[PATCH AUTOSEL 6.18-6.1] HID: bpf: Add Huion Inspiroy Frego M button quirk

[Sasha Levin <sashal@kernel.org>]

From: Nikhil Chatterjee <nikhilc1527@gmail.com>

[ Upstream commit 857e71cb0a538b1660743a4267a1e789575f7966 ]

The Huion Inspiroy Frego M pen report descriptor exposes the second
side button as Secondary Tip Switch instead of Secondary Barrel Switch.
This makes userspace see the control as the wrong pen button.

Add a HID-BPF report descriptor fixup for the Bluetooth 256c:8251
device and USB 256c:2012 L610 variant. The fixup matches the expected
pen descriptor and rewrites the offending usage from Secondary Tip
Switch to Secondary Barrel Switch.

Tested by building the HID-BPF object with:

  make -C drivers/hid/bpf/progs Huion__Inspiroy-Frego-M.bpf.o

Signed-off-by: Nikhil Chatterjee <nikhilc1527@gmail.com>
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

The background git searches finished and match the earlier analysis:

- **`git log --grep="Frego"` / `--grep="Inspiroy Frego"`** — no matches
  in this tree; the quirk is not present in 6.18.44.
- **`git log --grep="8251"` under `drivers/hid/`** — only an unrelated
  HID debugging commit, not this device.

That confirms the patch would be a new addition here, consistent with
the **YES** backport recommendation.

 .../bpf/progs/Huion__Inspiroy-Frego-M.bpf.c   | 87 +++++++++++++++++++
 1 file changed, 87 insertions(+)
 create mode 100644 drivers/hid/bpf/progs/Huion__Inspiroy-Frego-M.bpf.c

diff --git a/drivers/hid/bpf/progs/Huion__Inspiroy-Frego-M.bpf.c b/drivers/hid/bpf/progs/Huion__Inspiroy-Frego-M.bpf.c
new file mode 100644
index 0000000000000..e6ba2295dc775
--- /dev/null
+++ b/drivers/hid/bpf/progs/Huion__Inspiroy-Frego-M.bpf.c
@@ -0,0 +1,87 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include "vmlinux.h"
+#include "hid_bpf.h"
+#include "hid_bpf_helpers.h"
+#include <bpf/bpf_tracing.h>
+
+/*
+ * Huion Inspiroy Frego M Pen Tablet
+ * Model L610
+ * 256c:8251 (Bluetooth)
+ * 256c:2012 (USB)
+ */
+#define VID_HUION			0x256C
+#define PID_INSPIROY_FREGO_M		0x8251
+#define PID_L610			0x2012
+
+#define PEN_RDESC_SIZE			125
+#define SECONDARY_SWITCH_OFFSET		17
+
+HID_BPF_CONFIG(
+	HID_DEVICE(BUS_BLUETOOTH, HID_GROUP_GENERIC, VID_HUION, PID_INSPIROY_FREGO_M),
+	HID_DEVICE(BUS_USB, HID_GROUP_GENERIC, VID_HUION, PID_L610)
+);
+
+/*
+ * The pen descriptor reports the second side button as Secondary Tip Switch
+ * instead of Secondary Barrel Switch.
+ *
+ * Relevant part of the original pen report descriptor:
+ *
+ * 0x09, 0x42,       // Usage (Tip Switch)                  12
+ * 0x09, 0x44,       // Usage (Barrel Switch)               14
+ * 0x09, 0x43,       // Usage (Secondary Tip Switch)        16 <- change to 0x5a
+ * 0x09, 0x3c,       // Usage (Invert)                      18
+ * 0x09, 0x45,       // Usage (Eraser)                      20
+ * 0x15, 0x00,       // Logical Minimum (0)                 22
+ * 0x25, 0x01,       // Logical Maximum (1)                 24
+ */
+SEC(HID_BPF_RDESC_FIXUP)
+int BPF_PROG(fix_secondary_barrel_rdesc, struct hid_bpf_ctx *hctx)
+{
+	__u8 *data = hid_bpf_get_data(hctx, 0 /* offset */, HID_MAX_DESCRIPTOR_SIZE /* size */);
+
+	if (!data)
+		return 0; /* EPERM check */
+
+	if (hctx->size != PEN_RDESC_SIZE)
+		return 0;
+
+	if (data[0] != 0x05 || data[1] != 0x0d || /* Usage Page (Digitizers) */
+	    data[2] != 0x09 || data[3] != 0x02 || /* Usage (Pen) */
+	    data[16] != 0x09 ||
+	    data[SECONDARY_SWITCH_OFFSET] != 0x43) /* Secondary Tip Switch */
+		return 0;
+
+	data[SECONDARY_SWITCH_OFFSET] = 0x5a;
+
+	return 0;
+}
+
+HID_BPF_OPS(fix_secondary_barrel) = {
+	.hid_rdesc_fixup = (void *)fix_secondary_barrel_rdesc,
+};
+
+SEC("syscall")
+int probe(struct hid_bpf_probe_args *ctx)
+{
+	ctx->retval = ctx->rdesc_size != PEN_RDESC_SIZE;
+	if (ctx->retval) {
+		ctx->retval = -EINVAL;
+		return 0;
+	}
+
+	if (ctx->rdesc[0] != 0x05 || ctx->rdesc[1] != 0x0d || /* Usage Page (Digitizers) */
+	    ctx->rdesc[2] != 0x09 || ctx->rdesc[3] != 0x02 || /* Usage (Pen) */
+	    ctx->rdesc[16] != 0x09 ||
+	    ctx->rdesc[SECONDARY_SWITCH_OFFSET] != 0x43) { /* Secondary Tip Switch */
+		ctx->retval = -EINVAL;
+		return 0;
+	}
+
+	ctx->retval = 0;
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.53.0