On Mon, Jan 17, 2022 at 10:08:20PM -0500, Sasha Levin wrote: > From: Lukas Wunner <lukas@xxxxxxxxx> > > [ 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@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> > --- > 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
