Take the console into graphics mode while drawing

fb.go

@@ -12,6 +12,11 @@ const (
 	fbioGetVScreenInfo = 0x4600
 	fbioBlank          = 0x4611
 	fbBlankUnblank     = 0
+
+	kdSetMode   = 0x4b3a
+	kdText      = 0x00
+	kdGraphics  = 0x01
+	consolePath = "/dev/tty0"
 )
 
 // The variable screen info is a flat run of 40 u32s, so it is read as an array
@@ -28,6 +33,7 @@ const (
 
 type framebuffer struct {
 	file   *os.File
+	tty    *os.File
 	mem    []byte
 	back   []byte
 	w      int
@@ -38,6 +44,22 @@ type framebuffer struct {
 	bShift uint
 }
 
+// The kernel console draws into the same framebuffer, including a blinking
+// cursor and any keyboard echo, which fights every frame we write. Putting the
+// active console into graphics mode stops it touching the framebuffer at all.
+func takeConsole() (*os.File, error) {
+	tty, err := os.OpenFile(consolePath, os.O_RDWR, 0)
+	if err != nil {
+		return nil, err
+	}
+	if _, _, errno := unix.Syscall(unix.SYS_IOCTL, tty.Fd(),
+		kdSetMode, kdGraphics); errno != 0 {
+		tty.Close()
+		return nil, fmt.Errorf("console to graphics mode: %w", errno)
+	}
+	return tty, nil
+}
+
 func openFramebuffer(path string) (*framebuffer, error) {
 	f, err := os.OpenFile(path, os.O_RDWR, 0)
 	if err != nil {
@@ -81,12 +103,21 @@ func openFramebuffer(path string) (*framebuffer, error) {
 		return nil, fmt.Errorf("mmap: %w", err)
 	}
 	fb.back = make([]byte, fb.stride*fb.h)
+
+	fb.tty, err = takeConsole()
+	if err != nil {
+		unix.Munmap(fb.mem)
+		f.Close()
+		return nil, err
+	}
 	return fb, nil
 }
 
 func (fb *framebuffer) close() {
 	fb.fill(rgb{})
 	fb.flush()
+	unix.Syscall(unix.SYS_IOCTL, fb.tty.Fd(), kdSetMode, kdText)
+	fb.tty.Close()
 	unix.Munmap(fb.mem)
 	fb.file.Close()
 }