new file mode 100644
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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_PRO14_GEN2 0x095A
+#define PID_ARTIST_PRO16_GEN2 0x095B
+
+HID_BPF_CONFIG(
+ HID_DEVICE(BUS_USB, HID_GROUP_GENERIC, VID_UGEE, PID_ARTIST_PRO14_GEN2),
+ HID_DEVICE(BUS_USB, HID_GROUP_GENERIC, VID_UGEE, PID_ARTIST_PRO16_GEN2)
+);
+
+/*
+ * We need to amend the report descriptor for the following:
+ * - the device reports Eraser instead of using Secondary Barrel Switch
+ * - when the eraser button is pressed and the stylus is touching the tablet,
+ * the device sends Tip Switch instead of sending Eraser
+ *
+ * This descriptor uses physical dimensions of the 16" device.
+ */
+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) */
+ 0x09, 0x3c, // Usage (Invert) 18
+ 0x09, 0x45, // Usage (Eraser) 16 /* created over a padding bit at offset 29-33 */
+ 0x15, 0x00, // Logical Minimum (0) 20
+ 0x25, 0x01, // Logical Maximum (1) 22
+ 0x75, 0x01, // Report Size (1) 24
+ 0x95, 0x05, // Report Count (5) 26 /* changed from 4 to 5 */
+ 0x81, 0x02, // Input (Data,Var,Abs) 28
+ 0x09, 0x32, // Usage (In Range) 34
+ 0x15, 0x00, // Logical Minimum (0) 36
+ 0x25, 0x01, // Logical Maximum (1) 38
+ 0x95, 0x01, // Report Count (1) 40
+ 0x81, 0x02, // Input (Data,Var,Abs) 42
+ 0x95, 0x02, // Report Count (2) 44
+ 0x81, 0x03, // Input (Cnst,Var,Abs) 46
+ 0x75, 0x10, // Report Size (16) 48
+ 0x95, 0x01, // Report Count (1) 50
+ 0x35, 0x00, // Physical Minimum (0) 52
+ 0xa4, // Push 54
+ 0x05, 0x01, // Usage Page (Generic Desktop) 55
+ 0x09, 0x30, // Usage (X) 57
+ 0x65, 0x13, // Unit (EnglishLinear: in) 59
+ 0x55, 0x0d, // Unit Exponent (-3) 61
+ 0x46, 0xff, 0x34, // Physical Maximum (13567) 63
+ 0x26, 0xff, 0x7f, // Logical Maximum (32767) 66
+ 0x81, 0x02, // Input (Data,Var,Abs) 69
+ 0x09, 0x31, // Usage (Y) 71
+ 0x46, 0x20, 0x21, // Physical Maximum (8480) 73
+ 0x26, 0xff, 0x7f, // Logical Maximum (32767) 76
+ 0x81, 0x02, // Input (Data,Var,Abs) 79
+ 0xb4, // Pop 81
+ 0x09, 0x30, // Usage (Tip Pressure) 82
+ 0x45, 0x00, // Physical Maximum (0) 84
+ 0x26, 0xff, 0x3f, // Logical Maximum (16383) 86
+ 0x81, 0x42, // Input (Data,Var,Abs,Null) 89
+ 0x09, 0x3d, // Usage (X Tilt) 91
+ 0x15, 0x81, // Logical Minimum (-127) 93
+ 0x25, 0x7f, // Logical Maximum (127) 95
+ 0x75, 0x08, // Report Size (8) 97
+ 0x95, 0x01, // Report Count (1) 99
+ 0x81, 0x02, // Input (Data,Var,Abs) 101
+ 0x09, 0x3e, // Usage (Y Tilt) 103
+ 0x15, 0x81, // Logical Minimum (-127) 105
+ 0x25, 0x7f, // Logical Maximum (127) 107
+ 0x81, 0x02, // Input (Data,Var,Abs) 109
+ 0xc0, // End Collection 111
+ 0xc0, // End Collection 112
+};
+
+SEC("fmod_ret/hid_bpf_rdesc_fixup")
+int BPF_PROG(hid_fix_rdesc_xppen_artistpro16gen2, 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));
+
+ /* Fix the Physical maximum values for different sizes of the device
+ * The 14" screen device descriptor size is 11.874" x 7.421"
+ */
+ if (hctx->hid->product == PID_ARTIST_PRO14_GEN2) {
+ data[63] = 0x2e;
+ data[62] = 0x62;
+ data[73] = 0x1c;
