Wait after writing so userland gets expected speed

Constrain tty0tty's bandwidth by waiting the required amount of time
after writing bytes, so that userland can expect write/read speeds
consistent with the current baud rate, data bits, parity and stop
bits settings.

module/tty0tty.c

@@ -36,6 +36,7 @@
 #include <linux/tty_flip.h>
 #include <linux/serial.h>
 #include <linux/sched.h>
+#include <linux/delay.h>
 #include <linux/version.h>
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 11, 0)
 #include <linux/sched/signal.h>
@@ -97,6 +98,8 @@ struct tty0tty_serial {
  wait_queue_head_t wait;
  struct async_icount icount;
 
+ /* for timing control */
+ u64 nanosecs_per_byte;
 };
 
 static struct tty0tty_serial **tty0tty_table; /* initially all NULL */
@@ -224,6 +227,8 @@ static int tty0tty_write(struct tty_struct *tty, const unsigned char *buffer,
  struct tty0tty_serial *tty0tty = tty->driver_data;
  int retval = 0;
  struct tty_struct *ttyx = NULL;
+ u64 elapsed, delay;
+ ktime_t start_time = ktime_get_ns();
 
  if (!tty0tty)
  return -ENODEV;
@@ -261,6 +266,17 @@ static int tty0tty_write(struct tty_struct *tty, const unsigned char *buffer,
  retval = count;
  }
 
+ /* Compute time spent pushing this buffer */
+ elapsed = ktime_get_ns() - start_time;
+
+ /* Compute how much to wait to make sure this buffer is, from
+ * userland's point of view, pushed as slow as it would be on
+ * a real serial port */
+ delay = ((count * tty0tty->nanosecs_per_byte) - elapsed) / 1000;
+
+ if (delay > 1)
+ usleep_range(delay-1, delay);
+
 exit:
  up(&tty0tty->sem);
  return retval;
@@ -307,10 +323,13 @@ static void tty0tty_set_termios(struct tty_struct *tty,
  unsigned int iflag;
  unsigned int bits_per_byte;
  unsigned int baud_rate;
- u64 nanosecs_per_byte;
+ struct tty0tty_serial *tty0tty = tty->driver_data;
 
  DEBUG_PRINTK(KERN_DEBUG "%s - \n", __FUNCTION__);
 
+ if (!tty0tty)
+ return;
+
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(3,7,0)
  cflag = tty->termios.c_cflag;
  iflag = tty->termios.c_iflag;
@@ -410,8 +429,8 @@ static void tty0tty_set_termios(struct tty_struct *tty,
  DEBUG_PRINTK(KERN_DEBUG " - baud rate = %d\n", baud_rate);
 
  /* get the time a real serial port would require to push a byte */
- nanosecs_per_byte = 1000000000ULL / (baud_rate / bits_per_byte);
- DEBUG_PRINTK(KERN_DEBUG " - time per byte = %lluns\n", nanosecs_per_byte);
+ tty0tty->nanosecs_per_byte = 1000000000ULL / (baud_rate / bits_per_byte);
+ DEBUG_PRINTK(KERN_DEBUG " - time per byte = %lluns\n", tty0tty->nanosecs_per_byte);
 }
 
 //static int tty0tty_tiocmget(struct tty_struct *tty, struct file *file)