linux-kernel - [PATCH v3 3/6] serial: 8250: Get preserved flags using serial_lsr

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Date:   Tue,  7 Jun 2022 11:29:31 +0300
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     linux-serial@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Douglas Anderson <dianders@...omium.org>,
        Phil Edworthy <phil.edworthy@...esas.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        linux-kernel@...r.kernel.org
Cc:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Subject: [PATCH v3 3/6] serial: 8250: Get preserved flags using serial_lsr_in()

serial8250_handle_irq() assumes it's the first to read LSR register.
However, there are 8250 drivers which perform LSR read in their own irq
handler prior to calling serial8250_handle_irq(). As not all flags are
preserved across LSR reads, use serial_lsr_in() helper to get all the
preserved flags.

This commit might fix other commits too besides the ones for DW UART
mentioned below. It's just not clear to me which of the other devices
clear some of the LSR flags on read. AFAIK, nobody has complained about
this problem (either against DW or other devices) so it might not have
that bad impact in the end.

Fixes: 424d79183af0 ("serial: 8250_dw: Avoid "too much work" from bogus rx timeout interrupt")
Fixes: aa63d786cea2 ("serial: 8250: dw: Add support for DMA flow controlling devices")
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
---
 drivers/tty/serial/8250/8250_port.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index ec5abeb638eb..a0ea048eb2ad 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1916,7 +1916,7 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
 
 	spin_lock_irqsave(&port->lock, flags);
 
-	status = serial_port_in(port, UART_LSR);
+	status = serial_lsr_in(up);
 
 	/*
 	 * If port is stopped and there are no error conditions in the
-- 
2.30.2