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Benjamin TissoiresJiri Kosina
Benjamin Tissoires
authored and
Jiri Kosina
committed
HID: bpf: Add support for the Waltop Batteryless Tablet
Several bugs as outlined in udev-hid-bpf#66 and udev-hid-bpf!198: - pressure curve is far from linear - tilt range is ±60, not ±127 - pressing the second button sets both tip down and barrel switch Fix the second button by adding a Secondary Barrel Switch in the existing padding and check for the tip down/barrel switch down combo. When both values become true at the same time, set the Secondary Barrel Switch instead. Implement a custom pressure curve that maps the hardware range 0-102 linearly to the logical range 0-1224, and maps the hardware range 103-2047 logarithmically to the logical range 1232-2047. This mapping isn’t perfect, but it’s way more natural than the stock configuration. Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net> Signed-off-by: Jan Felix Langenbach <JanFelix.Langenbach@protonmail.com> Link: https://gitlab.freedesktop.org/libevdev/udev-hid-bpf/-/merge_requests/200 Signed-off-by: Benjamin Tissoires <bentiss@kernel.org> Signed-off-by: Jiri Kosina <jkosina@suse.com>
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‎drivers/hid/bpf/progs/WALTOP__Batteryless-Tablet.bpf.c‎

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// SPDX-License-Identifier: GPL-2.0-only
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/* Copyright (c) 2025 Red Hat
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*/
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#include "vmlinux.h"
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#include "hid_bpf.h"
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#include "hid_bpf_helpers.h"
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#include <bpf/bpf_tracing.h>
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#define VID_WALTOP 0x172F
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#define PID_BATTERYLESS_TABLET 0x0505
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HID_BPF_CONFIG(
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HID_DEVICE(BUS_USB, HID_GROUP_ANY, VID_WALTOP, PID_BATTERYLESS_TABLET)
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);
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#define EXPECTED_RDESC_SIZE 335
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#define PEN_REPORT_ID 16
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#define TIP_SWITCH BIT(0)
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#define BARREL_SWITCH BIT(1)
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#define SECONDARY_BARREL_SWITCH BIT(5)
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static __u8 last_button_state;
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static const __u8 fixed_rdesc[] = {
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0x05, 0x01, // Usage Page (Generic Desktop)
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0x09, 0x02, // Usage (Mouse)
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0xa1, 0x01, // Collection (Application)
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0x85, 0x01, // Report ID (1)
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0x09, 0x01, // Usage (Pointer)
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0xa1, 0x00, // Collection (Physical)
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0x05, 0x09, // Usage Page (Button)
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0x19, 0x01, // Usage Minimum (1)
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0x29, 0x05, // Usage Maximum (5)
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0x15, 0x00, // Logical Minimum (0)
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0x25, 0x01, // Logical Maximum (1)
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0x75, 0x01, // Report Size (1)
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0x95, 0x05, // Report Count (5)
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0x81, 0x02, // Input (Data,Var,Abs)
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0x75, 0x03, // Report Size (3)
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0x95, 0x01, // Report Count (1)
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0x81, 0x03, // Input (Cnst,Var,Abs)
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0x05, 0x01, // Usage Page (Generic Desktop)
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0x09, 0x30, // Usage (X)
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0x09, 0x31, // Usage (Y)
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0x09, 0x38, // Usage (Wheel)
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0x15, 0x81, // Logical Minimum (-127)
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0x25, 0x7f, // Logical Maximum (127)
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0x75, 0x08, // Report Size (8)
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0x95, 0x03, // Report Count (3)
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0x81, 0x06, // Input (Data,Var,Rel)
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0x05, 0x0c, // Usage Page (Consumer)
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0x15, 0x81, // Logical Minimum (-127)
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0x25, 0x7f, // Logical Maximum (127)
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0x75, 0x08, // Report Size (8)
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0x95, 0x01, // Report Count (1)
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0x0a, 0x38, 0x02, // Usage (AC Pan)
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0x81, 0x06, // Input (Data,Var,Rel)
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0xc0, // End Collection
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0xc0, // End Collection
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0x05, 0x0d, // Usage Page (Digitizers)
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0x09, 0x02, // Usage (Pen)
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0xa1, 0x01, // Collection (Application)
