all: add a flag to the command line to select the serial implementation

This can be very useful for some purposes:

  * It makes it possible to disable the UART in cases where it is not
    needed or needs to be disabled to conserve power.
  * It makes it possible to disable the serial output to reduce code
    size, which may be important for some chips. Sometimes, a few kB can
    be saved this way.
  * It makes it possible to override the default, for example you might
    want to use an actual UART to debug the USB-CDC implementation.

It also lowers the dependency on having machine.Serial defined, which is
often not defined when targeting a chip. Eventually, we might want to
make it possible to write `-target=nrf52` or `-target=atmega328p` for
example to target the chip itself with no board specific assumptions.

The defaults don't change. I checked this by running `make smoketest`
before and after and comparing the results.

Author: aykevl

Makefile

@@ -436,6 +436,8 @@ endif
 	@$(MD5SUM) test.hex
 	$(TINYGO) build -size short -o test.hex -target=pca10040 -opt=1     examples/blinky1
 	@$(MD5SUM) test.hex
+	$(TINYGO) build -size short -o test.hex -target=pca10040 -serial=none examples/echo
+	@$(MD5SUM) test.hex
 	$(TINYGO) build             -o test.nro -target=nintendoswitch      examples/serial
 	@$(MD5SUM) test.nro
 	$(TINYGO) build -size short -o test.hex -target=pca10040 -opt=0     ./testdata/stdlib.go

compileopts/config.go

@@ -55,7 +55,7 @@ func (c *Config) GOARCH() string {
 
 // BuildTags returns the complete list of build tags used during this build.
 func (c *Config) BuildTags() []string {
-	tags := append(c.Target.BuildTags, []string{"tinygo", "gc." + c.GC(), "scheduler." + c.Scheduler()}...)
+	tags := append(c.Target.BuildTags, []string{"tinygo", "gc." + c.GC(), "scheduler." + c.Scheduler(), "serial." + c.Serial()}...)
 	for i := 1; i <= c.GoMinorVersion; i++ {
 		tags = append(tags, fmt.Sprintf("go1.%d", i))
 	}
@@ -113,6 +113,18 @@ func (c *Config) Scheduler() string {
 	return "coroutines"
 }
 
+// Serial returns the serial implementation for this build configuration: uart,
+// usb (meaning USB-CDC), or none.
+func (c *Config) Serial() string {
+	if c.Options.Serial != "" {
+		return c.Options.Serial
+	}
+	if c.Target.Serial != "" {
+		return c.Target.Serial
+	}
+	return "none"
+}
+
 // OptLevels returns the optimization level (0-2), size level (0-2), and inliner
 // threshold as used in the LLVM optimization pipeline.
 func (c *Config) OptLevels() (optLevel, sizeLevel int, inlinerThreshold uint) {

compileopts/options.go

@@ -9,6 +9,7 @@ import (
 var (
 	validGCOptions            = []string{"none", "leaking", "extalloc", "conservative"}
 	validSchedulerOptions     = []string{"none", "tasks", "coroutines"}
+	validSerialOptions        = []string{"none", "uart", "usb"}
 	validPrintSizeOptions     = []string{"none", "short", "full"}
 	validPanicStrategyOptions = []string{"print", "trap"}
 	validOptOptions           = []string{"none", "0", "1", "2", "s", "z"}
@@ -22,6 +23,7 @@ type Options struct {
 	GC              string
 	PanicStrategy   string
 	Scheduler       string
+	Serial          string
 	PrintIR         bool
 	DumpSSA         bool
 	VerifyIR        bool
@@ -59,6 +61,15 @@ func (o *Options) Verify() error {
 		}
 	}
 
+	if o.Serial != "" {
+		valid := isInArray(validSerialOptions, o.Serial)
+		if !valid {
+			return fmt.Errorf(`invalid serial option '%s': valid values are %s`,
+				o.Serial,
+				strings.Join(validSerialOptions, ", "))
+		}
+	}
+
 	if o.PrintSizes != "" {
 		valid := isInArray(validPrintSizeOptions, o.PrintSizes)
 		if !valid {

compileopts/target.go

@@ -31,6 +31,7 @@ type TargetSpec struct {
 	BuildTags        []string `json:"build-tags"`
 	GC               string   `json:"gc"`
 	Scheduler        string   `json:"scheduler"`
+	Serial           string   `json:"serial"` // which serial output to use (uart, usb, none)
 	Linker           string   `json:"linker"`
 	RTLib            string   `json:"rtlib"` // compiler runtime library (libgcc, compiler-rt)
 	Libc             string   `json:"libc"`

main.go

@@ -1009,6 +1009,7 @@ func main() {
 	gc := flag.String("gc", "", "garbage collector to use (none, leaking, extalloc, conservative)")
 	panicStrategy := flag.String("panic", "print", "panic strategy (print, trap)")
 	scheduler := flag.String("scheduler", "", "which scheduler to use (none, coroutines, tasks)")
+	serial := flag.String("serial", "", "which serial output to use (none, uart, usb)")
 	printIR := flag.Bool("printir", false, "print LLVM IR")
 	dumpSSA := flag.Bool("dumpssa", false, "dump internal Go SSA")
 	verifyIR := flag.Bool("verifyir", false, "run extra verification steps on LLVM IR")
@@ -1081,6 +1082,7 @@ func main() {
 		GC:              *gc,
 		PanicStrategy:   *panicStrategy,
 		Scheduler:       *scheduler,
+		Serial:          *serial,
 		PrintIR:         *printIR,
 		DumpSSA:         *dumpSSA,
 		VerifyIR:        *verifyIR,

src/machine/board_arduino_mkr1000.go

@@ -47,8 +47,6 @@ const (
 	LED = D6
 )
 
-var Serial = USB
-
 // USBCDC pins
 const (
 	USBCDC_DM_PIN Pin = PA24

src/machine/board_arduino_zero.go

@@ -35,8 +35,6 @@ const (
 	LED3 Pin = PB03 // RX LED
 )
 
-var Serial = USB
-
 // ADC pins
 const (
 	AREF Pin = PA03

src/machine/board_atsamd21.go

@@ -74,5 +74,3 @@ const (
 	PB30 Pin = 62
 	PB31 Pin = 63
 )
-
-var Serial = USB

src/machine/board_atsame54-xpro.go

@@ -15,8 +15,6 @@ const (
 	BUTTON = PB31
 )
 
-var Serial = USB
-
 const (
 	// https://ww1.microchip.com/downloads/en/DeviceDoc/70005321A.pdf
 

src/machine/board_bluepill.go

@@ -17,7 +17,7 @@ const (
 	BUTTON = PA0
 )
 
-var Serial = UART1
+var DefaultUART = UART1
 
 // UART pins
 const (