From fce3c5c1a2d9cd888f2987662ce17c0c651916b2 Mon Sep 17 00:00:00 2001 From: Thomas Petazzoni Date: Thu, 1 Oct 2020 09:44:15 +0200 Subject: [PATCH] serial: max310x: rework RX interrupt handling Currently, the RX interrupt logic uses the RXEMPTY interrupt, with the RXEMPTYINV bit set, which means we get an RX interrupt as soon as the RX FIFO is non-empty. However, with the MAX310X having a FIFO of 128 bytes, this makes very poor use of the FIFO: we trigger an interrupt as soon as the RX FIFO has one byte, which means a lot of interrupts, each only collecting a few bytes from the FIFO, causing a significant CPU load. Instead this commit relies on two other RX interrupt events: - MAX310X_IRQ_RXFIFO_BIT, which triggers when the RX FIFO has reached a certain threshold, which we define to be half of the FIFO size. This ensure we get an interrupt before the RX FIFO fills up. - MAX310X_LSR_RXTO_BIT, which triggers when the RX FIFO has received some bytes, and then no more bytes are received for a certain time. Arbitrarily, this time is defined to the time is takes to receive 4 characters. On a Microchip SAMA5D3 platform that is receiving 20 bytes every 16ms over one MAX310X UART, this patch has allowed to reduce the CPU consumption of the interrupt handler thread from ~25% to 6-7%. Signed-off-by: Thomas Petazzoni Link: https://lore.kernel.org/r/20201001074415.349739-1-thomas.petazzoni@bootlin.com Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/max310x.c | 29 ++++++++++++++++++++++++----- 1 file changed, 24 insertions(+), 5 deletions(-) diff --git a/drivers/tty/serial/max310x.c b/drivers/tty/serial/max310x.c index 8434bd5a8ec7..21130af106bb 100644 --- a/drivers/tty/serial/max310x.c +++ b/drivers/tty/serial/max310x.c @@ -1056,9 +1056,9 @@ static int max310x_startup(struct uart_port *port) max310x_port_update(port, MAX310X_MODE1_REG, MAX310X_MODE1_TRNSCVCTRL_BIT, 0); - /* Configure MODE2 register & Reset FIFOs*/ - val = MAX310X_MODE2_RXEMPTINV_BIT | MAX310X_MODE2_FIFORST_BIT; - max310x_port_write(port, MAX310X_MODE2_REG, val); + /* Reset FIFOs */ + max310x_port_write(port, MAX310X_MODE2_REG, + MAX310X_MODE2_FIFORST_BIT); max310x_port_update(port, MAX310X_MODE2_REG, MAX310X_MODE2_FIFORST_BIT, 0); @@ -1086,8 +1086,27 @@ static int max310x_startup(struct uart_port *port) /* Clear IRQ status register */ max310x_port_read(port, MAX310X_IRQSTS_REG); - /* Enable RX, TX, CTS change interrupts */ - val = MAX310X_IRQ_RXEMPTY_BIT | MAX310X_IRQ_TXEMPTY_BIT; + /* + * Let's ask for an interrupt after a timeout equivalent to + * the receiving time of 4 characters after the last character + * has been received. + */ + max310x_port_write(port, MAX310X_RXTO_REG, 4); + + /* + * Make sure we also get RX interrupts when the RX FIFO is + * filling up quickly, so get an interrupt when half of the RX + * FIFO has been filled in. + */ + max310x_port_write(port, MAX310X_FIFOTRIGLVL_REG, + MAX310X_FIFOTRIGLVL_RX(MAX310X_FIFO_SIZE / 2)); + + /* Enable RX timeout interrupt in LSR */ + max310x_port_write(port, MAX310X_LSR_IRQEN_REG, + MAX310X_LSR_RXTO_BIT); + + /* Enable LSR, RX FIFO trigger, CTS change interrupts */ + val = MAX310X_IRQ_LSR_BIT | MAX310X_IRQ_RXFIFO_BIT | MAX310X_IRQ_TXEMPTY_BIT; max310x_port_write(port, MAX310X_IRQEN_REG, val | MAX310X_IRQ_CTS_BIT); return 0;