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0x85, 0x02, // Report ID (2)
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0x09, 0x20, // Usage (Stylus)
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0xa1, 0x00, // Collection (Physical)
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0x09, 0x00, // Usage (0x0000)
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0x15, 0x00, // Logical Minimum (0)
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0x26, 0xff, 0x00, // Logical Maximum (255)
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0x75, 0x08, // Report Size (8)
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0x95, 0x09, // Report Count (9)
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0x81, 0x02, // Input (Data,Var,Abs)
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0x09, 0x3f, // Usage (Azimuth)
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0x09, 0x40, // Usage (Altitude)
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0x15, 0x00, // Logical Minimum (0)
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0x26, 0xff, 0x00, // Logical Maximum (255)
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0x75, 0x08, // Report Size (8)
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0x95, 0x02, // Report Count (2)
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0xb1, 0x02, // Feature (Data,Var,Abs)
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0xc0, // End Collection
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0x85, 0x05, // Report ID (5)
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0x05, 0x0d, // Usage Page (Digitizers)
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0x09, 0x20, // Usage (Stylus)
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0xa1, 0x00, // Collection (Physical)
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0x09, 0x00, // Usage (0x0000)
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0x15, 0x00, // Logical Minimum (0)
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0x26, 0xff, 0x00, // Logical Maximum (255)
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0x75, 0x08, // Report Size (8)
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0x95, 0x07, // Report Count (7)
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0x81, 0x02, // Input (Data,Var,Abs)
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0xc0, // End Collection
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0x85, 0x0a, // Report ID (10)
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0x05, 0x0d, // Usage Page (Digitizers)
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0x09, 0x20, // Usage (Stylus)
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0xa1, 0x00, // Collection (Physical)
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0x09, 0x00, // Usage (0x0000)
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0x15, 0x00, // Logical Minimum (0)
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0x26, 0xff, 0x00, // Logical Maximum (255)
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0x75, 0x08, // Report Size (8)
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0x95, 0x07, // Report Count (7)
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0x81, 0x02, // Input (Data,Var,Abs)
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0xc0, // End Collection
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0x85, 0x10, // Report ID (16)
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0x09, 0x20, // Usage (Stylus)
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0xa1, 0x00, // Collection (Physical)
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0x09, 0x42, // Usage (Tip Switch)
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0x09, 0x44, // Usage (Barrel Switch)
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0x09, 0x3c, // Usage (Invert)
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0x09, 0x45, // Usage (Eraser)
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0x09, 0x32, // Usage (In Range)
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0x09, 0x5a, // Usage (Secondary Barrel Switch) <-- added
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0x15, 0x00, // Logical Minimum (0)
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0x25, 0x01, // Logical Maximum (1)
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0x75, 0x01, // Report Size (1)
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0x95, 0x06, // Report Count (6) <--- changed from 5
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0x81, 0x02, // Input (Data,Var,Abs)
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0x95, 0x02, // Report Count (2) <--- changed from 3
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0x81, 0x03, // Input (Cnst,Var,Abs)
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0x05, 0x01, // Usage Page (Generic Desktop)
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0x09, 0x30, // Usage (X)
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0x75, 0x10, // Report Size (16)
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0x95, 0x01, // Report Count (1)
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0xa4, // Push
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0x55, 0x0d, // Unit Exponent (-3)
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0x65, 0x33, // Unit (EnglishLinear: in³)
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0x15, 0x00, // Logical Minimum (0)
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0x26, 0x00, 0x7d, // Logical Maximum (32000)
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0x35, 0x00, // Physical Minimum (0)
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0x46, 0x00, 0x7d, // Physical Maximum (32000)
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0x81, 0x02, // Input (Data,Var,Abs)
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0x09, 0x31, // Usage (Y)
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0x15, 0x00, // Logical Minimum (0)
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0x26, 0x20, 0x4e, // Logical Maximum (20000)
