This is a note to let you know that I've just added the patch titled
serial: pl010: Drop CR register reset on set_termios
to the 5.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
serial-pl010-drop-cr-register-reset-on-set_termios.patch
and it can be found in the queue-5.10 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 85e46a962b2c31cd85c3eb3fcf895ab6c12e8ecd
Author: Lukas Wunner <lukas@xxxxxxxxx>
Date: Sun Jan 2 18:42:44 2022 +0100
serial: pl010: Drop CR register reset on set_termios
[ Upstream commit 08a0c6dff91c965e39905cf200d22db989203ccb ]
pl010_set_termios() briefly resets the CR register to zero.
Where does this register write come from?
The PL010 driver's IRQ handler ambauart_int() originally modified the CR
register without holding the port spinlock. ambauart_set_termios() also
modified that register. To prevent concurrent read-modify-writes by the
IRQ handler and to prevent transmission while changing baudrate,
ambauart_set_termios() had to disable interrupts. That is achieved by
writing zero to the CR register.
However in 2004 the PL010 driver was amended to acquire the port
spinlock in the IRQ handler, obviating the need to disable interrupts in
->set_termios():
https://git.kernel.org/history/history/c/157c0342e591
That rendered the CR register write obsolete. Drop it.
Cc: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>
Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx>
Link: https://lore.kernel.org/r/fcaff16e5b1abb4cc3da5a2879ac13f278b99ed0.1641128728.git.lukas@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/drivers/tty/serial/amba-pl010.c b/drivers/tty/serial/amba-pl010.c
index 3284f34e9dfe1..75d61e038a775 100644
--- a/drivers/tty/serial/amba-pl010.c
+++ b/drivers/tty/serial/amba-pl010.c
@@ -448,14 +448,11 @@ pl010_set_termios(struct uart_port *port, struct ktermios *termios,
if ((termios->c_cflag & CREAD) == 0)
uap->port.ignore_status_mask |= UART_DUMMY_RSR_RX;
- /* first, disable everything */
old_cr = readb(uap->port.membase + UART010_CR) & ~UART010_CR_MSIE;
if (UART_ENABLE_MS(port, termios->c_cflag))
old_cr |= UART010_CR_MSIE;
- writel(0, uap->port.membase + UART010_CR);
-
/* Set baud rate */
quot -= 1;
writel((quot & 0xf00) >> 8, uap->port.membase + UART010_LCRM);
