On Mon, Jan 17, 2022 at 10:08:20PM -0500, Sasha Levin wrote:
From: Lukas Wunner lukas@wunner.de
[ 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.
I'd recommend against backporting this particular patch for pl010 as it's merely a cleanup that eases future work on the driver, but doesn't actually fix anything.
You've also auto-selected a patch for the pl011 driver with the same subject. That other patch *does* actually fix an rs485 Transmit Enable glitch, so backporting it makes sense.
Thanks,
Lukas
Cc: Russell King rmk+kernel@armlinux.org.uk Signed-off-by: Lukas Wunner lukas@wunner.de Link: https://lore.kernel.org/r/fcaff16e5b1abb4cc3da5a2879ac13f278b99ed0.164112872... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Sasha Levin sashal@kernel.org
drivers/tty/serial/amba-pl010.c | 3 --- 1 file changed, 3 deletions(-)
diff --git a/drivers/tty/serial/amba-pl010.c b/drivers/tty/serial/amba-pl010.c index 5d41d5b92619a..7f4ba92739663 100644 --- a/drivers/tty/serial/amba-pl010.c +++ b/drivers/tty/serial/amba-pl010.c @@ -465,14 +465,11 @@ 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);
-- 2.34.1