+ data[72] = 0xfd;
+ }
+
+ return sizeof(fixed_rdesc);
+}
+
+SEC("fmod_ret/hid_bpf_device_event")
+int BPF_PROG(xppen_16_fix_eraser, struct hid_bpf_ctx *hctx)
+{
+ __u8 *data = hid_bpf_get_data(hctx, 0 /* offset */, 10 /* size */);
+
+ if (!data)
+ return 0; /* EPERM check */
+
+ if ((data[1] & 0x29) != 0x29) /* tip switch=1 invert=1 inrange=1 */
+ return 0;
+
+ /* xor bits 0,3 and 4: convert Tip Switch + Invert into Eraser only */
+ data[1] ^= 0x19;
+
+ return 0;
+}
+
+/*
+ * Static coordinate offset table based on positive only angles
+ * Two tables are needed, because the logical coordinates are scaled
+ *
+ * The table can be generated by Python like this:
+ * >>> full_scale = 11.874 # the display width/height in inches
+ * >>> tip_height = 0.055677699 # the center of the pen coil distance from screen in inch (empirical)
+ * >>> h = tip_height * (32767 / full_scale) # height of the coil in logical coordinates
+ * >>> [round(h*math.sin(math.radians(d))) for d in range(0, 128)]
+ * [0, 13, 26, ....]
+ */
+
+/* 14" inch screen 11.874" x 7.421" */
+static const __u16 angle_offsets_horizontal_14[128] = {
+ 0, 3, 5, 8, 11, 13, 16, 19, 21, 24, 27, 29, 32, 35, 37, 40, 42, 45, 47, 50, 53,
+ 55, 58, 60, 62, 65, 67, 70, 72, 74, 77, 79, 81, 84, 86, 88, 90, 92, 95, 97, 99,
+ 101, 103, 105, 107, 109, 111, 112, 114, 116, 118, 119, 121, 123, 124, 126, 127,
+ 129, 130, 132, 133, 134, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146,
+ 147, 148, 148, 149, 150, 150, 151, 151, 152, 152, 153, 153, 153, 153, 153, 154,
+ 154, 154, 154, 154, 153, 153, 153, 153, 153, 152, 152, 151, 151, 150, 150, 149,
+ 148, 148, 147, 146, 145, 144, 143, 142, 141, 140, 139, 138, 137, 136, 134, 133,
+ 132, 130, 129, 127, 126, 124, 123
+};
+static const __u16 angle_offsets_vertical_14[128] = {
+ 0, 4, 9, 13, 17, 21, 26, 30, 34, 38, 43, 47, 51, 55, 59, 64, 68, 72, 76, 80, 84,
+ 88, 92, 96, 100, 104, 108, 112, 115, 119, 123, 127, 130, 134, 137, 141, 145, 148,
+ 151, 155, 158, 161, 165, 168, 171, 174, 177, 180, 183, 186, 188, 191, 194, 196,
+ 199, 201, 204, 206, 208, 211, 213, 215, 217, 219, 221, 223, 225, 226, 228, 230,
+ 231, 232, 234, 235, 236, 237, 239, 240, 240, 241, 242, 243, 243, 244, 244, 245,
+ 245, 246, 246, 246, 246, 246, 246, 246, 245, 245, 244, 244, 243, 243, 242, 241,
+ 240, 240, 239, 237, 236, 235, 234, 232, 231, 230, 228, 226, 225, 223, 221, 219,
+ 217, 215, 213, 211, 208, 206, 204, 201, 199, 196
+};
+
+/* 16" inch screen 13.567" x 8.480" */
+static const __u16 angle_offsets_horizontal_16[128] = {
+ 0, 2, 5, 7, 9, 12, 14, 16, 19, 21, 23, 26, 28, 30, 33, 35, 37, 39, 42, 44, 46, 48,
+ 50, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79, 81, 83, 85, 86, 88, 90,
+ 92, 93, 95, 97, 98, 100, 101, 103, 105, 106, 107, 109, 110, 111, 113, 114, 115,
+ 116, 118, 119, 120, 121, 122, 123, 124, 125, 126, 126, 127, 128, 129, 129, 130,
+ 130, 131, 132, 132, 132, 133, 133, 133, 134, 134, 134, 134, 134, 134, 134, 134,
+ 134, 134, 134, 134, 134, 133, 133, 133, 132, 132, 132, 131, 130, 130, 129, 129,
