HID: bpf: add in-tree HID-BPF fix for the XBox Elite 2 over Bluetooth

When using the XBox Wireless Controller Elite 2 over Bluetooth,
the device exports the paddle on the back of the device as a single
bitfield value of usage "Assign Selection".

The kernel doesn't process those usages properly and report KEY_UNKNOWN
for it.

SDL doesn't know how to interprete that KEY_UNKNOWN and thus ignores the
paddles.

Given that over USB the kernel uses BTN_TRIGGER_HAPPY[5-8], we
can tweak the report descriptor to make the kernel interprete it properly:
- we need an application collection of gamepad (so we have to close the
  current Consumer Control one)
- we need to change the usage to be buttons from 0x15 to 0x18

Link: https://lore.kernel.org/r/20240410-bpf_sources-v1-7-a8bf16033ef8@kernel.orgReviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>

drivers/hid/bpf/progs/Microsoft__XBox-Elite-2.bpf.c

+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2024 Benjamin Tissoires
+ */
+
+#include "vmlinux.h"
+#include "hid_bpf.h"
+#include "hid_bpf_helpers.h"
+#include <bpf/bpf_tracing.h>
+
+#define VID_MICROSOFT 0x045e
+#define PID_XBOX_ELITE_2 0x0b22
+
+HID_BPF_CONFIG(
+	HID_DEVICE(BUS_BLUETOOTH, HID_GROUP_GENERIC, VID_MICROSOFT, PID_XBOX_ELITE_2)
+);
+
+/*
+ * When using the XBox Wireless Controller Elite 2 over Bluetooth,
+ * the device exports the paddle on the back of the device as a single
+ * bitfield value of usage "Assign Selection".
+ *
+ * The kernel doesn't process those usages properly and report KEY_UNKNOWN
+ * for it.
+ *
+ * SDL doesn't know how to interprete that KEY_UNKNOWN and thus ignores the paddles.
+ *
+ * Given that over USB the kernel uses BTN_TRIGGER_HAPPY[5-8], we
+ * can tweak the report descriptor to make the kernel interprete it properly:
+ * - we need an application collection of gamepad (so we have to close the current
+ *   Consumer Control one)
+ * - we need to change the usage to be buttons from 0x15 to 0x18
+ */
+
+#define OFFSET_ASSIGN_SELECTION		211
+#define ORIGINAL_RDESC_SIZE		464
+
+const __u8 rdesc_assign_selection[] = {
+	0x0a, 0x99, 0x00,              //   Usage (Media Select Security)     211
+	0x15, 0x00,                    //   Logical Minimum (0)               214
+	0x26, 0xff, 0x00,              //   Logical Maximum (255)             216
+	0x95, 0x01,                    //   Report Count (1)                  219
+	0x75, 0x04,                    //   Report Size (4)                   221
+	0x81, 0x02,                    //   Input (Data,Var,Abs)              223
+	0x15, 0x00,                    //   Logical Minimum (0)               225
+	0x25, 0x00,                    //   Logical Maximum (0)               227
+	0x95, 0x01,                    //   Report Count (1)                  229
+	0x75, 0x04,                    //   Report Size (4)                   231
+	0x81, 0x03,                    //   Input (Cnst,Var,Abs)              233
+	0x0a, 0x81, 0x00,              //   Usage (Assign Selection)          235
+	0x15, 0x00,                    //   Logical Minimum (0)               238
+	0x26, 0xff, 0x00,              //   Logical Maximum (255)             240
+	0x95, 0x01,                    //   Report Count (1)                  243
+	0x75, 0x04,                    //   Report Size (4)                   245
+	0x81, 0x02,                    //   Input (Data,Var,Abs)              247
+};
+
+/*
+ * we replace the above report descriptor extract
+ * with the one below.
+ * To make things equal in size, we take out a larger
+ * portion than just the "Assign Selection" range, because
+ * we need to insert a new application collection to force
+ * the kernel to use BTN_TRIGGER_HAPPY[4-7].
+ */
+const __u8 fixed_rdesc_assign_selection[] = {
+	0x0a, 0x99, 0x00,              //   Usage (Media Select Security)     211
+	0x15, 0x00,                    //   Logical Minimum (0)               214
+	0x26, 0xff, 0x00,              //   Logical Maximum (255)             216
+	0x95, 0x01,                    //   Report Count (1)                  219
+	0x75, 0x04,                    //   Report Size (4)                   221
+	0x81, 0x02,                    //   Input (Data,Var,Abs)              223
+	/* 0x15, 0x00, */              //   Logical Minimum (0)               ignored
+	0x25, 0x01,                    //   Logical Maximum (1)               225
+	0x95, 0x04,                    //   Report Count (4)                  227
+	0x75, 0x01,                    //   Report Size (1)                   229
+	0x81, 0x03,                    //   Input (Cnst,Var,Abs)              231
+	0xc0,                          //  End Collection                     233
+	0x05, 0x01,                    //  Usage Page (Generic Desktop)       234
+	0x0a, 0x05, 0x00,              //  Usage (Game Pad)                   236
+	0xa1, 0x01,                    //  Collection (Application)           239
+	0x05, 0x09,                    //   Usage Page (Button)               241
+	0x19, 0x15,                    //   Usage Minimum (21)                243
+	0x29, 0x18,                    //   Usage Maximum (24)                245
+	/* 0x15, 0x00, */              //  Logical Minimum (0)                ignored
+	/* 0x25, 0x01, */              //  Logical Maximum (1)                ignored
+	/* 0x95, 0x01, */              //  Report Size (1)                    ignored
+	/* 0x75, 0x04, */              //  Report Count (4)                   ignored
+	0x81, 0x02,                    //   Input (Data,Var,Abs)              247
+};
+
+_Static_assert(sizeof(rdesc_assign_selection) == sizeof(fixed_rdesc_assign_selection),
+	       "Rdesc and fixed rdesc of different size");
+_Static_assert(sizeof(rdesc_assign_selection) + OFFSET_ASSIGN_SELECTION < ORIGINAL_RDESC_SIZE,
+	       "Rdesc at given offset is too big");
+
+SEC("fmod_ret/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 */
+
+	/* Check that the device is compatible */
+	if (__builtin_memcmp(data + OFFSET_ASSIGN_SELECTION,
+			     rdesc_assign_selection,
+			     sizeof(rdesc_assign_selection)))
+		return 0;
+
+	__builtin_memcpy(data + OFFSET_ASSIGN_SELECTION,
+			 fixed_rdesc_assign_selection,
+			 sizeof(fixed_rdesc_assign_selection));
+
+	return 0;
+}
+
+SEC("syscall")
+int probe(struct hid_bpf_probe_args *ctx)
+{
+	/* only bind to the keyboard interface */
+	ctx->retval = ctx->rdesc_size != ORIGINAL_RDESC_SIZE;
+	if (ctx->retval)
+		ctx->retval = -EINVAL;
+
+	if (__builtin_memcmp(ctx->rdesc + OFFSET_ASSIGN_SELECTION,
+			     rdesc_assign_selection,
+			     sizeof(rdesc_assign_selection)))
+		ctx->retval = -EINVAL;
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";