	unsigned int n = floor_log2_32(x);
	unsigned int b = 1 << n;

	/* Fixed-point fraction in [0, 1), linearly
	 * interpolated using delta-y = 1 and
	 * delta-x = (2b - b) = b.
	 */
	unsigned int frac = (x - b) << YMAX;
	unsigned int lerp = frac / b;
	unsigned int log2 = (n << YMAX) + lerp;

	return ((log2 * XMAX) / YMAX) >> YMAX;
}

SEC(HID_BPF_RDESC_FIXUP)
int BPF_PROG(hid_fix_rdesc, struct hid_bpf_ctx *hctx)
{
	__u8 *data = hid_bpf_get_data(hctx, 0 /* offset */, 4096 /* size */);

	if (!data)
		return 0; /* EPERM check */

	__builtin_memcpy(data, fixed_rdesc, sizeof(fixed_rdesc));

	return sizeof(fixed_rdesc);
}

SEC(HID_BPF_DEVICE_EVENT)
int BPF_PROG(waltop_fix_events, struct hid_bpf_ctx *hctx)
{
	__u8 *data = hid_bpf_get_data(hctx, 0 /* offset */, 10 /* size */);

	if (!data)
		return 0; /* EPERM check */

	__u8 report_id = data[0];

	if (report_id != PEN_REPORT_ID)
		return 0;

	/* On this tablet if the secondary barrel switch is pressed,
	 * the tablet sends tip down and barrel down. Change this to
	 * just secondary barrel down when there is no ambiguity.
	 *
	 * It's possible that there is a bug in the firmware and the
	 * device intends to set invert + eraser instead (i.e. the
	 * pysical button is an eraser button) but since
	 * the pressure is always zero, said eraser button
	 * would be useless anyway.
	 *
	 * So let's just change the button to secondary barrel down.
	 */

	__u8 tip_switch = data[1] & TIP_SWITCH;
	__u8 barrel_switch = data[1] & BARREL_SWITCH;

	__u8 tip_held = last_button_state & TIP_SWITCH;
	__u8 barrel_held = last_button_state & BARREL_SWITCH;

	if (tip_switch && barrel_switch && !tip_held && !barrel_held) {
		data[1] &= ~(TIP_SWITCH | BARREL_SWITCH); /* release tip and barrel */
		data[1] |= SECONDARY_BARREL_SWITCH; /* set secondary barrel switch */
	}

	last_button_state = data[1];

	/* The pressure sensor on this tablet maps around half of the
	 * logical pressure range into the interval [0-100]. Further
	 * pressure causes the sensor value to increase exponentially
	 * up to a maximum value of 2047.
	 *
	 * The values 12 and 102 were chosen to have an integer slope
	 * with smooth transition between the two curves around the
	 * value 100.
	 */

	__u16 pressure = (((__u16)data[6]) << 0) | (((__u16)data[7]) << 8);

	if (pressure <= 102)
		pressure *= 12;
	else
		pressure = scaled_log2(pressure);

	data[6] = pressure >> 0;
	data[7] = pressure >> 8;

	return 0;
}

HID_BPF_OPS(waltop_batteryless) = {
	.hid_device_event = (void *)waltop_fix_events,
	.hid_rdesc_fixup = (void *)hid_fix_rdesc,
};

SEC("syscall")
int probe(struct hid_bpf_probe_args *ctx)
{
	if (ctx->rdesc_size == EXPECTED_RDESC_SIZE)
		ctx->retval = 0;
	else
		ctx->retval = -EINVAL;

	return 0;
}

char _license[] SEC("license") = "GPL";