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// SPDX-License-Identifier: GPL-2.0-only
/* Copyright (c) 2023 Benjamin Tissoires
*/
#include "vmlinux.h"
#include "hid_bpf.h"
#include "hid_bpf_helpers.h"
#include <bpf/bpf_tracing.h>
#define VID_UGEE 0x28BD /* VID is shared with SinoWealth and Glorious and prob others */
#define PID_ARTIST_24 0x093A
#define PID_ARTIST_24_PRO 0x092D
HID_BPF_CONFIG(
HID_DEVICE(BUS_USB, HID_GROUP_GENERIC, VID_UGEE, PID_ARTIST_24),
HID_DEVICE(BUS_USB, HID_GROUP_GENERIC, VID_UGEE, PID_ARTIST_24_PRO)
);
/*
* We need to amend the report descriptor for the following:
* - the device reports Eraser instead of using Secondary Barrel Switch
* - the pen doesn't have a rubber tail, so basically we are removing any
* eraser/invert bits
*/
static const __u8 fixed_rdesc[] = {
0x05, 0x0d, // Usage Page (Digitizers) 0
0x09, 0x02, // Usage (Pen) 2
0xa1, 0x01, // Collection (Application) 4
0x85, 0x07, // Report ID (7) 6
0x09, 0x20, // Usage (Stylus) 8
0xa1, 0x00, // Collection (Physical) 10
0x09, 0x42, // Usage (Tip Switch) 12
0x09, 0x44, // Usage (Barrel Switch) 14
0x09, 0x5a, // Usage (Secondary Barrel Switch) 16 /* changed from 0x45 (Eraser) to 0x5a (Secondary Barrel Switch) */
0x15, 0x00, // Logical Minimum (0) 18
0x25, 0x01, // Logical Maximum (1) 20
0x75, 0x01, // Report Size (1) 22
0x95, 0x03, // Report Count (3) 24
0x81, 0x02, // Input (Data,Var,Abs) 26
0x95, 0x02, // Report Count (2) 28
0x81, 0x03, // Input (Cnst,Var,Abs) 30
0x09, 0x32, // Usage (In Range) 32
0x95, 0x01, // Report Count (1) 34
0x81, 0x02, // Input (Data,Var,Abs) 36
0x95, 0x02, // Report Count (2) 38
0x81, 0x03, // Input (Cnst,Var,Abs) 40
0x75, 0x10, // Report Size (16) 42
0x95, 0x01, // Report Count (1) 44
0x35, 0x00, // Physical Minimum (0) 46
0xa4, // Push 48
0x05, 0x01, // Usage Page (Generic Desktop) 49
0x09, 0x30, // Usage (X) 51
0x65, 0x13, // Unit (EnglishLinear: in) 53
0x55, 0x0d, // Unit Exponent (-3) 55
0x46, 0xf0, 0x50, // Physical Maximum (20720) 57
0x26, 0xff, 0x7f, // Logical Maximum (32767) 60
0x81, 0x02, // Input (Data,Var,Abs) 63
0x09, 0x31, // Usage (Y) 65
0x46, 0x91, 0x2d, // Physical Maximum (11665) 67
0x26, 0xff, 0x7f, // Logical Maximum (32767) 70
0x81, 0x02, // Input (Data,Var,Abs) 73
0xb4, // Pop 75
0x09, 0x30, // Usage (Tip Pressure) 76
0x45, 0x00, // Physical Maximum (0) 78
0x26, 0xff, 0x1f, // Logical Maximum (8191) 80
0x81, 0x42, // Input (Data,Var,Abs,Null) 83
0x09, 0x3d, // Usage (X Tilt) 85
0x15, 0x81, // Logical Minimum (-127) 87
0x25, 0x7f, // Logical Maximum (127) 89
0x75, 0x08, // Report Size (8) 91
0x95, 0x01, // Report Count (1) 93
0x81, 0x02, // Input (Data,Var,Abs) 95
0x09, 0x3e, // Usage (Y Tilt) 97
0x15, 0x81, // Logical Minimum (-127) 99
0x25, 0x7f, // Logical Maximum (127) 101
0x81, 0x02, // Input (Data,Var,Abs) 103
0xc0, // End Collection 105
0xc0, // End Collection 106
};
#define TIP_SWITCH BIT(0)
#define BARREL_SWITCH BIT(1)
#define ERASER BIT(2)
/* padding BIT(3) */
/* padding BIT(4) */
#define IN_RANGE BIT(5)
/* padding BIT(6) */
/* padding BIT(7) */
#define U16(index) (data[index] | (data[index + 1] << 8))
SEC(HID_BPF_RDESC_FIXUP)
int BPF_PROG(hid_fix_rdesc_xppen_artist24, 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);
}
static __u8 prev_state = 0;
/*
* There are a few cases where the device is sending wrong event
