Re: [PATCH v2 6/6] USB: serial: ch341: Simulate break condition if not supported

[Johan Hovold <johan@kernel.org>]

On Tue, Mar 31, 2020 at 11:37:22PM +0000, Michael Hanselmann wrote:
> 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 drawback of this approach is that the duration of the break
> can't be controlled by userland.
> 
> 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>
> ---
>  drivers/usb/serial/ch341.c | 95 ++++++++++++++++++++++++++++++++++----
>  1 file changed, 85 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c
> index c820772e6a07..fc8ef816d143 100644
> --- a/drivers/usb/serial/ch341.c
> +++ b/drivers/usb/serial/ch341.c
> @@ -25,6 +25,7 @@
>  #define DEFAULT_TIMEOUT   1000
>  
>  #define CH341_QUIRK_LIMITED_PRESCALER 0x01
> +#define CH341_QUIRK_SIMULATE_BREAK    0x02
>
>  /* flags for IO-Bits */
>  #define CH341_BIT_RTS (1 << 6)
> @@ -94,6 +95,7 @@ struct ch341_private {
>  	u8 msr;
>  	u8 lcr;
>  	u8 quirks;
> +	unsigned long break_end;
>  };
>  
>  static void ch341_set_termios(struct tty_struct *tty,
> @@ -203,14 +205,11 @@ static int ch341_calc_divisor(speed_t speed, unsigned int ps,
>   *		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)
>  {
> -	speed_t speed;
>  	unsigned int fact, div, clk_div;
>  	int ps;
>  
> -	speed = priv->baud_rate;
> -
>  	/*
>  	 * Clamp to supported range, making the later range sanity checks
>  	 * redundant.
> @@ -282,15 +281,16 @@ 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,
> +				  unsigned baud_rate, u8 lcr)

Use speed_t.

>  {
>  	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;
>  
> @@ -369,7 +369,7 @@ 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;
>  
> @@ -401,7 +401,7 @@ static int ch341_detect_quirks(struct usb_device *dev)
>  	if (r == -EPIPE) {
>  		dev_dbg(&dev->dev, "%s - reading break condition register"
>  			" failed (%d)\n", __func__, r);
> -		r = CH341_QUIRK_LIMITED_PRESCALER;
> +		r = CH341_QUIRK_LIMITED_PRESCALER | CH341_QUIRK_SIMULATE_BREAK;
>  		goto out;
>  	}
>  
> @@ -579,7 +579,8 @@ 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);
> @@ -598,15 +599,89 @@ 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.
> + *
> + * 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 1s / 46bd * 10bit = 217ms.
> + */
> +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 delay;
> +	int r;
> +
> +	if (break_state != 0) {
> +		dev_dbg(&port->dev, "%s - Enter break state requested\n",
> +			__func__);
> +
> +		r = ch341_set_baudrate_lcr(port->serial->dev, priv,
> +			CH341_MIN_BPS,
> +			CH341_LCR_ENABLE_RX | CH341_LCR_ENABLE_TX |

Hmm. This would corrupt incoming data as well during the break.

> +			CH341_LCR_CS8);
> +		if (r < 0) {
> +			dev_err(&port->dev, "%s - baud rate status %d\n",
> +				__func__, r);

Spell out error messages instead of relying on __func__; "failed to
change baudrate: %d\n"?

> +			goto restore;
> +		}
> +
> +		r = tty_put_char(tty, '\0');
> +		if (r < 0) {
> +			dev_err(&port->dev, "%s - write status %d\n",
> +				__func__, r);

ditto

> +			goto restore;
> +		}
> +
> +		priv->break_end = jiffies + (10 * HZ / CH341_MIN_BPS);
> +
> +		return;
> +	}
> +
> +	dev_dbg(&port->dev, "%s - Leave break state requested\n", __func__);
> +
> +	if (time_before(jiffies, priv->break_end)) {
> +		/* Wait until NUL byte is written */
> +		delay = min_t(unsigned long, HZ, priv->break_end - jiffies);

Looks like this can underflow if you're preempted after the check.

> +
> +		dev_dbg(&port->dev, "%s - sleep for %d ms\n", __func__,
> +			jiffies_to_msecs(delay));
> +		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, "%s - baud rate status %d\n", __func__, 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) {
> +		dev_warn_once(&port->dev,
> +			      "%s - hardware doesn't support real break"
> +			      " condition, simulating instead\n",
> +			      __func__);

Don't break the string, and drop the __func__.

> +		ch341_simulate_break(tty, break_state);
> +		return;
> +	}
> +
>  	break_reg = kmalloc(2, GFP_KERNEL);
>  	if (!break_reg)
>  		return;

Johan