From ce7f1a7f7aa0dbdc8d4ae1536db8f9605b4e4702 Mon Sep 17 00:00:00 2001
From: Sam Anthony <sam@samanthony.xyz>
Date: Fri, 5 Sep 2025 17:55:43 -0400
Subject: rework usb state machine

---
 fw/usb.c | 159 ++++++++++++++++++++++++++++++++-------------------------------
 1 file changed, 82 insertions(+), 77 deletions(-)

diff --git a/fw/usb.c b/fw/usb.c
index a843945..bdf3b64 100644
--- a/fw/usb.c
+++ b/fw/usb.c
@@ -10,6 +10,8 @@
 
 #include "usb.h"
 
+/***** Constants *****/
+
 // Line coding
 // See struct USB_CDC_LINE_CODING
 enum {
@@ -19,24 +21,84 @@ enum {
 	DATA_BITS = 8,
 };
 
-// State
-typedef enum {
-	IDLE,
-	ECHO,
-	WRITE_EEPROM,
-	READ_EEPROM,
+/***** Types *****/
+
+// A state is a function that executes the state's action and returns the next state.
+typedef struct State {
+	struct State* (*next)(void);
 } State;
 
+/***** Function Declarations *****/
+
+// States
+static State *idleState(void);
+static State *echoState(void);
+static State *writeEepromState(void);
+static State *readEepromState(void);
+
+/***** Global Variables *****/
+
 static uint8_t readBuf[CDC_DATA_OUT_EP_SIZE];
 static uint8_t writeBuf[CDC_DATA_IN_EP_SIZE];
 static uint8_t readLen = 0u;
 
+/***** Function Definitions *****/
+
+void
+usbTask(void) {
+	static State state = {idleState};
+
+	USBDeviceTasks();
+
+	if (USBGetDeviceState() < CONFIGURED_STATE) {
+		return;
+	}
+	if (USBIsDeviceSuspended()) {
+		return;
+	}
+
+	if (USBUSARTIsTxTrfReady()) {
+		// Execute action and transition to next state
+		state = *state.next();
+	}
+
+	CDCTxService();
+}
+
+// Read (the start of) a command from USB.
+// Leaves <args> at the beginning of the read buffer.
+static State *
+idleState(void) {
+	static State state;
+	uint8_t opcode;
+
+	readLen = getsUSBUSART(readBuf, sizeof(readBuf));
+	if (readLen >= 2u) {
+		opcode = readBuf[0u];
+
+		// skip <opcode> <space>
+		memmove(readBuf, readBuf+2u, readLen-2u);
+		readLen -= 2u;
+
+		switch (opcode) {
+		case 'e': state.next = echoState; break;
+		case 'w': state.next = writeEepromState; break;
+		case 'r': state.next = readEepromState; break;
+		default: state.next =  idleState; break; // invalid command
+		}
+	} else {
+		// Invalid command. Must start with <opcode> <space>
+		state.next = idleState;
+	}
+
+	return &state;
+}
+
 // Handle "e" echo command.
-// Returns the next state.
-static State
-echo(void) {
+static State *
+echoState(void) {
+	static State state = {echoState};
 	uint8_t i;
-	State state = ECHO;
 
 	if (readLen == 0u) {
 		readLen = getsUSBUSART(readBuf, sizeof(readBuf));
@@ -46,7 +108,7 @@ echo(void) {
 		writeBuf[i] = readBuf[i];
 		if (readBuf[i] == '\n') {
 			// End of command
-			state = IDLE;
+			state.next = idleState;
 			i++;
 			break;
 		}
@@ -57,80 +119,23 @@ echo(void) {
 	}
 	readLen = 0u;
 
-	return state;
+	return &state;
 }
 
 // Handle "w" write eeprom command.
-// Returns the next state.
-static State
-writeEeprom(void) {
+static State *
+writeEepromState(void) {
+	static State state = {idleState};
 	// TODO
-	return IDLE;
+	return &state;
 }
 
 // Handle "r" read eeprom command.
-// Returns the next state.
-static State
-readEeprom(void) {
+static State *
+readEepromState(void) {
+	static State state = {idleState};
 	// TODO
-	return IDLE;
-}
-
-static State
-cmdState(uint8_t opcode) {
-	switch (opcode) {
-	case 'e': return ECHO;
-	case 'w': return WRITE_EEPROM;
-	case 'r': return READ_EEPROM;
-	default: return IDLE; // invalid command
-	}
-}
-
-// Read (the start of) a command from USB.
-// Leaves <args> at the beginning of the read buffer.
-// Returns the next state based on the opcode of the command.
-static State
-readCmd(void) {
-	uint8_t opcode;
-
-	readLen = getsUSBUSART(readBuf, sizeof(readBuf));
-	if (readLen < 2u) {
-		// Invalid command. Must start with <opcode> <space>
-		return IDLE;
-	}
-
-	opcode = readBuf[0u];
-
-	// skip <opcode> <space>
-	memmove(readBuf, readBuf+2u, readLen-2u);
-	readLen -= 2u;
-
-	return cmdState(opcode);
-}
-
-void
-usbTask(void) {
-	static State state = IDLE;
-
-	USBDeviceTasks();
-
-	if (USBGetDeviceState() < CONFIGURED_STATE) {
-		return;
-	}
-	if (USBIsDeviceSuspended()) {
-		return;
-	}
-
-	if (USBUSARTIsTxTrfReady()) {
-		switch (state) {
-		case IDLE: state = readCmd(); break;
-		case ECHO: state = echo(); break;
-		case WRITE_EEPROM: state = writeEeprom(); break;
-		case READ_EEPROM: state = readEeprom(); break;
-		}
-	}
-
-	CDCTxService();
+	return &state;
 }
 
 static void
-- 
cgit v1.2.3