[PATCH v3 0/4] Improve error handling for qnap-mcu transfers

[PATCH v3 0/4] Improve error handling for qnap-mcu transfers

[Heiko Stuebner]

Digging deeper into how that MCU behaves, I found out it can return
more status codes than the "@0" for "ok".

The additional codes can report a failed checksum verification and
some "general" error for the command execution.

This also explains sporadic command timeout messages, I have seen
over time, when the controller sends an error code while we expect
a longer reply from it.

So while I'm not sure yet why it reports an error, with these changes
we at least handle the error return code in a meaningful way.

changes in v3:
- fix stupid mistake of not running checkpatch after reorganizing
  the error handling code and having the function opening { not
  on the next line

changes in v2:
- rebase on top of 6.18-rc1
- include improvement suggestions from Lee

Heiko Stuebner (4):
  mfd: qnap-mcu: Calculate the checksum on the actual number of bytes
    received
  mfd: qnap-mcu: Use EPROTO in stead of EIO on checksum errors
  mfd: qnap-mcu: Move checksum verification to its own function
  mfd: qnap-mcu: Add proper error handling for command errors

 drivers/mfd/qnap-mcu.c | 79 +++++++++++++++++++++++++++++++++++++++---
 1 file changed, 74 insertions(+), 5 deletions(-)

-- 
2.47.2

[PATCH v3 1/4] mfd: qnap-mcu: Calculate the checksum on the actual number of bytes received

[Heiko Stuebner]

In the case of an error message, the number of received bytes can be
less than originally expected but still contain a valid message.

If the transfer itself ended in an error we would exit earlier already.

So calculate the checksum on the number of received bytes and not the
number of expected bytes.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
---
 drivers/mfd/qnap-mcu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/mfd/qnap-mcu.c b/drivers/mfd/qnap-mcu.c
index 2be429a50611..e3210541ec56 100644
--- a/drivers/mfd/qnap-mcu.c
+++ b/drivers/mfd/qnap-mcu.c
@@ -175,8 +175,8 @@ int qnap_mcu_exec(struct qnap_mcu *mcu,
 		return -ETIMEDOUT;
 	}
 
-	crc = qnap_mcu_csum(rx, reply_data_size);
-	if (crc != rx[reply_data_size]) {
+	crc = qnap_mcu_csum(rx, reply->received - QNAP_MCU_CHECKSUM_SIZE);
+	if (crc != rx[reply->received - QNAP_MCU_CHECKSUM_SIZE]) {
 		dev_err(&mcu->serdev->dev, "Invalid Checksum received\n");
 		return -EIO;
 	}
-- 
2.47.2

[PATCH v3 2/4] mfd: qnap-mcu: Use EPROTO in stead of EIO on checksum errors

[Heiko Stuebner]

EPROTO stands for protocol error and a lot of driver already use it
to designate errors in the sent or received data from a peripheral.

So use it in the qnap-mcu as well for checksum errors.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
---
 drivers/mfd/qnap-mcu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/mfd/qnap-mcu.c b/drivers/mfd/qnap-mcu.c
index e3210541ec56..b4b630f7d413 100644
--- a/drivers/mfd/qnap-mcu.c
+++ b/drivers/mfd/qnap-mcu.c
@@ -178,7 +178,7 @@ int qnap_mcu_exec(struct qnap_mcu *mcu,
 	crc = qnap_mcu_csum(rx, reply->received - QNAP_MCU_CHECKSUM_SIZE);
 	if (crc != rx[reply->received - QNAP_MCU_CHECKSUM_SIZE]) {
 		dev_err(&mcu->serdev->dev, "Invalid Checksum received\n");
-		return -EIO;
+		return -EPROTO;
 	}
 
 	memcpy(reply_data, rx, reply_data_size);
-- 
2.47.2

[PATCH v3 3/4] mfd: qnap-mcu: Move checksum verification to its own function

[Heiko Stuebner]