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0x35, 0x00, // Physical Minimum (0)
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0x46, 0x20, 0x4e, // Physical Maximum (20000)
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0x81, 0x02, // Input (Data,Var,Abs)
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0x05, 0x0d, // Usage Page (Digitizers)
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0x09, 0x30, // Usage (Tip Pressure)
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0x15, 0x00, // Logical Minimum (0)
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0x26, 0xff, 0x07, // Logical Maximum (2047)
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0x35, 0x00, // Physical Minimum (0)
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0x46, 0xff, 0x07, // Physical Maximum (2047)
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0x81, 0x02, // Input (Data,Var,Abs)
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0x05, 0x0d, // Usage Page (Digitizers)
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0x09, 0x3d, // Usage (X Tilt)
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0x09, 0x3e, // Usage (Y Tilt)
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0x15, 0xc4, // Logical Minimum (-60) <- changed from -127
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0x25, 0x3c, // Logical Maximum (60) <- changed from 127
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0x75, 0x08, // Report Size (8)
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0x95, 0x02, // Report Count (2)
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0x81, 0x02, // Input (Data,Var,Abs)
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0xc0, // End Collection
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0xc0, // End Collection
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0x05, 0x01, // Usage Page (Generic Desktop)
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0x09, 0x06, // Usage (Keyboard)
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0xa1, 0x01, // Collection (Application)
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0x85, 0x0d, // Report ID (13)
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0x05, 0x07, // Usage Page (Keyboard/Keypad)
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0x19, 0xe0, // Usage Minimum (224)
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0x29, 0xe7, // Usage Maximum (231)
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0x15, 0x00, // Logical Minimum (0)
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0x25, 0x01, // Logical Maximum (1)
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0x75, 0x01, // Report Size (1)
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0x95, 0x08, // Report Count (8)
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0x81, 0x02, // Input (Data,Var,Abs)
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0x75, 0x08, // Report Size (8)
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0x95, 0x01, // Report Count (1)
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0x81, 0x01, // Input (Cnst,Arr,Abs)
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0x05, 0x07, // Usage Page (Keyboard/Keypad)
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0x19, 0x00, // Usage Minimum (0)
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0x29, 0x65, // Usage Maximum (101)
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0x15, 0x00, // Logical Minimum (0)
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0x25, 0x65, // Logical Maximum (101)
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0x75, 0x08, // Report Size (8)
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0x95, 0x05, // Report Count (5)
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0x81, 0x00, // Input (Data,Arr,Abs)
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0xc0, // End Collection
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0x05, 0x0c, // Usage Page (Consumer)
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0x09, 0x01, // Usage (Consumer Control)
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0xa1, 0x01, // Collection (Application)
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0x85, 0x0c, // Report ID (12)
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0x09, 0xe9, // Usage (Volume Increment)
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0x09, 0xea, // Usage (Volume Decrement)
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0x09, 0xe2, // Usage (Mute)
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0x15, 0x00, // Logical Minimum (0)
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0x25, 0x01, // Logical Maximum (1)
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0x75, 0x01, // Report Size (1)
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0x95, 0x03, // Report Count (3)
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0x81, 0x06, // Input (Data,Var,Rel)
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0x75, 0x05, // Report Size (5)
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0x95, 0x01, // Report Count (1)
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0x81, 0x07, // Input (Cnst,Var,Rel)
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0xc0, // End Collection
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};
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static inline unsigned int bitwidth32(__u32 x)
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{
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return 32 - __builtin_clzg(x, 32);
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}
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static inline unsigned int floor_log2_32(__u32 x)
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{
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return bitwidth32(x) - 1;
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}
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/* Maps the interval [0, 2047] to itself using a scaled
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* approximation of the function log2(x+1).
