1// SPDX-License-Identifier: GPL-2.0-only
2/* Copyright (c) 2024 Benjamin Tissoires
3 */
4
5#include "vmlinux.h"
6#include "hid_bpf.h"
7#include "hid_bpf_helpers.h"
8#include <bpf/bpf_tracing.h>
9
10#define VID_WACOM		0x056a
11#define ART_PEN_ID		0x0804
12#define PID_INTUOS_PRO_2_M	0x0357
13
14HID_BPF_CONFIG(
15	HID_DEVICE(BUS_USB, HID_GROUP_GENERIC, VID_WACOM, PID_INTUOS_PRO_2_M)
16);
17
18/*
19 * This filter is here for the Art Pen stylus only:
20 * - when used on some Wacom devices (see the list of attached PIDs), this pen
21 *   reports pressure every other events.
22 * - to solve that, given that we know that the next event will be the same as
23 *   the current one, we can emulate a smoother pressure reporting by reporting
24 *   the mean of the previous value and the current one.
25 *
26 * We are effectively delaying the pressure by one event every other event, but
27 * that's less of an annoyance compared to the chunkiness of the reported data.
28 *
29 * For example, let's assume the following set of events:
30 * <Tip switch 0> <X 0> <Y 0> <Pressure    0 > <Tooltype 0x0804>
31 * <Tip switch 1> <X 1> <Y 1> <Pressure  100 > <Tooltype 0x0804>
32 * <Tip switch 1> <X 2> <Y 2> <Pressure  100 > <Tooltype 0x0804>
33 * <Tip switch 1> <X 3> <Y 3> <Pressure  200 > <Tooltype 0x0804>
34 * <Tip switch 1> <X 4> <Y 4> <Pressure  200 > <Tooltype 0x0804>
35 * <Tip switch 0> <X 5> <Y 5> <Pressure    0 > <Tooltype 0x0804>
36 *
37 * The filter will report:
38 * <Tip switch 0> <X 0> <Y 0> <Pressure    0 > <Tooltype 0x0804>
39 * <Tip switch 1> <X 1> <Y 1> <Pressure * 50*> <Tooltype 0x0804>
40 * <Tip switch 1> <X 2> <Y 2> <Pressure  100 > <Tooltype 0x0804>
41 * <Tip switch 1> <X 3> <Y 3> <Pressure *150*> <Tooltype 0x0804>
42 * <Tip switch 1> <X 4> <Y 4> <Pressure  200 > <Tooltype 0x0804>
43 * <Tip switch 0> <X 5> <Y 5> <Pressure    0 > <Tooltype 0x0804>
44 *
45 */
46
47struct wacom_params {
48	__u16 pid;
49	__u16 rdesc_len;
50	__u8 report_id;
51	__u8 report_len;
52	struct {
53		__u8 tip_switch;
54		__u8 pressure;
55		__u8 tool_type;
56	} offsets;
57};
58
59/*
60 * Multiple device can support the same stylus, so
61 * we need to know which device has which offsets
62 */
63static const struct wacom_params devices[] = {
64	{
65		.pid = PID_INTUOS_PRO_2_M,
66		.rdesc_len = 949,
67		.report_id = 16,
68		.report_len = 27,
69		.offsets = {
70			.tip_switch = 1,
71			.pressure = 8,
72			.tool_type = 25,
73		},
74	},
75};
76
77static struct wacom_params params = { 0 };
78
79/* HID-BPF reports a 64 bytes chunk anyway, so this ensures
80 * the verifier to know we are addressing the memory correctly
81 */
82#define PEN_REPORT_LEN		64
83
84/* only odd frames are modified */
85static bool odd;
86
87static __u16 prev_pressure;
88
89static inline void *get_bits(__u8 *data, unsigned int byte_offset)
90{
91	return data + byte_offset;
92}
93
94static inline __u16 *get_u16(__u8 *data, unsigned int offset)
95{
96	return (__u16 *)get_bits(data, offset);
97}
98
99static inline __u8 *get_u8(__u8 *data, unsigned int offset)
100{
101	return (__u8 *)get_bits(data, offset);
102}
103
104SEC("fmod_ret/hid_bpf_device_event")
105int BPF_PROG(artpen_pressure_interpolate, struct hid_bpf_ctx *hctx)
106{
107	__u8 *data = hid_bpf_get_data(hctx, 0 /* offset */, PEN_REPORT_LEN /* size */);
108	__u16 *pressure, *tool_type;
109	__u8 *tip_switch;
110
111	if (!data)
112		return 0; /* EPERM check */
113
114	if (data[0] != params.report_id ||
115	    params.offsets.tip_switch >= PEN_REPORT_LEN ||
116	    params.offsets.pressure >= PEN_REPORT_LEN - 1 ||
117	    params.offsets.tool_type >= PEN_REPORT_LEN - 1)
118		return 0; /* invalid report or parameters */
119
120	tool_type = get_u16(data, params.offsets.tool_type);
121	if (*tool_type != ART_PEN_ID)
122		return 0;
123
124	tip_switch = get_u8(data, params.offsets.tip_switch);
125	if ((*tip_switch & 0x01) == 0) {
126		prev_pressure = 0;
127		odd = true;
128		return 0;
129	}
130
131	pressure = get_u16(data, params.offsets.pressure);
132
133	if (odd)
134		*pressure = (*pressure + prev_pressure) / 2;
135
136	prev_pressure = *pressure;
137	odd = !odd;
138
139	return 0;
140}
141
142SEC("syscall")
143int probe(struct hid_bpf_probe_args *ctx)
144{
145	struct hid_bpf_ctx *hid_ctx;
146	__u16 pid;
147	int i;
148
149	/* get a struct hid_device to access the actual pid of the device */
150	hid_ctx = hid_bpf_allocate_context(ctx->hid);
151	if (!hid_ctx) {
152		ctx->retval = -ENODEV;
153		return -1; /* EPERM check */
154	}
155	pid = hid_ctx->hid->product;
156
157	ctx->retval = -EINVAL;
158
159	/* Match the given device with the list of known devices */
160	for (i = 0; i < ARRAY_SIZE(devices); i++) {
161		const struct wacom_params *device = &devices[i];
162
163		if (device->pid == pid && device->rdesc_len == ctx->rdesc_size) {
164			params = *device;
165			ctx->retval = 0;
166		}
167	}
168
169	hid_bpf_release_context(hid_ctx);
170	return 0;
171}
172
173char _license[] SEC("license") = "GPL";
174