[PATCH v2 2/2] platform/x86: dell-uart-backlight: recover from stray EC response byte

[chitranshmandhaniya@gmail.com]

From: Chitransh Mandhaniya <Chitransh_Mandhaniya@dell.com>

On some Dell AIO machines the EC occasionally inserts a spurious byte
after the cmd-echo in UART backlight response frames.  A get-brightness
reply "04 0c 0a e5" arrives as "04 0c 0d 0a e5". The length field still
reflects the original frame size, so the parser consumes the stray byte
as payload and mistakes the real payload for the checksum, returning
-EIO.

On the first checksum failure for a frame longer than the minimum size,
drop the byte at the first payload offset, shift the remaining data down
by one, and call dell_uart_bl_receive_data() again to re-fetch the
checksum byte from the wire and re-validate.

Limit this single-byte resync to once per command via a resync_used flag
cleared at the start of each transaction. A second mismatch is still a
hard error.  Well-formed replies pass on the first check and never enter
the recovery path.

Tested on Dell OptiPlex AIO 7410 65W (0TKWC4) where the EC inserts a
stray byte on every get-brightness response -- resync fixes it
transparently.

Signed-off-by: Chitransh Mandhaniya <Chitransh_Mandhaniya@dell.com>
---
 .../platform/x86/dell/dell-uart-backlight.c   | 23 ++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/drivers/platform/x86/dell/dell-uart-backlight.c b/drivers/platform/x86/dell/dell-uart-backlight.c
index b61499d7b3f8..085dcc055554 100644
--- a/drivers/platform/x86/dell/dell-uart-backlight.c
+++ b/drivers/platform/x86/dell/dell-uart-backlight.c
@@ -60,6 +60,7 @@ struct dell_uart_backlight {
 	u8 resp_len;
 	u8 resp_max_len;
 	u8 pending_cmd;
+	bool resync_used;
 	int status;
 	int power;
 };
@@ -91,6 +92,7 @@ static int dell_uart_bl_command(struct dell_uart_backlight *dell_bl,
 	dell_bl->resp_len = -1; /* Invalid / unset */
 	dell_bl->resp_max_len = resp_max_len;
 	dell_bl->pending_cmd = cmd[1];
+	dell_bl->resync_used = false;
 
 	/* The TTY buffer should be big enough to take the entire cmd in one go */
 	ret = serdev_device_write_buf(to_serdev_device(dell_bl->dev), cmd, cmd_len);
@@ -270,8 +272,27 @@ static size_t dell_uart_bl_receive(struct serdev_device *serdev, const u8 *data,
 	if (ret < 0)
 		return len; /* Response not complete yet */
 
+	/* Stray byte after cmd-echo? Drop it, shift data down, re-fetch checksum. */
+	if (dell_bl->status == -EIO &&
+	    !dell_bl->resync_used && dell_bl->resp_len > MIN_RESP_LEN) {
+		dell_bl->resync_used = true;
+		dell_bl->status = -EBUSY;
+
+		dev_dbg(dell_bl->dev, "Resync: dropping stray byte 0x%02x at offset %d\n",
+			dell_bl->resp[RESP_DATA], RESP_DATA);
+
+		memmove(&dell_bl->resp[RESP_DATA], &dell_bl->resp[RESP_DATA + 1],
+			dell_bl->resp_len - RESP_DATA - 1);
+		dell_bl->resp_idx = dell_bl->resp_len - 1;
+
+		/* Re-fetch checksum byte from remaining data */
+		ret = dell_uart_bl_receive_data(dell_bl, data + ret, len - ret);
+		if (ret < 0)
+			return len; /* Need more data for checksum byte */
+	}
+
 	wake_up(&dell_bl->wait_queue);
-	return ret;
+	return len;
 }
 
 static const struct serdev_device_ops dell_uart_bl_serdev_ops = {
-- 
2.43.0