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[74.123.28.25]) by smtp-relay.gmail.com with ESMTPS id ada2fe7eead31-785f800dbd3sm28652137.17.2026.08.28.07.46.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 28 Aug 2026 07:46:58 -0700 (PDT) X-Relaying-Domain: arista.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=arista.com; s=Arista-A; t=1787928417; bh=YA8nPi3ueKnPwe6mZk4m/RrVVcVGq0t5B55x+7fBTtQ=; h=From:To:Cc:Subject:Date:From; b=EN5ec2xjEDQzo5TUwvsImgohFuK2Ucq51bvpqk5Ichgg9ZdWA+bfElliISOYYeY87 JdMvH5TkgUgl+vCrQSimMFgYY6cW5rVlKUdA0wK6qhwU+/21sIPmEbgmBhDAu2eS3A iGEWBtiMsn9ARvnwbubsmopAfOV2yGVn2waBVZMGnF4waUOZaxmvqvkK6Z451wB+ET 4gy6/SH2Kpwro4Woi37KpBUkaGuBZoOmqUcUDCpn/38dpIBJ0x9yTGZUmQRd1uLz0j WC2BUcnVI9wRAwogxR/j/eduSwbDzf0+AQ8JVvDSvQ+fr5sgH/agqeOrHMx3c82D7v t9VK2pj06Z0ig== Received: from vm-whiteberg-ubuntu.aristanetworks.com (dhcp-244-90-77.sjc.aristanetworks.com [10.244.90.77]) by smtp.aristanetworks.com (Postfix) with ESMTPS id 260B31003C7; Fri, 28 Aug 2026 14:46:57 +0000 (UTC) X-SMTP-Authentication: Allow-List-permitted X-SMTP-Authentication: Allow-List-permitted From: Tate Whiteberg To: Greg Kroah-Hartman , Jiri Slaby , Andy Shevchenko Cc: Tate Whiteberg , linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org Subject: [PATCH] serial: 8250_mid: wait for LSR tx empty before setting termios Date: Fri, 28 Aug 2026 14:45:17 +0000 Message-ID: <20260828144617.1210599-2-whiteberg@arista.com> Content-Transfer-Encoding: quoted-printable Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" If mid8250_set_termios is called while data is still in transmission, the corresponding register updates will corrupt the transmission. Fix this by locking the port and and waiting for the transmitter to empty before performing updates. It is necessary to wait for both UART_LSR_THRE and UART_LSR_TEMT to ensure the final character is sent. Signed-off-by: Tate Whiteberg --- drivers/tty/serial/8250/8250.h | 1 + drivers/tty/serial/8250/8250_mid.c | 5 +++++ drivers/tty/serial/8250/8250_port.c | 13 +++++++------ 3 files changed, 13 insertions(+), 6 deletions(-) diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h index 9337fec9394e..f99620b1cab7 100644 --- a/drivers/tty/serial/8250/8250.h +++ b/drivers/tty/serial/8250/8250.h @@ -180,6 +180,7 @@ void serial8250_clear_and_reinit_fifos(struct uart_8250= _port *p); void serial8250_fifo_wait_for_lsr_thre(struct uart_8250_port *up, struct nbcon_write_context *wctxt, unsigned int count); +void serial8250_wait_for_xmitr(struct uart_8250_port *up, int bits); =20 void serial8250_rpm_get(struct uart_8250_port *p); void serial8250_rpm_put(struct uart_8250_port *p); diff --git a/drivers/tty/serial/8250/8250_mid.c b/drivers/tty/serial/8250/8= 250_mid.c index 82656645b8a6..c77e117b7822 100644 --- a/drivers/tty/serial/8250/8250_mid.c +++ b/drivers/tty/serial/8250/8250_mid.c @@ -210,6 +210,7 @@ static void mid8250_set_termios(struct uart_port *p, st= ruct ktermios *termios, { unsigned int baud =3D tty_termios_baud_rate(termios); struct mid8250 *mid =3D p->private_data; + struct uart_8250_port *up =3D up_to_u8250p(p); unsigned short ps =3D 16; unsigned long fuart =3D baud * ps; unsigned long w =3D