Wacom__ArtPen.bpf.c

// SPDX-License-Identifier: GPL-2.0-only
/* Copyright (c) 2024 Benjamin Tissoires
 */

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

#define VID_WACOM		0x056a
#define ART_PEN_ID		0x0804
#define PID_INTUOS_PRO_2_M	0x0357

HID_BPF_CONFIG(
	HID_DEVICE(BUS_USB, HID_GROUP_GENERIC, VID_WACOM, PID_INTUOS_PRO_2_M)
);

/*
 * This filter is here for the Art Pen stylus only:
 * - when used on some Wacom devices (see the list of attached PIDs), this pen
 *   reports pressure every other events.
 * - to solve that, given that we know that the next event will be the same as
 *   the current one, we can emulate a smoother pressure reporting by reporting
 *   the mean of the previous value and the current one.
 *
 * We are effectively delaying the pressure by one event every other event, but
 * that's less of an annoyance compared to the chunkiness of the reported data.
 *
 * For example, let's assume the following set of events:
 * <Tip switch 0> <X 0> <Y 0> <Pressure    0 > <Tooltype 0x0804>
 * <Tip switch 1> <X 1> <Y 1> <Pressure  100 > <Tooltype 0x0804>
 * <Tip switch 1> <X 2> <Y 2> <Pressure  100 > <Tooltype 0x0804>
 * <Tip switch 1> <X 3> <Y 3> <Pressure  200 > <Tooltype 0x0804>
 * <Tip switch 1> <X 4> <Y 4> <Pressure  200 > <Tooltype 0x0804>
 * <Tip switch 0> <X 5> <Y 5> <Pressure    0 > <Tooltype 0x0804>
 *
 * The filter will report:
 * <Tip switch 0> <X 0> <Y 0> <Pressure    0 > <Tooltype 0x0804>
 * <Tip switch 1> <X 1> <Y 1> <Pressure * 50*> <Tooltype 0x0804>
 * <Tip switch 1> <X 2> <Y 2> <Pressure  100 > <Tooltype 0x0804>
 * <Tip switch 1> <X 3> <Y 3> <Pressure *150*> <Tooltype 0x0804>
 * <Tip switch 1> <X 4> <Y 4> <Pressure  200 > <Tooltype 0x0804>
 * <Tip switch 0> <X 5> <Y 5> <Pressure    0 > <Tooltype 0x0804>
 *
 */

struct wacom_params {
	__u16 pid;
	__u16 rdesc_len;
	__u8 report_id;
	__u8 report_len;
	struct {
		__u8 tip_switch;
		__u8 pressure;
		__u8 tool_type;
	} offsets;
};

/*
 * Multiple device can support the same stylus, so
 * we need to know which device has which offsets
 */
static const struct wacom_params devices[] = {
	{
		.pid = PID_INTUOS_PRO_2_M,
		.rdesc_len = 949,
		.report_id = 16,
		.report_len = 27,
		.offsets = {
			.tip_switch = 1,
			.pressure = 8,
			.tool_type = 25,
		},
	},
};

static struct wacom_params params = { 0 };

/* HID-BPF reports a 64 bytes chunk anyway, so this ensures
 * the verifier to know we are addressing the memory correctly
 */
#define PEN_REPORT_LEN		64

/* only odd frames are modified */
static bool odd;

static __u16 prev_pressure;

static inline void *get_bits(__u8 *data, unsigned int byte_offset)
{
	return data + byte_offset;
}

static inline __u16 *get_u16(__u8 *data, unsigned int offset)
{
	return (__u16 *)get_bits(data, offset);
}

static inline __u8 *get_u8(__u8 *data, unsigned int offset)
{
	return (__u8 *)get_bits(data, offset);
}

SEC(HID_BPF_DEVICE_EVENT)
int BPF_PROG(artpen_pressure_interpolate, struct hid_bpf_ctx *hctx)
{
	__u8 *data = hid_bpf_get_data(hctx, 0 /* offset */, PEN_REPORT_LEN /* size */);
	__u16 *pressure, *tool_type;
	__u8 *tip_switch;

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

	if (data[0] != params.report_id ||
	    params.offsets.tip_switch >= PEN_REPORT_LEN ||
	    params.offsets.pressure >= PEN_REPORT_LEN - 1 ||
	    params.offsets.tool_type >= PEN_REPORT_LEN - 1)
		return 0; /* invalid report or parameters */

	tool_type = get_u16(data, params.offsets.tool_type);
	if (*tool_type != ART_PEN_ID)
		return 0;

	tip_switch = get_u8(data, params.offsets.tip_switch);
	if ((*tip_switch & 0x01) == 0) {
		prev_pressure = 0;
		odd = true;
		return 0;
	}

	pressure = get_u16(data, params.offsets.pressure);

	if (odd)
		*pressure = (*pressure + prev_pressure) / 2;

	prev_pressure = *pressure;
	odd = !odd;

	return 0;
}

HID_BPF_OPS(wacom_artpen) = {
	.hid_device_event = (void *)artpen_pressure_interpolate,
};

SEC("syscall")
int probe(struct hid_bpf_probe_args *ctx)
{
	struct hid_bpf_ctx *hid_ctx;
	__u16 pid;
	int i;

	/* get a struct hid_device to access the actual pid of the device */
	hid_ctx = hid_bpf_allocate_context(ctx->hid);
	if (!hid_ctx) {
		ctx->retval = -ENODEV;
		return -1; /* EPERM check */
	}
	pid = hid_ctx->hid->product;

	ctx->retval = -EINVAL;

	/* Match the given device with the list of known devices */
	for (i = 0; i < ARRAY_SIZE(devices); i++) {
		const struct wacom_params *device = &devices[i];

		if (device->pid == pid && device->rdesc_len == ctx->rdesc_size) {
			params = *device;
			ctx->retval = 0;
		}
	}

	hid_bpf_release_context(hid_ctx);
	return 0;
}

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