Commit 040adbe8 authored by Benjamin Tissoires's avatar Benjamin Tissoires Committed by Jiri Kosina
Browse files

HID: bpf: Add support for the Waltop Batteryless Tablet 



Several bugs as outlined in udev-hid-bpf#66 and udev-hid-bpf!198:
- pressure curve is far from linear
- tilt range is ±60, not ±127
- pressing the second button sets both tip down and
  barrel switch

Fix the second button by adding a Secondary Barrel Switch in the
existing padding and check for the tip down/barrel switch down combo.
When  both values become true at the same time, set the Secondary Barrel
Switch instead.

Implement a custom pressure curve that maps the hardware range 0-102
linearly to the logical range 0-1224, and maps the hardware range
103-2047 logarithmically to the logical range 1232-2047. This mapping
isn’t perfect, but it’s way more natural than the stock configuration.

Signed-off-by: default avatarPeter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: default avatarJan Felix Langenbach <JanFelix.Langenbach@protonmail.com>
Link: https://gitlab.freedesktop.org/libevdev/udev-hid-bpf/-/merge_requests/200


Signed-off-by: default avatarBenjamin Tissoires <bentiss@kernel.org>
Signed-off-by: default avatarJiri Kosina <jkosina@suse.com>
parent 029dff1c

drivers/hid/bpf/progs/WALTOP__Batteryless-Tablet.bpf.c

0 → 100644
+321 −0
Original line number Diff line number Diff line 
// 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";