esp32: fix I2C timeouts on larger transfers (#5674)

* esp32: fix I2C timeouts on larger transfers

* fix: clearer comment

Author: pottekkat

src/machine/machine_esp32_i2c.go

@@ -242,6 +242,7 @@ func (i2c *I2C) transmit(addr uint16, cmd []i2cCommand, timeoutMS int) error {
 	timeoutNS := int64(timeoutMS) * 1000000
 	needAddress := true
 	needRestart := false
+	isRead := false
 	var readTo []byte
 	for cmdIdx, reg := 0, &i2c.Bus.COMD0; cmdIdx < len(cmd); {
 		c := &cmd[cmdIdx]
@@ -253,6 +254,7 @@ func (i2c *I2C) transmit(addr uint16, cmd []i2cCommand, timeoutMS int) error {
 			cmdIdx++
 
 		case i2cCMD_WRITE:
+			isRead = false
 			count := 32
 			if needAddress {
 				needAddress = false
@@ -266,15 +268,17 @@ func (i2c *I2C) transmit(addr uint16, cmd []i2cCommand, timeoutMS int) error {
 			reg.Set(i2cCMD_WRITE | uint32(32-count))
 			reg = nextAddress(reg)
 
-			if c.head < len(c.data) {
-				reg.Set(i2cCMD_END)
-				reg = nil
-			} else {
+			// Always end a write segment with END, so STOP goes out alone
+			// in its own segment (ref: ESP32 TRM v5.8, section 21.3.5).
+			reg.Set(i2cCMD_END)
+			reg = nil
+			if c.head >= len(c.data) {
 				cmdIdx++
 			}
 			needRestart = true
 
 		case i2cCMD_READ:
+			isRead = true
 			if needAddress {
 				needAddress = false
 				i2c.Bus.SetDATA_FIFO_RDATA((uint32(addr)&0x7f)<<1 | 1)
@@ -308,10 +312,16 @@ func (i2c *I2C) transmit(addr uint16, cmd []i2cCommand, timeoutMS int) error {
 				reg = nextAddress(reg)
 			}
 
-			if split {
+			if split && bytes > 0 {
+				reg.Set(i2cCMD_END)
+				readTo = c.data[c.head : c.head+bytes]
+				reg = nil
+			} else if split {
 				reg.Set(i2cCMD_READLAST | 1)
 				reg = nextAddress(reg)
-				readTo = c.data[c.head : c.head+bytes+1] // read bytes + 1 last byte
+				reg.Set(i2cCMD_END)
+				readTo = c.data[c.head : c.head+1]
+				reg = nil
 				cmdIdx++
 			} else {
 				reg.Set(i2cCMD_END)
@@ -333,7 +343,7 @@ func (i2c *I2C) transmit(addr uint16, cmd []i2cCommand, timeoutMS int) error {
 				if nanotime() > end {
 					// timeout leaves the bus in an undefined state, reset
 					i2c.resetBus()
-					if readTo != nil {
+					if isRead {
 						return errI2CReadTimeout
 					}
 					return errI2CWriteTimeout
@@ -345,7 +355,7 @@ func (i2c *I2C) transmit(addr uint16, cmd []i2cCommand, timeoutMS int) error {
 			case mask&esp.I2C_INT_STATUS_TIME_OUT_INT_ST_Msk != 0:
 				// timeout leaves the bus in an undefined state, reset
 				i2c.resetBus()
-				if readTo != nil {
+				if isRead {
 					return errI2CReadTimeout
 				}
 				return errI2CWriteTimeout