From 39d307074f919aed4fe7b1c131e624f34fec3d51 Mon Sep 17 00:00:00 2001 From: John Ogness Date: Mon, 11 May 2026 17:33:01 +0206 Subject: [PATCH] serial: 8250: Check LSR timeout on console flow control wait_for_xmitr() calls wait_for_lsr() to wait for the transmission registers to be empty. wait_for_lsr() can timeout after a reasonable amount of time. When console flow control is active, wait_for_xmitr() additionally polls CTS, waiting for the peer to signal that it is ready to receive more data. If hardware flow control is enabled (auto CTS) and the peer deasserts CTS, wait_for_lsr() will timeout. If additionally console flow control is active and while polling CTS the peer asserts CTS, the console will assume it can immediately transmit, even though the transmission registers may not be empty. This can lead to data loss. Avoid this problem by performing an extra wait_for_lsr() upon CTS assertion if wait_for_lsr() previously timed out. Signed-off-by: John Ogness Link: https://patch.msgid.link/20260511152706.151498-3-john.ogness@linutronix.de Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/8250/8250_port.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c index e4e6a53ebea3..fe2e0f1e66c2 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -1986,16 +1986,20 @@ static bool wait_for_lsr(struct uart_8250_port *up, int bits) static void wait_for_xmitr(struct uart_8250_port *up, int bits) { unsigned int tmout; + bool tx_ready; - wait_for_lsr(up, bits); + tx_ready = wait_for_lsr(up, bits); /* Wait up to 1s for flow control if necessary */ if (uart_cons_flow_enabled(&up->port)) { for (tmout = 1000000; tmout; tmout--) { unsigned int msr = serial_in(up, UART_MSR); up->msr_saved_flags |= msr & MSR_SAVE_FLAGS; - if (msr & UART_MSR_CTS) + if (msr & UART_MSR_CTS) { + if (!tx_ready) + wait_for_lsr(up, bits); break; + } udelay(1); touch_nmi_watchdog(); }