+ 128, 127, 126, 126, 125, 124, 123, 122, 121, 120, 119, 118, 116, 115, 114, 113,
+ 111, 110, 109, 107
+};
+static const __u16 angle_offsets_vertical_16[128] = {
+ 0, 4, 8, 11, 15, 19, 22, 26, 30, 34, 37, 41, 45, 48, 52, 56, 59, 63, 66, 70, 74,
+ 77, 81, 84, 88, 91, 94, 98, 101, 104, 108, 111, 114, 117, 120, 123, 126, 129, 132,
+ 135, 138, 141, 144, 147, 149, 152, 155, 157, 160, 162, 165, 167, 170, 172, 174,
+ 176, 178, 180, 182, 184, 186, 188, 190, 192, 193, 195, 197, 198, 199, 201, 202,
+ 203, 205, 206, 207, 208, 209, 210, 210, 211, 212, 212, 213, 214, 214, 214, 215,
+ 215, 215, 215, 215, 215, 215, 215, 215, 214, 214, 214, 213, 212, 212, 211, 210,
+ 210, 209, 208, 207, 206, 205, 203, 202, 201, 199, 198, 197, 195, 193, 192, 190,
+ 188, 186, 184, 182, 180, 178, 176, 174, 172
+};
+
+static void compensate_coordinates_by_tilt(__u8 *data, const __u8 idx,
+ const __s8 tilt, const __u16 (*compensation_table)[128])
+{
+ __u16 coords = data[idx+1];
+
+ coords <<= 8;
+ coords += data[idx];
+
+ __u8 direction = tilt > 0 ? 0 : 1; /* Positive tilt means we need to subtract the compensation (vs. negative angle where we need to add) */
+ __u8 angle = tilt > 0 ? tilt : -tilt;
+
+ if (angle > 127)
+ return;
+
+ __u16 compensation = (*compensation_table)[angle];
+
+ if (direction == 0) {
+ coords = (coords > compensation) ? coords - compensation : 0;
+ } else {
+ const __u16 logical_maximum = 32767;
+ __u16 max = logical_maximum - compensation;
+
+ coords = (coords < max) ? coords + compensation : logical_maximum;
+ }
+
+ data[idx] = coords & 0xff;
+ data[idx+1] = coords >> 8;
+}
+
+SEC("fmod_ret/hid_bpf_device_event")
+int BPF_PROG(xppen_16_fix_angle_offset, struct hid_bpf_ctx *hctx)
+{
+ __u8 *data = hid_bpf_get_data(hctx, 0 /* offset */, 10 /* size */);
+
+ if (!data)
+ return 0; /* EPERM check */
+
+ /*
+ * Compensate X and Y offset caused by tilt.
+ *
+ * The magnetic center moves when the pen is tilted, because the coil
+ * is not touching the screen.
+ *
+ * a (tilt angle)
+ * | /... h (coil distance from tip)
+ * | /
+ * |/______
+ * |x (position offset)
+ *
+ * x = sin a * h
+ *
+ * Subtract the offset from the coordinates. Use the precomputed table!
+ *
+ * bytes 0 - report id
+ * 1 - buttons
+ * 2-3 - X coords (logical)
+ * 4-5 - Y coords
+ * 6-7 - pressure (ignore)
+ * 8 - tilt X
+ * 9 - tilt Y
+ */
+
+ __s8 tilt_x = (__s8) data[8];
+ __s8 tilt_y = (__s8) data[9];
+
+ if (hctx->hid->product == PID_ARTIST_PRO14_GEN2) {
+ compensate_coordinates_by_tilt(data, 2, tilt_x, &angle_offsets_horizontal_14);
+ compensate_coordinates_by_tilt(data, 4, tilt_y, &angle_offsets_vertical_14);
+ } else if (hctx->hid->product == PID_ARTIST_PRO16_GEN2) {
+ compensate_coordinates_by_tilt(data, 2, tilt_x, &angle_offsets_horizontal_16);
+ compensate_coordinates_by_tilt(data, 4, tilt_y, &angle_offsets_vertical_16);
+ }
+
+ return 0;
+}
+
+SEC("syscall")
+int probe(struct hid_bpf_probe_args *ctx)
+{
+ /*
+ * The device exports 3 interfaces.
+ */
+ ctx->retval = ctx->rdesc_size != 113;
+ 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";