HID: bpf: Add support for the XP-PEN Deco Mini 4

The XP-PEN Deco Mini 4 is a UGEE device with a frame with 6 buttons.
Its pen has 2 buttons and supports pressure reporting.

Fix their report descriptors and transform the frame button events to
support it.

Link: https://gitlab.freedesktop.org/libevdev/udev-hid-bpf/-/merge_requests/88
Signed-off-by: José Expósito <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Benjamin Tissoires <[e-mail]>

drivers/hid/bpf/progs/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";