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*/
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static unsigned int scaled_log2(__u16 v)
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{
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const unsigned int XMAX = 2047;
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const unsigned int YMAX = 11; /* log2(2048) = 11 */
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unsigned int x = v + 1;
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unsigned int n = floor_log2_32(x);
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unsigned int b = 1 << n;
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/* Fixed-point fraction in [0, 1), linearly
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* interpolated using delta-y = 1 and
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* delta-x = (2b - b) = b.
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*/
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unsigned int frac = (x - b) << YMAX;
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unsigned int lerp = frac / b;
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unsigned int log2 = (n << YMAX) + lerp;
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return ((log2 * XMAX) / YMAX) >> YMAX;
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}
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SEC(HID_BPF_RDESC_FIXUP)
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int BPF_PROG(hid_fix_rdesc, struct hid_bpf_ctx *hctx)
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{
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__u8 *data = hid_bpf_get_data(hctx, 0 /* offset */, 4096 /* size */);
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if (!data)
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return 0; /* EPERM check */
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__builtin_memcpy(data, fixed_rdesc, sizeof(fixed_rdesc));
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return sizeof(fixed_rdesc);
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}
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SEC(HID_BPF_DEVICE_EVENT)
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int BPF_PROG(waltop_fix_events, struct hid_bpf_ctx *hctx)
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{
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__u8 *data = hid_bpf_get_data(hctx, 0 /* offset */, 10 /* size */);
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if (!data)
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return 0; /* EPERM check */
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__u8 report_id = data[0];
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if (report_id != PEN_REPORT_ID)
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return 0;
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/* On this tablet if the secondary barrel switch is pressed,
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* the tablet sends tip down and barrel down. Change this to
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* just secondary barrel down when there is no ambiguity.
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*
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* It's possible that there is a bug in the firmware and the
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* device intends to set invert + eraser instead (i.e. the
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* pysical button is an eraser button) but since
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* the pressure is always zero, said eraser button
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* would be useless anyway.
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*
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* So let's just change the button to secondary barrel down.
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*/
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__u8 tip_switch = data[1] & TIP_SWITCH;
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__u8 barrel_switch = data[1] & BARREL_SWITCH;
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__u8 tip_held = last_button_state & TIP_SWITCH;
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__u8 barrel_held = last_button_state & BARREL_SWITCH;
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if (tip_switch && barrel_switch && !tip_held && !barrel_held) {
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data[1] &= ~(TIP_SWITCH | BARREL_SWITCH); /* release tip and barrel */
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data[1] |= SECONDARY_BARREL_SWITCH; /* set secondary barrel switch */
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}
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last_button_state = data[1];
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/* The pressure sensor on this tablet maps around half of the
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* logical pressure range into the interval [0-100]. Further
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* pressure causes the sensor value to increase exponentially
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* up to a maximum value of 2047.
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*
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* The values 12 and 102 were chosen to have an integer slope
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* with smooth transition between the two curves around the
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* value 100.
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*/
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__u16 pressure = (((__u16)data[6]) << 0) | (((__u16)data[7]) << 8);
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if (pressure <= 102)
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pressure *= 12;
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else
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pressure = scaled_log2(pressure);
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data[6] = pressure >> 0;
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data[7] = pressure >> 8;
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return 0;
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}
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HID_BPF_OPS(waltop_batteryless) = {
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.hid_device_event = (void *)waltop_fix_events,
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.hid_rdesc_fixup = (void *)hid_fix_rdesc,
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};
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SEC("syscall")
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int probe(struct hid_bpf_probe_args *ctx)
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{
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if (ctx->rdesc_size == EXPECTED_RDESC_SIZE)
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ctx->retval = 0;
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else
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ctx->retval = -EINVAL;
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return 0;
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}
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char _license[] SEC("license") = "GPL";

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