BIT(24) - 1; @@ -218,6 +219,7 @@ static void mid8250_set_termios(struct uart_port *p, st= ruct ktermios *termios, /* Gracefully handle the B0 case: fall back to B9600 */ fuart =3D fuart ? fuart : 9600 * 16; =20 + uart_port_lock_irq(p); if (mid->board->freq < fuart) { /* Find prescaler value that satisfies Fuart < Fref */ if (mid->board->freq > baud) @@ -231,11 +233,14 @@ static void mid8250_set_termios(struct uart_port *p, = struct ktermios *termios, } =20 rational_best_approximation(fuart, mid->board->freq, w, w, &mul, &div); + + serial8250_wait_for_xmitr(up, UART_LSR_BOTH_EMPTY); p->uartclk =3D fuart * 16 / ps; /* core uses ps =3D 16 always */ =20 writel(ps, p->membase + INTEL_MID_UART_PS); /* set PS */ writel(mul, p->membase + INTEL_MID_UART_MUL); /* set MUL */ writel(div, p->membase + INTEL_MID_UART_DIV); + uart_port_unlock_irq(p); =20 serial8250_do_set_termios(p, termios, old); } diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/= 8250_port.c index 38fa45e74a37..4eaf417f1fd7 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -2024,7 +2024,7 @@ static bool wait_for_lsr(struct uart_8250_port *up, i= nt bits) } =20 /* Wait for transmitter and holding register to empty with timeout */ -static void wait_for_xmitr(struct uart_8250_port *up, int bits) +void serial8250_wait_for_xmitr(struct uart_8250_port *up, int bits) { unsigned int tmout; bool tx_ready; @@ -2052,6 +2052,7 @@ static void wait_for_xmitr(struct uart_8250_port *up,= int bits) } } } +EXPORT_SYMBOL_NS_GPL(serial8250_wait_for_xmitr, "SERIAL_8250"); =20 #ifdef CONFIG_CONSOLE_POLL /* @@ -2098,7 +2099,7 @@ static void serial8250_put_poll_char(struct uart_port= *port, ier =3D serial_port_in(port, UART_IER); __serial8250_clear_IER(up); =20 - wait_for_xmitr(up, UART_LSR_BOTH_EMPTY); + serial8250_wait_for_xmitr(up, UART_LSR_BOTH_EMPTY); /* * Send the character out. */ @@ -2108,7 +2109,7 @@ static void serial8250_put_poll_char(struct uart_port= *port, * Finally, wait for transmitter to become empty * and restore the IER */ - wait_for_xmitr(up, UART_LSR_BOTH_EMPTY); + serial8250_wait_for_xmitr(up, UART_LSR_BOTH_EMPTY); serial_port_out(port, UART_IER, ier); } =20 @@ -2223,7 +2224,7 @@ static void serial8250_THRE_test(struct uart_port *po= rt) * Synchronize UART_IER access against the console. */ scoped_guard(uart_port_lock_irqsave, port) { - wait_for_xmitr(up, UART_LSR_THRE); + serial8250_wait_for_xmitr(up, UART_LSR_THRE); serial_port_out_sync(port, UART_IER, UART_IER_THRI); udelay(1); /* allow THRE to set */ iir_noint1 =3D serial_port_in(port, UART_IIR) & UART_IIR_NO_INT; @@ -3293,7 +3294,7 @@ static void serial8250_console_wait_putchar(struct ua= rt_port *port, unsigned cha { struct uart_8250_port *up =3D up_to_u8250p(port); =20 - wait_for_xmitr(up, UART_LSR_THRE); + serial8250_wait_for_xmitr(up, UART_LSR_THRE); serial8250_console_putchar(port, ch); } =20 @@ -3504,7 +3505,7 @@ void serial8250_console_write(struct uart_8250_port *= up, * Finally, wait for transmitter to become empty * and restore the IER */ - wait_for_xmitr(up, UART_LSR_BOTH_EMPTY); + serial8250_wait_for_xmitr(up, UART_LSR_BOTH_EMPTY); =20 if (em485) { mdelay(port->rs485.delay_rts_after_send); --=20 2.43.0