[PATCH] HID: letsketch: skip string descriptors the firmware does not implement

["Eren Çakar" <hey@erencakar.com>]

The Starcom GC-422 is a rebranded LetSketch WP9620C with the same USB
ID (6161:4d15), so it binds to this driver. Its firmware
("Vision_V201_180530") does not implement every string descriptor index
that the raw mode handshake reads. For those indices usb_string() keeps
returning an empty string or -EPIPE, letsketch_get_string() gives up
after its retries, and the probe fails:

  letsketch ...: probe with driver letsketch failed with error -5

The tablet then does not work at all.

The handshake only needs the reads to happen, not their contents. Once
the retries are used up, report an index that still reads as empty or
STALLs as missing with -ENOENT, and let the probe skip it. Other errors
still fail the probe as before, and the retries stay in place, so
firmware that only fails a read now and then behaves as it did.

With this change the GC-422 completes the handshake, switches to raw
mode and reports pen position, pressure and the five pad buttons.

The change and this message were written with the help of an AI coding
assistant (Claude Code); I reviewed them and tested the result on the
hardware.

Fixes: 33a5c2793451 ("HID: Add new Letsketch tablet driver")
Signed-off-by: Eren Çakar <hey@erencakar.com>
---
 drivers/hid/hid-letsketch.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/drivers/hid/hid-letsketch.c b/drivers/hid/hid-letsketch.c
index b52e93a..6a93245 100644
--- a/drivers/hid/hid-letsketch.c
+++ b/drivers/hid/hid-letsketch.c
@@ -213,6 +213,12 @@ static int letsketch_raw_event(struct hid_device *hdev,
  * string descriptors. But the firmware is buggy and does not like it if
  * we do this too fast. Even if we go slow sometimes the usb_string() call
  * fails. Ignore errors and retry it a couple of times if necessary.
+ *
+ * Some firmware revisions (e.g. "Vision_V201_180530", sold as the Starcom
+ * GC-422) do not implement every string descriptor index the handshake
+ * reads. For those indices usb_string() keeps returning an empty string or
+ * -EPIPE (STALL). Report such an index as missing with -ENOENT once the
+ * retries are used up, so that the caller can skip it.
  */
 static int letsketch_get_string(struct usb_device *udev, int index, char *buf, int size)
 {
@@ -225,9 +231,14 @@ static int letsketch_get_string(struct usb_device *udev, int index, char *buf, i
 			return 0;
 	}
 
+	if (ret == 0 || ret == -EPIPE) {
+		dev_dbg(&udev->dev, "String descriptor %d not implemented\n", index);
+		return -ENOENT;
+	}
+
 	dev_err(&udev->dev, "Max retries (%d) exceeded reading string descriptor %d\n",
 		LETSKETCH_GET_STRING_RETRIES, index);
-	return ret ? ret : -EIO;
+	return ret;
 }
 
 static int letsketch_probe(struct hid_device *hdev, const struct hid_device_id *id)
@@ -256,6 +267,8 @@ static int letsketch_probe(struct hid_device *hdev, const struct hid_device_id *
 	 */
 	for (i = LETSKETCH_INFO_STR_IDX_BEGIN; i <= LETSKETCH_INFO_STR_IDX_END; i++) {
 		ret = letsketch_get_string(udev, i, buf, sizeof(buf));
+		if (ret == -ENOENT)
+			continue;
 		if (ret)
 			return ret;
 
@@ -264,16 +277,16 @@ static int letsketch_probe(struct hid_device *hdev, const struct hid_device_id *
 
 	for (i = 1; i <= 250; i++) {
 		ret = letsketch_get_string(udev, i, buf, sizeof(buf));
-		if (ret)
+		if (ret && ret != -ENOENT)
 			return ret;
 	}
 
 	ret = letsketch_get_string(udev, 0x64, buf, sizeof(buf));
-	if (ret)
+	if (ret && ret != -ENOENT)
 		return ret;
 
 	ret = letsketch_get_string(udev, LETSKETCH_INFO_STR_IDX_BEGIN, buf, sizeof(buf));
-	if (ret)
+	if (ret && ret != -ENOENT)
 		return ret;
 
 	/*
-- 
2.56.1