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SubjectRe: [PATCH] serial: max310x: rework RX interrupt handling
On Wed, Sep 30, 2020 at 4:51 PM Thomas Petazzoni
<thomas.petazzoni@bootlin.com> wrote:
>
> 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%.

Was it always like this?
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
See one nit-pick below.

> Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
> ---
> 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 8434bd5a8ec78..a1c80850d77ed 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*/

Add a space before the asterisk.

> + 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;
> --
> 2.26.2
>


--
With Best Regards,
Andy Shevchenko

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