[PATCH 6.18 049/227] serial: 8250_pci: Fix broken RS485 for F81504/508/512

Greg Kroah-Hartman
2026-01-28T15:52:54Z

From: Marnix Rijnart <marnix.rijnart@iwell.eu>

commit 27aff0a56b3c77ea1a73641c9b3c4172a8f7238f upstream.

Fintek F81504/508/512 can support both RTS_ON_SEND and RTS_AFTER_SEND,
but pci_fintek_rs485_supported only announces the former.

This makes it impossible to unset SER_RS485_RTS_ON_SEND from
userspace because of uart_sanitize_serial_rs485(). Some devices
with these chips need RTS low on TX, so they are effectively broken.

Fix this by announcing the support for SER_RS485_RTS_AFTER_SEND,
similar to commit 068d35a7be65 ("serial: sc16is7xx: announce support
for SER_RS485_RTS_ON_SEND").

Fixes: 4afeced55baa ("serial: core: fix sanitizing check for RTS settings")
Cc: stable <stable@kernel.org>
Signed-off-by: Marnix Rijnart <marnix.rijnart@iwell.eu>
Link: https://patch.msgid.link/20260112000931.61703-1-marnix.rijnart@iwell.eu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/tty/serial/8250/8250_pci.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/tty/serial/8250/8250_pci.c
+++ b/drivers/tty/serial/8250/8250_pci.c
@@ -1650,7 +1650,7 @@ static int pci_fintek_rs485_config(struc
}

static const struct serial_rs485 pci_fintek_rs485_supported = {
- .flags = SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND,
+ .flags = SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND | SER_RS485_RTS_AFTER_SEND,
/* F81504/508/512 does not support RTS delay before or after send */
};