From: Hugo Villeneuve hvilleneuve@dimonoff.com

Commit cc4c1d05eb10 ("sc16is7xx: Properly resume TX after stop") changed behavior to unconditionnaly set the THRI interrupt in sc16is7xx_tx_proc().

For example when sending a 65 bytes message, and assuming the Tx FIFO is initially empty, sc16is7xx_handle_tx() will write the first 64 bytes of the message to the FIFO and sc16is7xx_tx_proc() will then activate THRI. When the THRI IRQ is fired, the driver will write the remaining byte of the message to the FIFO, and disable THRI by calling sc16is7xx_stop_tx().

When sending a 2 bytes message, sc16is7xx_handle_tx() will write the 2 bytes of the message to the FIFO and call sc16is7xx_stop_tx(), disabling THRI. After sc16is7xx_handle_tx() exits, control returns to sc16is7xx_tx_proc() which will unconditionally set THRI. When the THRI IRQ is fired, the driver simply acknowledges the interrupt and does nothing more, since all the data has already been written to the FIFO. This results in 2 register writes and 4 register reads all for nothing and taking precious cycles from the I2C/SPI bus.

Fix this by enabling the THRI interrupt only when we fill the Tx FIFO to its maximum capacity and there are remaining bytes to send in the message.

Fixes: cc4c1d05eb10 ("sc16is7xx: Properly resume TX after stop") Cc: stable@vger.kernel.org Signed-off-by: Hugo Villeneuve hvilleneuve@dimonoff.com --- drivers/tty/serial/sc16is7xx.c | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c index 7e4b9b52841d..e40e4a99277e 100644 --- a/drivers/tty/serial/sc16is7xx.c +++ b/drivers/tty/serial/sc16is7xx.c @@ -687,6 +687,8 @@ static void sc16is7xx_handle_tx(struct uart_port *port)

if (uart_circ_empty(xmit)) sc16is7xx_stop_tx(port); + else + sc16is7xx_ier_set(port, SC16IS7XX_IER_THRI_BIT); uart_port_unlock_irqrestore(port, flags); }

@@ -815,7 +817,6 @@ static void sc16is7xx_tx_proc(struct kthread_work *ws) { struct uart_port *port = &(to_sc16is7xx_one(ws, tx_work)->port); struct sc16is7xx_one *one = to_sc16is7xx_one(port, port); - unsigned long flags;

if ((port->rs485.flags & SER_RS485_ENABLED) && (port->rs485.delay_rts_before_send > 0)) @@ -824,10 +825,6 @@ static void sc16is7xx_tx_proc(struct kthread_work *ws) mutex_lock(&one->efr_lock); sc16is7xx_handle_tx(port); mutex_unlock(&one->efr_lock); - - uart_port_lock_irqsave(port, &flags); - sc16is7xx_ier_set(port, SC16IS7XX_IER_THRI_BIT); - uart_port_unlock_irqrestore(port, flags); }

static void sc16is7xx_reconf_rs485(struct uart_port *port)