Fix sff panic deadlock, share the evdev probe, assorted review cleanups

internal/evdev/evdev.go

@@ -0,0 +1,91 @@
+// Package evdev is the kernel input-event ABI.
+package evdev
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+)
+
+const (
+	Syn       = 0x00
+	Key       = 0x01
+	Abs       = 0x03
+	SynReport = 0x00
+	BtnTouch  = 0x14a
+	AbsX      = 0x00
+	AbsY      = 0x01
+
+	propDirect = 0x01
+
+	EventSize = 24
+)
+
+func ioc(nr, size uintptr) uintptr {
+	const iocRead = 2
+	return iocRead<<30 | size<<16 | 'E'<<8 | nr
+}
+
+type Touchscreen struct {
+	F                      *os.File
+	MinX, MaxX, MinY, MaxY int32
+}
+
+// Picks the direct-input device that reports absolute X and Y, which is the
+// touchscreen; a mouse is relative and a keyboard has no absolute axes.
+func FindTouchscreen(flag int) (*Touchscreen, error) {
+	paths, err := filepath.Glob("/dev/input/event*")
+	if err != nil {
+		return nil, err
+	}
+	for _, p := range paths {
+		f, err := os.OpenFile(p, flag, 0)
+		if err != nil {
+			continue
+		}
+		var props, absBits uint64
+		if _, _, errno := unix.Syscall(unix.SYS_IOCTL, f.Fd(),
+			ioc(0x09, 8), uintptr(unsafe.Pointer(&props))); errno != 0 {
+			f.Close()
+			continue
+		}
+		if _, _, errno := unix.Syscall(unix.SYS_IOCTL, f.Fd(),
+			ioc(0x20+Abs, 8), uintptr(unsafe.Pointer(&absBits))); errno != 0 {
+			f.Close()
+			continue
+		}
+		if props&(1<<propDirect) == 0 ||
+			absBits&(1<<AbsX) == 0 || absBits&(1<<AbsY) == 0 {
+			f.Close()
+			continue
+		}
+		t := &Touchscreen{F: f}
+		if t.MinX, t.MaxX, err = axisRange(f, AbsX); err != nil {
+			f.Close()
+			return nil, fmt.Errorf("%s x range: %w", p, err)
+		}
+		if t.MinY, t.MaxY, err = axisRange(f, AbsY); err != nil {
+			f.Close()
+			return nil, fmt.Errorf("%s y range: %w", p, err)
+		}
+		if t.MaxX <= t.MinX || t.MaxY <= t.MinY {
+			f.Close()
+			return nil, fmt.Errorf("%s: degenerate axes x %d..%d y %d..%d",
+				p, t.MinX, t.MaxX, t.MinY, t.MaxY)
+		}
+		return t, nil
+	}
+	return nil, fmt.Errorf("no touchscreen found among /dev/input/event*")
+}
+
+func axisRange(f *os.File, axis uintptr) (min, max int32, err error) {
+	var info [6]int32
+	if _, _, errno := unix.Syscall(unix.SYS_IOCTL, f.Fd(),
+		ioc(0x40+axis, 24), uintptr(unsafe.Pointer(&info[0]))); errno != 0 {
+		return 0, 0, errno
+	}
+	return info[1], info[2], nil
+}