fix(isotp): add delay for flow-control frame

the remote (in our case the Nvidia Jetson) seems to have trouble sending
 messages over iso-tp in specific cases
using candump, we can see that the flow-control (FC) frame is received
before the last frame from the block (bs=8) is sent, which is not
expected.

A dump:
```
can0  3F2 [8]  27 2C 2D 2E 2F 30 31 32  <- frame zephyrproject-rtos#27 = 7/8
can0  1F2 [3]  30 08 00                 <- FC frame
can0  3F2 [8]  28 33 34 35 36 37 38 39  <- frame zephyrproject-rtos#28 = 8/8
```

Probing the can bus, we can see that frame zephyrproject-rtos#28 is sent before the FC
frame is responded to the Jetson (as per iso-tp protocol).
But the Jetson doesn't seem to correctly order these frames and each
time it happens, transmission stalls.

Adding some delay for the FC response on the mcu side seems to fix the
issue as the Jetson now has more time to set up the reception of the FC
frame.

Signed-off-by: Cyril Fougeray <[email protected]>

Author: fouge

subsys/canbus/isotp/Kconfig

@@ -26,6 +26,15 @@ config ISOTP_WFTMAX
 	  This value defines the maximum number of WAIT frames before the transmission
 	  is aborted.
 
+config ISOTP_FC_DELAY_US
+    int "Delay in microseconds before sending the FC frame"
+    default 0
+    range 0 10000
+    help
+        The sender might need time to set up reception of the FC frame. This config allows such case.
+        Resolution of that timer might not be ensured because the lower bound on the duration of a sleep is
+        the duration of a tick.
+
 config ISOTP_BS_TIMEOUT
 	int "Bs timeout [ms] (timeout for receiving the frame control)"
 	default 1000

subsys/canbus/isotp/isotp.c

@@ -161,6 +161,15 @@ static void receive_send_fc(struct isotp_recv_ctx *rctx, uint8_t fs)
 	frame.dlc = can_bytes_to_dlc(payload_len);
 #endif
 
+#if defined(CONFIG_ISOTP_FC_DELAY_US) && CONFIG_ISOTP_FC_DELAY_US != 0
+	// @fouge
+	// Remote might need some time to set up reception of the FC frame.
+	// As seen on Jetson, the last CF of the block was seen after
+	// the FC frame by candump on the Jetson 🤯. Each time, it leads
+	// to error during transmission of the message of iso-tp.
+	k_usleep(CONFIG_ISOTP_FC_DELAY_US);
+#endif
+
 	ret = can_send(rctx->can_dev, &frame, K_MSEC(ISOTP_A_TIMEOUT_MS), receive_can_tx, rctx);
 	if (ret) {
 		LOG_ERR("Can't send FC, (%d)", ret);