Take the line rate as a constant so nothing has to be plugged in, and drop the config box

main.go

@@ -6,7 +6,6 @@ import (
 	"net"
 	"os"
 	"os/signal"
-	"strings"
 	"sync"
 	"sync/atomic"
 	"syscall"
@@ -18,12 +17,11 @@ import (
 const wireOverhead = 24
 
 type endpoint struct {
-	name  string
+	name string
-	tag   string
+	tag  string
-	idx   int
+	idx  int
-	mac   [6]byte
+	mac  [6]byte
-	mtu   int
+	mtu  int
-	speed float64
 }
 
 func (e endpoint) macString() string {
@@ -185,14 +183,7 @@ func lookupEndpoint(name string) (endpoint, error) {
 	}
 	var mac [6]byte
 	copy(mac[:], ifi.HardwareAddr)
-	// The rate columns are graded against this, so a guessed speed would silently
+	return endpoint{name: name, idx: ifi.Index, mac: mac, mtu: ifi.MTU}, nil
-	// grade every reading against the wrong target.
-	v, ok := readUint("/sys/class/net/" + name + "/speed")
-	if !ok || v == 0 {
-		return endpoint{}, fmt.Errorf("%s: cannot read link speed", name)
-	}
-	return endpoint{name: name, idx: ifi.Index, mac: mac, mtu: ifi.MTU,
-		speed: float64(v) / 1000}, nil
 }
 
 func (d *direction) snapshot() sample {
@@ -517,6 +508,10 @@ const (
 	batchSize  = 64
 
 	probeEther uint16 = etherBase + numStreams
+
+	// A constant rather than the negotiated speed, since this has to come up
+	// with no cable in the port and nothing to negotiate.
+	linkSpeed = 10.0
 )
 
 var frameSizes = []int{64, 128, 256, 512, 1024, 1280, 1514}
@@ -617,33 +612,16 @@ func run(aName, bName string, nsPerM float64) error {
 	for _, e := range []endpoint{a, b} {
 		linkRows = append(linkRows, []string{
 			paint(e.tag, cCyan), e.name, e.macString(),
-			fmt.Sprintf("%.0f Gb/s", e.speed), fmt.Sprintf("%d", e.mtu),
+			fmt.Sprintf("%.0f Gb/s", linkSpeed), fmt.Sprintf("%d", e.mtu),
 		})
 	}
 	fmt.Println(renderBox("LINKS",
 		[]string{"TAG", "INTERFACE", "MAC", "SPEED", "MTU"},
 		[]bool{false, false, false, true, true}, linkRows))
+	fmt.Println()
 
 	// One row carries both directions, so line rate is both links at once.
-	target := a.speed * float64(len(dirs))
+	target := linkSpeed * float64(len(dirs))
-	sizeStrs := make([]string, len(frameSizes))
-	for i, s := range frameSizes {
-		sizeStrs[i] = fmt.Sprintf("%d", s)
-	}
-	fmt.Println(renderBox("CONFIG",
-		[]string{"SETTING", "VALUE"},
-		[]bool{false, false}, [][]string{
-			{"frame sizes", strings.Join(sizeStrs, " ")},
-			{"streams", fmt.Sprintf("%d per direction, ethertypes 0x%04x-0x%04x",
-				numStreams, ethertypes[0], ethertypes[len(ethertypes)-1])},
-			{"probe", fmt.Sprintf("ethertype 0x%04x every %s", probeEther, probeInterval)},
-			{"batch", fmt.Sprintf("%d frames per syscall", batchSize)},
-			{"calibration", fmt.Sprintf("%g ns/m, zero taken from the shortest delay seen so far", nsPerM)},
-			{"buffers", fmt.Sprintf("sndbuf %sB, rcvbuf %sB",
-				scaleCount(uint64(sockBufSize(dirs[0].txFDs[0], unix.SO_SNDBUF))),
-				scaleCount(uint64(sockBufSize(dirs[0].rxFDs[0], unix.SO_RCVBUF))))},
-		}))
-	fmt.Println()
 
 	var doneTx, doneRx atomic.Bool
 	var wg sync.WaitGroup

sock.go

@@ -22,14 +22,6 @@ func setBufForce(fd, forceOpt, opt, size int) error {
 	return unix.SetsockoptInt(fd, unix.SOL_SOCKET, opt, size)
 }
 
-func sockBufSize(fd, opt int) int {
-	v, err := unix.GetsockoptInt(fd, unix.SOL_SOCKET, opt)
-	if err != nil {
-		panic(fmt.Sprintf("reading socket buffer size: %v", err))
-	}
-	return v
-}
-
 const (
 	sndbufBytes = 8 << 20
 	rcvbufBytes = 64 << 20

system.go

@@ -585,6 +585,9 @@ func withIoctlSocket(fn func(fd int) []checkResult) []checkResult {
 	return fn(fd)
 }
 
+// Nothing here waits for a carrier: the two ports are the two ends of the cable
+// under test, so with no cable there will never be one, and a dead wire is a
+// result to report rather than a reason to refuse to start.
 func configureSystem(ifnames []string, ethertypes []uint16) []checkResult {
 	return withIoctlSocket(func(fd int) []checkResult {
 		out := []checkResult{checkGovernor(wantGovernor)}