[PATCH] serial: mvebu-uart: fix tx lost characters

[Gabriel Matni]

From: Gabriel Matni <gabriel.matni at exfo.com>

Fixes missing characters on kernel console at low baud rates (i.e.9600).
The driver should poll TX_RDY or TX_FIFO_EMP instead of TX_EMP to ensure
that the transmitter holding register (THR) is ready to receive a new byte.

TX_EMP tells us when it is possible to send a break sequence via
SND_BRK_SEQ. While this also indicates that both the THR and the TSR are
empty, it does not guarantee that a new byte can be written just yet.

Fixes: 30530791a7a0 ("serial: mvebu-uart: initial support for Armada-3700
      serial port")
Reviewed-by: Miquel Raynal <miquel.raynal at bootlin.com> 
Acked-by: Gregory CLEMENT <gregory.clement at bootlin.com>  
Signed-off-by: Gabriel Matni <gabriel.matni at exfo.com> 

---
 drivers/tty/serial/mvebu-uart.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/serial/mvebu-uart.c b/drivers/tty/serial/mvebu-uart.c
index a100e98259d7..f0df0640208e 100644
--- a/drivers/tty/serial/mvebu-uart.c
+++ b/drivers/tty/serial/mvebu-uart.c
@@ -618,7 +618,7 @@ static void wait_for_xmitr(struct uart_port *port)
 	u32 val;
 
 	readl_poll_timeout_atomic(port->membase + UART_STAT, val,
-				  (val & STAT_TX_EMP), 1, 10000);
+				  (val & STAT_TX_RDY(port)), 1, 10000);
 }
 
 static void mvebu_uart_console_putchar(struct uart_port *port, int ch)
-- 
2.7.4


> -----Original Message-----
> From: Miquel Raynal <miquel.raynal at bootlin.com>
> Sent: March 15, 2018 4:22 PM
> To: Gabriel Matni <gabriel.matni at exfo.com>
> Cc: gregkh at linuxfoundation.org; linux-serial at vger.kernel.org; linux-arm-
> kernel at lists.infradead.org; stable at vger.kernel.org; Grégory Clement
> <gregory.clement at bootlin.com>; Thomas Petazzoni
> <thomas.petazzoni at bootlin.com>
> Subject: Re: [PATCH] serial: mvebu-uart: fix tx lost characters
> 
> Hi Gabriel,
> 
> On Thu, 15 Mar 2018 18:55:25 +0000, Gabriel Matni
> <gabriel.matni at exfo.com> wrote:
> 
> > From: Gabriel Matni <gabriel.matni at exfo.com>
> >
> > Fixes missing characters on kernel console at low baud rates (i.e.9600).
> > The driver should poll TX_RDY or TX_FIFO_EMP instead of TX_EMP to
> ensure
> > that the transmitter holding register (THR) is ready to receive a new byte.
> >
> > TX_EMP tells us when it is possible to send a break sequence via
> > SND_BRK_SEQ. While this also indicates that both the THR and the TSR are
> > empty, it does not guarantee that a new byte can be written just yet.
> >
> > Fixes: 30530791a7a0 ("serial: mvebu-uart: initial support for Armada-3700
> >       serial port")
> > Reviewed-by: Miquel Raynal <miquel.raynal at bootlin.com>
> > Acked-by: Gregory CLEMENT <gregory.clement at bootlin.com>
> > Signed-off-by: Gabriel Matni <gabriel.matni at exfo.com>
> >
> > ---
> >  drivers/tty/serial/mvebu-uart.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/tty/serial/mvebu-uart.c b/drivers/tty/serial/mvebu-
> uart.c
> > index a100e98259d7..400e1bc558b2 100644
> > --- a/drivers/tty/serial/mvebu-uart.c
> > +++ b/drivers/tty/serial/mvebu-uart.c
> > @@ -618,7 +618,7 @@ static void wait_for_xmitr(struct uart_port *port)
> >  	u32 val;
> >
> >  	readl_poll_timeout_atomic(port->membase + UART_STAT, val,
> > -				  (val & STAT_TX_EMP), 1, 10000);
> > +             (val & STAT_TX_RDY(port)), 1, 10000);
> 
> I think this line should be indented like the one you replaced.
> 
> Thanks,
> Miquèl
> 
> --
> Miquel Raynal, Bootlin (formerly Free Electrons)
> Embedded Linux and Kernel engineering
> https://bootlin.com