We'll need the checksum check in a second place in the future, so
move the verification code to a separate function.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
---
 drivers/mfd/qnap-mcu.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/mfd/qnap-mcu.c b/drivers/mfd/qnap-mcu.c
index b4b630f7d413..cd836bdd44a8 100644
--- a/drivers/mfd/qnap-mcu.c
+++ b/drivers/mfd/qnap-mcu.c
@@ -78,6 +78,13 @@ static u8 qnap_mcu_csum(const u8 *buf, size_t size)
 	return csum;
 }
 
+static bool qnap_mcu_verify_checksum(const u8 *buf, size_t size)
+{
+	u8 crc = qnap_mcu_csum(buf, size - QNAP_MCU_CHECKSUM_SIZE);
+
+	return crc == buf[size - QNAP_MCU_CHECKSUM_SIZE];
+}
+
 static int qnap_mcu_write(struct qnap_mcu *mcu, const u8 *data, u8 data_size)
 {
 	unsigned char tx[QNAP_MCU_TX_BUFFER_SIZE];
@@ -150,7 +157,6 @@ int qnap_mcu_exec(struct qnap_mcu *mcu,
 	size_t length = reply_data_size + QNAP_MCU_CHECKSUM_SIZE;
 	struct qnap_mcu_reply *reply = &mcu->reply;
 	int ret = 0;
-	u8 crc;
 
 	if (length > sizeof(rx)) {
 		dev_err(&mcu->serdev->dev, "expected data too big for receive buffer");
@@ -175,8 +181,7 @@ int qnap_mcu_exec(struct qnap_mcu *mcu,
 		return -ETIMEDOUT;
 	}
 
-	crc = qnap_mcu_csum(rx, reply->received - QNAP_MCU_CHECKSUM_SIZE);
-	if (crc != rx[reply->received - QNAP_MCU_CHECKSUM_SIZE]) {
+	if (!qnap_mcu_verify_checksum(rx, reply->received)) {
 		dev_err(&mcu->serdev->dev, "Invalid Checksum received\n");
 		return -EPROTO;
 	}
-- 
2.47.2

[PATCH v3 4/4] mfd: qnap-mcu: Add proper error handling for command errors

[Heiko Stuebner]

Further investigation revealed that the MCU in QNAP devices may return
two error states. One "@8" for a checksum error in the submitted command
and one "@9" for any generic (and sadly unspecified) error.

These error codes with 2 data character can of course also be shorter
then the expected reply length for the submitted command, so we'll
need to check the received data for error codes and exit the receive
portion early in that case.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
---
 drivers/mfd/qnap-mcu.c | 66 +++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 65 insertions(+), 1 deletion(-)

diff --git a/drivers/mfd/qnap-mcu.c b/drivers/mfd/qnap-mcu.c
index cd836bdd44a8..f81c69f22254 100644
--- a/drivers/mfd/qnap-mcu.c
+++ b/drivers/mfd/qnap-mcu.c
@@ -19,6 +19,7 @@
 /* The longest command found so far is 5 bytes long */
 #define QNAP_MCU_MAX_CMD_SIZE		5
 #define QNAP_MCU_MAX_DATA_SIZE		36
+#define QNAP_MCU_ERROR_SIZE		2
 #define QNAP_MCU_CHECKSUM_SIZE		1
 
 #define QNAP_MCU_RX_BUFFER_SIZE		\
@@ -103,6 +104,48 @@ static int qnap_mcu_write(struct qnap_mcu *mcu, const u8 *data, u8 data_size)
 	return serdev_device_write(mcu->serdev, tx, length, HZ);
 }
 
