Quelle  Microsoft__Xbox-Elite-2.bpf.c   Sprache: 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 paddles on the back of the device as a single
 * bitfield value of usage "Assign Selection".
 *
 * The kernel doesn't process the paddles usage properly and reports KEY_UNKNOWN.
 *
 * SDL doesn't know how to interpret 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 interpret 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(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;
}

HID_BPF_OPS(xbox_elite_2) = {
 .hid_rdesc_fixup = (void *)hid_fix_rdesc,
};

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";