On Tue, Jul 12, 2022 at 01:53:05PM +0200, Marek Behún wrote:
> From: Pali Rohár <pali@xxxxxxxxxx>
>
> When ASYNC_SPD_CUST is used, update custom_divisor and c_*speed fields
> so that they correspond to the newly set baud rate value, so that
> userspace GET ioctls will see the true baud rate that is being used.
No, this is wrong.
In fact, there's a long-standing bug in this driver which started
reporting back the actual baud rate when using SPD_CUST. The rate should
be left unchanged at 38400 in that case.
> Signed-off-by: Pali Rohár <pali@xxxxxxxxxx>
> Tested-by: Marek Behún <kabel@xxxxxxxxxx>
> Signed-off-by: Marek Behún <kabel@xxxxxxxxxx>
> ---
> drivers/usb/serial/ftdi_sio.c | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
>
> diff --git a/drivers/usb/serial/ftdi_sio.c b/drivers/usb/serial/ftdi_sio.c
> index 5db1293bb7a2..39e8c5d06157 100644
> --- a/drivers/usb/serial/ftdi_sio.c
> +++ b/drivers/usb/serial/ftdi_sio.c
> @@ -1303,6 +1303,7 @@ static u32 get_ftdi_divisor(struct tty_struct *tty,
> {
> struct ftdi_private *priv = usb_get_serial_port_data(port);
> struct device *dev = &port->dev;
> + int fix_custom_divisor = 0;
> int div_value = 0;
> int div_okay = 1;
> int baud;
> @@ -1317,6 +1318,7 @@ static u32 get_ftdi_divisor(struct tty_struct *tty,
> if (baud == 38400 &&
> ((priv->flags & ASYNC_SPD_MASK) == ASYNC_SPD_CUST) &&
> (priv->custom_divisor)) {
> + fix_custom_divisor = 1;
> baud = DIV_ROUND_CLOSEST(priv->baud_base, priv->custom_divisor);
> dev_dbg(dev, "%s - custom divisor %d sets baud rate to %d\n",
> __func__, priv->custom_divisor, baud);
> @@ -1401,7 +1403,19 @@ static u32 get_ftdi_divisor(struct tty_struct *tty,
> ftdi_chip_name[priv->chip_type]);
> }
>
> + /* Fix deprecated async-compatible custom_divisor hack and update tty baud rate */
> + if (fix_custom_divisor) {
> + priv->custom_divisor = DIV_ROUND_CLOSEST(priv->baud_base, baud);
> + old_baud = baud;
> + baud = 38400;
> + }
> +
> tty_encode_baud_rate(tty, baud, baud);
> +
> + /* For async-compatible custom_divisor store into TCGETS2 c_*speed fields real baud rate */
> + if (fix_custom_divisor)
> + tty->termios.c_ispeed = tty->termios.c_ospeed = old_baud;
> +
> return div_value;
> }
>
> @@ -2674,6 +2688,8 @@ static void ftdi_set_termios(struct tty_struct *tty,
> dev_dbg(ddev, "%s: forcing baud rate for this device\n", __func__);
> tty_encode_baud_rate(tty, priv->force_baud,
> priv->force_baud);
> + termios->c_ispeed = termios->c_ospeed =
> + DIV_ROUND_CLOSEST(priv->baud_base, priv->custom_divisor);
> }
>
> /* Force RTS-CTS if this device requires it. */
Johan
