// 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";