+static bool qnap_mcu_is_error_msg(size_t size)
+{
+	return (size == QNAP_MCU_ERROR_SIZE + QNAP_MCU_CHECKSUM_SIZE);
+}
+
+static bool qnap_mcu_reply_is_generic_error(unsigned char *buf, size_t size)
+{
+	if (!qnap_mcu_is_error_msg(size))
+		return false;
+
+	if (buf[0] == '@' && buf[1] == '9')
+		return true;
+
+	return false;
+}
+
+static bool qnap_mcu_reply_is_checksum_error(unsigned char *buf, size_t size)
+{
+	if (!qnap_mcu_is_error_msg(size))
+		return false;
+
+	if (buf[0] == '@' && buf[1] == '8')
+		return true;
+
+	return false;
+}
+
+static bool qnap_mcu_reply_is_any_error(struct qnap_mcu *mcu, unsigned char *buf, size_t size)
+{
+	if (qnap_mcu_reply_is_generic_error(buf, size)) {
+		dev_err(&mcu->serdev->dev, "Controller sent generic error response\n");
+		return true;
+	}
+
+	if (qnap_mcu_reply_is_checksum_error(buf, size)) {
+		dev_err(&mcu->serdev->dev, "Controller received invalid checksum for the command\n");
+		return true;
+	}
+
+	return false;
+}
+
 static size_t qnap_mcu_receive_buf(struct serdev_device *serdev, const u8 *buf, size_t size)
 {
 	struct device *dev = &serdev->dev;
@@ -136,6 +179,24 @@ static size_t qnap_mcu_receive_buf(struct serdev_device *serdev, const u8 *buf,
 		}
 	}
 
+	/*
+	 * We received everything the uart had to offer for now.
+	 * This could mean that either the uart will send more in a 2nd
+	 * receive run, or that the MCU cut the reply short because it
+	 * sent an error code instead of the expected reply.
+	 *
+	 * So check if the received data has the correct size for an error
+	 * reply and if it matches, is an actual error code.
+	 */
+	if (qnap_mcu_is_error_msg(reply->received) &&
+	    qnap_mcu_verify_checksum(reply->data, reply->received) &&
+	    qnap_mcu_reply_is_any_error(mcu, reply->data, reply->received)) {
+		/* The reply was an error code, we're done */
+		reply->length = 0;
+
+		complete(&reply->done);
+	}
+
 	/*
 	 * The only way to get out of the above loop and end up here
 	 * is through consuming all of the supplied data, so here we
@@ -182,10 +243,13 @@ int qnap_mcu_exec(struct qnap_mcu *mcu,
 	}
 
 	if (!qnap_mcu_verify_checksum(rx, reply->received)) {
-		dev_err(&mcu->serdev->dev, "Invalid Checksum received\n");
+		dev_err(&mcu->serdev->dev, "Invalid Checksum received from controller\n");
 		return -EPROTO;
 	}
 
+	if (qnap_mcu_reply_is_any_error(mcu, rx, reply->received))
+		return -EPROTO;
+
 	memcpy(reply_data, rx, reply_data_size);
 
 	return 0;
-- 
2.47.2

Re: [PATCH v3 0/4] Improve error handling for qnap-mcu transfers

[Lee Jones]

On Thu, 13 Nov 2025 17:52:14 +0100, Heiko Stuebner wrote:
> Digging deeper into how that MCU behaves, I found out it can return
> more status codes than the "@0" for "ok".
> 
> The additional codes can report a failed checksum verification and
> some "general" error for the command execution.
> 
> This also explains sporadic command timeout messages, I have seen
> over time, when the controller sends an error code while we expect
> a longer reply from it.
> 
> [...]

Applied, thanks!

[1/4] mfd: qnap-mcu: Calculate the checksum on the actual number of bytes received
      commit: b4881070a02b017aea84592c424d5a980ed261c4
[2/4] mfd: qnap-mcu: Use EPROTO in stead of EIO on checksum errors
      commit: c94fce30e190555d74e2769b5fe4a932d0ad432e
[3/4] mfd: qnap-mcu: Move checksum verification to its own function
      commit: c3223f562586307b1bcb014475d0b71913972145
[4/4] mfd: qnap-mcu: Add proper error handling for command errors
      commit: 56c1245d51faab70bf68cc3a5cd3925768e6375b

--
Lee Jones [李琼斯]