From b0e0bd9d0d891c1811f52e836ef54fff486db866 Mon Sep 17 00:00:00 2001 From: Tomasz Moń Date: Mon, 28 Feb 2022 06:49:11 +0100 Subject: serial: core: fix tcdrain() with CTS enabled MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Do not set timeout to twice the approximate amount of time to send the entire FIFO if CTS is enabled. If the caller requested no timeout, e.g. when userspace program called tcdrain(), then wait without any timeout. Premature return from tcdrain() was observed on imx based system which has 32 character long transmitter FIFO with hardware CTS handling. Simple userspace application that reproduces problem has to: * Open tty device, enable hardware flow control (CRTSCTS) * Write data, e.g. 26 bytes * Call tcdrain() to wait for the transmitter * Close tty device The other side of serial connection has to: * Receive some data, e.g. 10 bytes * Set RTS output (CTS input from sender perspective) inactive for at least twice the port timeout * Try to receive remaining data Without this patch, userspace application will finish without any error while the other side of connection will never receive remaining data. Signed-off-by: Tomasz Moń Link: https://lore.kernel.org/r/20220228054911.1420221-1-tomasz.mon@camlingroup.com Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/serial_core.c | 26 ++++++++++++++------------ 1 file changed, 14 insertions(+), 12 deletions(-) (limited to 'drivers/tty') diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c index d067b44f1425..01823ce87801 100644 --- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -1650,17 +1650,19 @@ static void uart_wait_until_sent(struct tty_struct *tty, int timeout) if (timeout && timeout < char_time) char_time = timeout; - /* - * If the transmitter hasn't cleared in twice the approximate - * amount of time to send the entire FIFO, it probably won't - * ever clear. This assumes the UART isn't doing flow - * control, which is currently the case. Hence, if it ever - * takes longer than port->timeout, this is probably due to a - * UART bug of some kind. So, we clamp the timeout parameter at - * 2*port->timeout. - */ - if (timeout == 0 || timeout > 2 * port->timeout) - timeout = 2 * port->timeout; + if (!uart_cts_enabled(port)) { + /* + * If the transmitter hasn't cleared in twice the approximate + * amount of time to send the entire FIFO, it probably won't + * ever clear. This assumes the UART isn't doing flow + * control, which is currently the case. Hence, if it ever + * takes longer than port->timeout, this is probably due to a + * UART bug of some kind. So, we clamp the timeout parameter at + * 2*port->timeout. + */ + if (timeout == 0 || timeout > 2 * port->timeout) + timeout = 2 * port->timeout; + } expire = jiffies + timeout; @@ -1676,7 +1678,7 @@ static void uart_wait_until_sent(struct tty_struct *tty, int timeout) msleep_interruptible(jiffies_to_msecs(char_time)); if (signal_pending(current)) break; - if (time_after(jiffies, expire)) + if (timeout && time_after(jiffies, expire)) break; } uart_port_deref(port); -- cgit