XPPen__DecoMini4.bpf.c

// SPDX-License-Identifier: GPL-2.0-only
/* Copyright (c) 2024 José Expósito
 */

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

#define VID_UGEE	0x28BD
#define PID_DECO_MINI_4	0x0929
#define RDESC_SIZE_PAD	177
#define RDESC_SIZE_PEN	109
#define PAD_REPORT_ID	0x06

/*
 * XP-Pen devices return a descriptor with the values the driver should use when
 * one of its interfaces is queried. For this device the descriptor is:
 *
 * 0E 03 60 4F 88 3B 06 00 FF 1F D8 13
 *       ----- -----       ----- -----
 *         |     |           |     |
 *         |     |           |     `- Resolution: 5080 (13d8)
 *         |     |           `- Maximum pressure: 8191 (1FFF)
 *         |     `- Logical maximum Y: 15240 (3B88)
 *         `- Logical maximum X: 20320 (4F60)
 *
 * The physical maximum is calculated as (logical_max * 1000) / resolution.
 */
#define LOGICAL_MAX_X	0x60, 0x4F
#define LOGICAL_MAX_Y	0x88, 0x3B
#define PHYSICAL_MAX_X	0xA0, 0x0F
#define PHYSICAL_MAX_Y	0xB8, 0x0B
#define PRESSURE_MAX	0xFF, 0x1F

HID_BPF_CONFIG(
	HID_DEVICE(BUS_USB, HID_GROUP_GENERIC, VID_UGEE, PID_DECO_MINI_4)
);

/*
 * The tablet send these values when the pad buttons are pressed individually:
 *
 *   Buttons released: 06 00 00 00 00 00 00 00
 *   Button 1:         06 00 05 00 00 00 00 00 -> b
 *   Button 2:         06 00 08 00 00 00 00 00 -> e
 *   Button 3:         06 04 00 00 00 00 00 00 -> LAlt
 *   Button 4:         06 00 2c 00 00 00 00 00 -> Space
 *   Button 5:         06 01 16 00 00 00 00 00 -> LControl + s
 *   Button 6:         06 01 1d 00 00 00 00 00 -> LControl + z
 *
 * When multiple buttons are pressed at the same time, the values used to
 * identify the buttons are identical, but they appear in different bytes of the
 * record. For example, when button 2 (0x08) and button 1 (0x05) are pressed,
 * this is the report:
 *
 *   Buttons 2 and 1:  06 00 08 05 00 00 00 00 -> e + b
 *
 * Buttons 1, 2, 4, 5 and 6 can be matched by finding their values in the
 * report.
 *
 * Button 3 is pressed when the 3rd bit is 1. For example, pressing buttons 3
 * and 5 generates this report:
 *
 *   Buttons 3 and 5:  06 05 16 00 00 00 00 00 -> LControl + LAlt + s
 *                        -- --
 *                         |  |
 *                         |  `- Button 5 (0x16)
 *                         `- 0x05 = 0101. Button 3 is pressed
 *                                    ^
 *
 * pad_buttons contains a list of buttons that can be matched in
 * HID_BPF_DEVICE_EVENT. Button 3 as it has a dedicated bit.
 */
static const __u8 pad_buttons[] = { 0x05, 0x08, 0x00, 0x2C, 0x16, 0x1D };

static const __u8 fixed_pad_rdesc[] = {
	0x05, 0x01,           /*  Usage Page (Desktop),                   */
	0x09, 0x07,           /*  Usage (Keypad),                         */
	0xA1, 0x01,           /*  Collection (Application),               */
	0x85, 0x06,           /*      Report ID (6),                      */
	0x05, 0x0D,           /*      Usage Page (Digitizer),             */
	0x09, 0x39,           /*      Usage (Tablet Function Keys),       */
	0xA0,                 /*      Collection (Physical),              */
	0x05, 0x09,           /*          Usage Page (Button),            */
	0x75, 0x01,           /*          Report Size (1),                */
	0x95, 0x06,           /*          Report Count (6),               */
	0x19, 0x01,           /*          Usage Minimum (01h),            */
	0x29, 0x06,           /*          Usage Maximum (06h),            */
	0x14,                 /*          Logical Minimum (0),            */
	0x25, 0x01,           /*          Logical Maximum (1),            */
	0x81, 0x02,           /*          Input (Variable),               */
	0x95, 0x32,           /*          Report Count (50),              */
	0x81, 0x01,           /*          Input (Constant),               */
	0xC0,                 /*      End Collection,                     */
	0xC0                  /*  End Collection                          */
};

