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 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";