From 302fbbb4fcbdeac2dc8c63a56c1c4e38c4781958 Mon Sep 17 00:00:00 2001 From: John Ogness Date: Tue, 7 Jul 2026 16:16:04 +0206 Subject: [PATCH] serial: 8250: Ignore flow control on suspend/resume with no_console_suspend If no_console_suspend is specified, on suspend the 8250 console driver uses a scratch register (UART_SCR) to store a special canary value. This is used during the resume path to identify a printk() call before the driver's own ->resume() callback. In this case, serial8250_console_restore() is called to quickly re-init the 8250 for console printing. See commit 4516d50aabed ("serial: 8250: Use canary to restart console after suspend") for the original motivation. Unfortunately, this canary workaround does not work in all cases (such as suspend to mem) because the scratch register will not reset. This has not been a real issue until now because it could simply lead to some garbage characters upon resume. However, with the introduction of console flow control it becomes a real problem because a failed suspend/resume detection when flow control is enabled leads to all characters hitting the flow control timeout. Workaround this issue by temporarily ignoring console flow control when the debug canary suspend/resume detection is active. Fixes: 5e6dfb87b191 ("serial: 8250: Add support for console flow control") Signed-off-by: John Ogness Link: https://patch.msgid.link/20260707141032.5074-1-john.ogness@linutronix.de Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/8250/8250_port.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c index 630deb7dd344..e94a0802cbdd 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -2000,8 +2000,14 @@ static void wait_for_xmitr(struct uart_8250_port *up, int bits) tx_ready = wait_for_lsr(up, bits); - /* Wait up to 1s for flow control if necessary */ - if (uart_console_hwflow_active(&up->port)) { + /* + * Wait up to 1s for flow control if necessary. + * When 'no_console_suspend' is active (in the window between + * suspend() and resume()), flow control is temporarily ignored + * because the canary workaround is not reliable in all situations, + * leading to flow control timeouts for every character. + */ + if (uart_console_hwflow_active(&up->port) && !up->canary) { for (tmout = 1000000; tmout; tmout--) { unsigned int msr = serial_in(up, UART_MSR); up->msr_saved_flags |= msr & MSR_SAVE_FLAGS; -- 2.47.3