static const __u8 fixed_pen_rdesc[] = {
	0x05, 0x0d,           /*  Usage Page (Digitizers),                */
	0x09, 0x01,           /*  Usage (Digitizer),                      */
	0xa1, 0x01,           /*  Collection (Application),               */
	0x85, 0x07,           /*      Report ID (7),                      */
	0x09, 0x20,           /*      Usage (Stylus),                     */
	0xa1, 0x00,           /*      Collection (Physical),              */
	0x09, 0x42,           /*          Usage (Tip Switch),             */
	0x09, 0x44,           /*          Usage (Barrel Switch),          */
	0x09, 0x46,           /*          Usage (Tablet Pick),            */
	0x75, 0x01,           /*          Report Size (1),                */
	0x95, 0x03,           /*          Report Count (3),               */
	0x14,                 /*          Logical Minimum (0),            */
	0x25, 0x01,           /*          Logical Maximum (1),            */
	0x81, 0x02,           /*          Input (Variable),               */
	0x95, 0x02,           /*          Report Count (2),               */
	0x81, 0x03,           /*          Input (Constant, Variable),     */
	0x09, 0x32,           /*          Usage (In Range),               */
	0x95, 0x01,           /*          Report Count (1),               */
	0x81, 0x02,           /*          Input (Variable),               */
	0x95, 0x02,           /*          Report Count (2),               */
	0x81, 0x03,           /*          Input (Constant, Variable),     */
	0x75, 0x10,           /*          Report Size (16),               */
	0x95, 0x01,           /*          Report Count (1),               */
	0x35, 0x00,           /*          Physical Minimum (0),           */
	0xa4,                 /*          Push,                           */
	0x05, 0x01,           /*          Usage Page (Desktop),           */
	0x09, 0x30,           /*          Usage (X),                      */
	0x65, 0x13,           /*          Unit (Inch),                    */
	0x55, 0x0d,           /*          Unit Exponent (-3),             */
	0x26, LOGICAL_MAX_X,  /*          Logical Maximum,                */
	0x46, PHYSICAL_MAX_X, /*          Physical Maximum,               */
	0x81, 0x02,           /*          Input (Variable),               */
	0x09, 0x31,           /*          Usage (Y),                      */
	0x26, LOGICAL_MAX_Y,  /*          Logical Maximum,                */
	0x46, PHYSICAL_MAX_Y, /*          Physical Maximum,               */
	0x81, 0x02,           /*          Input (Variable),               */
	0xb4,                 /*          Pop,                            */
	0x09, 0x30,           /*          Usage (Tip Pressure),           */
	0x45, 0x00,           /*          Physical Maximum (0),           */
	0x26, PRESSURE_MAX,   /*          Logical Maximum,                */
	0x75, 0x0D,           /*          Report Size (13),               */
	0x95, 0x01,           /*          Report Count (1),               */
	0x81, 0x02,           /*          Input (Variable),               */
	0x75, 0x01,           /*          Report Size (1),                */
	0x95, 0x13,           /*          Report Count (19),              */
	0x81, 0x01,           /*          Input (Constant),               */
	0xc0,                 /*      End Collection,                     */
	0xc0,                 /*  End Collection                          */
};

static const size_t fixed_pad_rdesc_size = sizeof(fixed_pad_rdesc);
static const size_t fixed_pen_rdesc_size = sizeof(fixed_pen_rdesc);

SEC(HID_BPF_RDESC_FIXUP)
int BPF_PROG(hid_rdesc_fixup_xppen_deco_mini_4, struct hid_bpf_ctx *hctx)
{
	__u8 *data = hid_bpf_get_data(hctx, 0, HID_MAX_DESCRIPTOR_SIZE);

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

	if (hctx->size == RDESC_SIZE_PAD) {
		__builtin_memcpy(data, fixed_pad_rdesc, fixed_pad_rdesc_size);
		return fixed_pad_rdesc_size;
	} else if (hctx->size == RDESC_SIZE_PEN) {
		__builtin_memcpy(data, fixed_pen_rdesc, fixed_pen_rdesc_size);
		return fixed_pen_rdesc_size;
	}

	return 0;
}

SEC(HID_BPF_DEVICE_EVENT)
int BPF_PROG(hid_device_event_xppen_deco_mini_4, struct hid_bpf_ctx *hctx)
{
	__u8 *data = hid_bpf_get_data(hctx, 0 /* offset */, 8 /* size */);
	__u8 button_mask = 0;
	int d, b;

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

	if (data[0] != PAD_REPORT_ID)
		return 0;

	/* data[1] stores the status of BTN_2 in the 3rd bit*/
	if (data[1] & BIT(2))
		button_mask |= BIT(2);

	/* The rest of the descriptor stores the buttons as in pad_buttons */
	for (d = 2; d < 8; d++) {
		for (b = 0; b < sizeof(pad_buttons); b++) {
			if (data[d] != 0 && data[d] == pad_buttons[b])
				button_mask |= BIT(b);
		}
	}

	__u8 report[8] = {PAD_REPORT_ID, button_mask, 0x00};

	__builtin_memcpy(data, report, sizeof(report));

	return 0;
}

HID_BPF_OPS(deco_mini_4) = {
	.hid_device_event = (void *)hid_device_event_xppen_deco_mini_4,
	.hid_rdesc_fixup = (void *)hid_rdesc_fixup_xppen_deco_mini_4,
};

SEC("syscall")
int probe(struct hid_bpf_probe_args *ctx)
{
	/*
	 * The device has 2 modes: The compatibility mode, enabled by default,
	 * and the raw mode, that can be activated by sending a buffer of magic
	 * data to a certain USB endpoint.
	 *
	 * Depending on the mode, different interfaces of the device are used:
	 * - First interface:  Pad in compatibility mode
	 * - Second interface: Pen in compatibility mode
	 * - Third interface:  Only used in raw mode
	 *
	 * We'll use the device in compatibility mode.
	 */
	ctx->retval = ctx->rdesc_size != RDESC_SIZE_PAD &&
		      ctx->rdesc_size != RDESC_SIZE_PEN;
	if (ctx->retval)
		ctx->retval = -EINVAL;

	return 0;
}

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