USB: serial: ch341: simulate break condition if not supported

A subset of all CH341 devices don't support a real break condition. This
fact is already used in the "ch341_detect_quirks" function. With this
change a quirk is implemented to simulate a break condition by
temporarily lowering the baud rate and sending a NUL byte.

The primary drawbacks of this approach are that the duration of the
break can't be controlled by userland and that data incoming during
a simulated break is corrupted.

The "TTY_DRIVER_HARDWARE_BREAK" serial driver flag was investigated as
an alternative. It's a driver-wide flag and would've required
significant changes to the serial and USB-serial driver frameworks to
expose it for individual USB-serial adapters.

Tested by sending a break condition and watching the TX pin using an
oscilloscope.
Signed-off-by: Michael Hanselmann <public@hansmi.ch>
Link: https://lore.kernel.org/r/f34a9b6e-ec2a-0873-e97b-2d5b2170e2ff@msgid.hansmi.ch
[ johan: condense info message ]
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/ch341.c

@@ -78,6 +78,7 @@
 #define CH341_LCR_CS5          0x00
 
 #define CH341_QUIRK_LIMITED_PRESCALER	BIT(0)
+#define CH341_QUIRK_SIMULATE_BREAK	BIT(1)
 
 static const struct usb_device_id id_table[] = {
 	{ USB_DEVICE(0x4348, 0x5523) },
@@ -94,6 +95,7 @@ struct ch341_private {
 	u8 msr;
 	u8 lcr;
 	unsigned long quirks;
+	unsigned long break_end;
 };
 
 static void ch341_set_termios(struct tty_struct *tty,
@@ -170,10 +172,9 @@ static const speed_t ch341_min_rates[] = {
  *		2 <= div <= 256 if fact = 0, or
  *		9 <= div <= 256 if fact = 1
  */
-static int ch341_get_divisor(struct ch341_private *priv)
+static int ch341_get_divisor(struct ch341_private *priv, speed_t speed)
 {
 	unsigned int fact, div, clk_div;
-	speed_t speed = priv->baud_rate;
 	bool force_fact0 = false;
 	int ps;
 
@@ -236,15 +237,16 @@ static int ch341_get_divisor(struct ch341_private *priv)
 }
 
 static int ch341_set_baudrate_lcr(struct usb_device *dev,
-				  struct ch341_private *priv, u8 lcr)
+				  struct ch341_private *priv,
+				  speed_t baud_rate, u8 lcr)
 {
 	int val;
 	int r;
 
-	if (!priv->baud_rate)
+	if (!baud_rate)
 		return -EINVAL;
 
-	val = ch341_get_divisor(priv);
+	val = ch341_get_divisor(priv, baud_rate);
 	if (val < 0)
 		return -EINVAL;
 
@@ -324,7 +326,7 @@ static int ch341_configure(struct usb_device *dev, struct ch341_private *priv)
 	if (r < 0)
 		goto out;
 
-	r = ch341_set_baudrate_lcr(dev, priv, priv->lcr);
+	r = ch341_set_baudrate_lcr(dev, priv, priv->baud_rate, priv->lcr);
 	if (r < 0)
 		goto out;
 
@@ -357,8 +359,8 @@ static int ch341_detect_quirks(struct usb_serial_port *port)
 			    USB_TYPE_VENDOR | USB_RECIP_DEVICE | USB_DIR_IN,
 			    CH341_REG_BREAK, 0, buffer, size, DEFAULT_TIMEOUT);
 	if (r == -EPIPE) {
-		dev_dbg(&port->dev, "break control not supported\n");
-		quirks = CH341_QUIRK_LIMITED_PRESCALER;
+		dev_info(&port->dev, "break control not supported, using simulated break\n");
+		quirks = CH341_QUIRK_LIMITED_PRESCALER | CH341_QUIRK_SIMULATE_BREAK;
 		r = 0;
 		goto out;
 	}
@@ -539,7 +541,8 @@ static void ch341_set_termios(struct tty_struct *tty,
 	if (baud_rate) {
 		priv->baud_rate = baud_rate;
 