* sequences, all related to the second button (the pen doesn't
* have an eraser switch on the tail end):
*
* whenever the second button gets pressed or released, an
* out-of-proximity event is generated and then the firmware
* compensate for the missing state (and the firmware uses
* eraser for that button):
*
* - if the pen is in range, an extra out-of-range is sent
* when the second button is pressed/released:
* // Pen is in range
* E: InRange
*
* // Second button is pressed
* E:
* E: Eraser InRange
*
* // Second button is released
* E:
* E: InRange
*
* This case is ignored by this filter, it's "valid"
* and userspace knows how to deal with it, there are just
* a few out-of-prox events generated, but the user doesn´t
* see them.
*
* - if the pen is in contact, 2 extra events are added when
* the second button is pressed/released: an out of range
* and an in range:
*
* // Pen is in contact
* E: TipSwitch InRange
*
* // Second button is pressed
* E: <- false release, needs to be filtered out
* E: Eraser InRange <- false release, needs to be filtered out
* E: TipSwitch Eraser InRange
*
* // Second button is released
* E: <- false release, needs to be filtered out
* E: InRange <- false release, needs to be filtered out
* E: TipSwitch InRange
*
*/
SEC(HID_BPF_DEVICE_EVENT)
int BPF_PROG(xppen_24_fix_eraser, struct hid_bpf_ctx *hctx)
{
__u8 *data = hid_bpf_get_data(hctx, 0 /* offset */, 10 /* size */);
__u8 current_state, changed_state;
bool prev_tip;
if (!data)
return 0; /* EPERM check */
current_state = data[1];
/* if the state is identical to previously, early return */
if (current_state == prev_state)
return 0;
prev_tip = !!(prev_state & TIP_SWITCH);
/*
* Illegal transition: pen is in range with the tip pressed, and
* it goes into out of proximity.
*
* Ideally we should hold the event, start a timer and deliver it
* only if the timer ends, but we are not capable of that now.
*
* And it doesn't matter because when we are in such cases, this
* means we are detecting a false release.
*/
if ((current_state & IN_RANGE) == 0) {
if (prev_tip)
return HID_IGNORE_EVENT;
return 0;
}
/*
* XOR to only set the bits that have changed between
* previous and current state
*/
changed_state = prev_state ^ current_state;
/* Store the new state for future processing */
prev_state = current_state;
/*
* We get both a tipswitch and eraser change in the same HID report:
* this is not an authorized transition and is unlikely to happen
* in real life.
* This is likely to be added by the firmware to emulate the
* eraser mode so we can skip the event.
*/
if ((changed_state & (TIP_SWITCH | ERASER)) == (TIP_SWITCH | ERASER)) /* we get both a tipswitch and eraser change at the same time */
return HID_IGNORE_EVENT;
return 0;
}
HID_BPF_OPS(xppen_artist_24) = {
.hid_rdesc_fixup = (void *)hid_fix_rdesc_xppen_artist24,
.hid_device_event = (void *)xppen_24_fix_eraser,
};
SEC("syscall")
int probe(struct hid_bpf_probe_args *ctx)
{
/*
* The device exports 3 interfaces.
*/
ctx->retval = ctx->rdesc_size != 107;
if (ctx->retval)
ctx->retval = -EINVAL;
/* ensure the kernel isn't fixed already */
if (ctx->rdesc[17] != 0x45) /* Eraser */
ctx->retval = -EINVAL;
return 0;
}
char _license[] SEC("license") = "GPL";
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