[PATCH v2] platform/chrome: cros_ec_proto: Send command again when timeout occurs

[Patryk Duda]

Sometimes kernel is trying to probe Fingerprint MCU (FPMCU) when it
hasn't initialized SPI yet. This can happen because FPMCU is restarted
during system boot and kernel can send message in short window
eg. between sysjump to RW and SPI initialization.

Cc: <stable@xxxxxxxxxxxxxxx> # 4.4+
Signed-off-by: Patryk Duda <pdk@xxxxxxxxxxxx>
---
Fingerprint MCU is rebooted during system startup by AP firmware (coreboot).
During cold boot kernel can query FPMCU in a window just after jump to RW
section of firmware but before SPI is initialized. The window was
shortened to <1ms, but it can't be eliminated completly.

Communication with FPMCU (and all devices based on EC) is bi-directional.
When kernel sends message, EC will send EC_SPI* status codes. When EC is
not able to process command one of bytes will be eg. EC_SPI_NOT_READY.
This mechanism won't work when SPI is not initailized on EC side. In fact,
buffer is filled with 0xFF bytes, so from kernel perspective device is not
responding. To avoid this problem, we can query device once again. We are
already waiting EC_MSG_DEADLINE_MS for response, so we can send command
immediately.

Best regards,
Patryk
v1 -> v2
- Removed message about timeout
 drivers/platform/chrome/cros_ec_proto.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/platform/chrome/cros_ec_proto.c b/drivers/platform/chrome/cros_ec_proto.c
index aa7f7aa77297..a7404d69b2d3 100644
--- a/drivers/platform/chrome/cros_ec_proto.c
+++ b/drivers/platform/chrome/cros_ec_proto.c
@@ -279,6 +279,15 @@ static int cros_ec_host_command_proto_query(struct cros_ec_device *ec_dev,
 	msg->insize = sizeof(struct ec_response_get_protocol_info);
 
 	ret = send_command(ec_dev, msg);
+	/*
+	 * Send command once again when timeout occurred.
+	 * Fingerprint MCU (FPMCU) is restarted during system boot which
+	 * introduces small window in which FPMCU won't respond for any
+	 * messages sent by kernel. There is no need to wait before next
+	 * attempt because we waited at least EC_MSG_DEADLINE_MS.
+	 */
+	if (ret == -ETIMEDOUT)
+		ret = send_command(ec_dev, msg);
 
 	if (ret < 0) {
 		dev_dbg(ec_dev->dev,
-- 
2.31.1.751.gd2f1c929bd-goog