-		r = ch341_set_baudrate_lcr(port->serial->dev, priv, lcr);
+		r = ch341_set_baudrate_lcr(port->serial->dev, priv,
+					   priv->baud_rate, lcr);
 		if (r < 0 && old_termios) {
 			priv->baud_rate = tty_termios_baud_rate(old_termios);
 			tty_termios_copy_hw(&tty->termios, old_termios);
@@ -558,15 +561,96 @@ static void ch341_set_termios(struct tty_struct *tty,
 	ch341_set_handshake(port->serial->dev, priv->mcr);
 }
 
+/*
+ * A subset of all CH34x devices don't support a real break condition and
+ * reading CH341_REG_BREAK fails (see also ch341_detect_quirks). This function
+ * simulates a break condition by lowering the baud rate to the minimum
+ * supported by the hardware upon enabling the break condition and sending
+ * a NUL byte.
+ *
+ * Incoming data is corrupted while the break condition is being simulated.
+ *
+ * Normally the duration of the break condition can be controlled individually
+ * by userspace using TIOCSBRK and TIOCCBRK or by passing an argument to
+ * TCSBRKP. Due to how the simulation is implemented the duration can't be
+ * controlled. The duration is always about (1s / 46bd * 9bit) = 196ms.
+ */
+static void ch341_simulate_break(struct tty_struct *tty, int break_state)
+{
+	struct usb_serial_port *port = tty->driver_data;
+	struct ch341_private *priv = usb_get_serial_port_data(port);
+	unsigned long now, delay;
+	int r;
+
+	if (break_state != 0) {
+		dev_dbg(&port->dev, "enter break state requested\n");
+
+		r = ch341_set_baudrate_lcr(port->serial->dev, priv,
+				CH341_MIN_BPS,
+				CH341_LCR_ENABLE_RX | CH341_LCR_ENABLE_TX | CH341_LCR_CS8);
+		if (r < 0) {
+			dev_err(&port->dev,
+				"failed to change baud rate to %u: %d\n",
+				CH341_MIN_BPS, r);
+			goto restore;
+		}
+
+		r = tty_put_char(tty, '\0');
+		if (r < 0) {
+			dev_err(&port->dev,
+				"failed to write NUL byte for simulated break condition: %d\n",
+				r);
+			goto restore;
+		}
+
+		/*
+		 * Compute expected transmission duration and add a single bit
+		 * of safety margin (the actual NUL byte transmission is 8 bits
+		 * plus one stop bit).
+		 */
+		priv->break_end = jiffies + (10 * HZ / CH341_MIN_BPS);
+
+		return;
+	}
+
+	dev_dbg(&port->dev, "leave break state requested\n");
+
+	now = jiffies;
+
+	if (time_before(now, priv->break_end)) {
+		/* Wait until NUL byte is written */
+		delay = priv->break_end - now;
+		dev_dbg(&port->dev,
+			"wait %d ms while transmitting NUL byte at %u baud\n",
+			jiffies_to_msecs(delay), CH341_MIN_BPS);
+		schedule_timeout_interruptible(delay);
+	}
+
+restore:
+	/* Restore original baud rate */
+	r = ch341_set_baudrate_lcr(port->serial->dev, priv, priv->baud_rate,
+				   priv->lcr);
+	if (r < 0)
+		dev_err(&port->dev,
+			"restoring original baud rate of %u failed: %d\n",
+			priv->baud_rate, r);
+}
+
 static void ch341_break_ctl(struct tty_struct *tty, int break_state)
 {
 	const uint16_t ch341_break_reg =
 			((uint16_t) CH341_REG_LCR << 8) | CH341_REG_BREAK;
 	struct usb_serial_port *port = tty->driver_data;
+	struct ch341_private *priv = usb_get_serial_port_data(port);
 	int r;
 	uint16_t reg_contents;
 	uint8_t *break_reg;
 
+	if (priv->quirks & CH341_QUIRK_SIMULATE_BREAK) {
+		ch341_simulate_break(tty, break_state);
+		return;
+	}
+
 	break_reg = kmalloc(2, GFP_KERNEL);
 	if (!break_reg)
 		return;