new file mode 100644
@@ -0,0 +1,133 @@
+// 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";
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 Signed-off-by: Benjamin Tissoires <bentiss@kernel.org> --- .../hid/bpf/progs/Microsoft__XBox-Elite-2.bpf.c | 133 +++++++++++++++++++++ 1 file changed, 133 insertions(+)