Re: [PATCH v3 13/14] serial: liteuart: add IRQ support for the TX path

Gabriel L. Somlo
2022-11-13T12:06:25Z

On Sat, Nov 12, 2022 at 04:21:24PM -0500, Gabriel Somlo wrote:
> Modify the TX path to operate in an IRQ-compatible way, while
> maintaining support for polling mode via the poll timer.
>
> Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>
> ---
> drivers/tty/serial/liteuart.c | 67 ++++++++++++++++++++++++-----------
> 1 file changed, 47 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/tty/serial/liteuart.c b/drivers/tty/serial/liteuart.c
> index e30adb30277f..307c27398e30 100644
> --- a/drivers/tty/serial/liteuart.c
> +++ b/drivers/tty/serial/liteuart.c
> @@ -46,6 +46,7 @@ struct liteuart_port {
> struct uart_port port;
> struct timer_list timer;
> u32 id;
> + bool poll_tx_started;
> };
>
> #define to_liteuart_port(port) container_of(port, struct liteuart_port, port)
> @@ -78,29 +79,24 @@ static void liteuart_putchar(struct uart_port *port, unsigned char ch)
>
> static void liteuart_stop_tx(struct uart_port *port)
> {
> - /* not used in LiteUART, but called unconditionally from serial_core */
> + if (port->irq) {
> + u8 irq_mask = litex_read8(port->membase + OFF_EV_ENABLE);
> + litex_write8(port->membase + OFF_EV_ENABLE, irq_mask & ~EV_TX);
> + } else {
> + struct liteuart_port *uart = to_liteuart_port(port);
> + uart->poll_tx_started = false;
> + }
> }
>
> static void liteuart_start_tx(struct uart_port *port)
> {
> - struct circ_buf *xmit = &port->state->xmit;
> - unsigned char ch;
> -
> - if (unlikely(port->x_char)) {
> - litex_write8(port->membase + OFF_RXTX, port->x_char);
> - port->icount.tx++;
> - port->x_char = 0;
> - } else if (!uart_circ_empty(xmit)) {
> - while (xmit->head != xmit->tail) {
> - ch = xmit->buf[xmit->tail];

I just realized that this:

> - xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
> - port->icount.tx++;

... conflicts with another accepted patch (by Ilpo) that's currently
making its way to being released:
https://lore.kernel.org/all/20221019091151.6692-20-ilpo.jarvinen@linux.intel.com/

> - liteuart_putchar(port, ch);
> - }
> + if (port->irq) {
> + u8 irq_mask = litex_read8(port->membase + OFF_EV_ENABLE);
> + litex_write8(port->membase + OFF_EV_ENABLE, irq_mask | EV_TX);
> + } else {
> + struct liteuart_port *uart = to_liteuart_port(port);
> + uart->poll_tx_started = true;
> }
> -
> - if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
> - uart_write_wakeup(port);
> }
>
> static void liteuart_stop_rx(struct uart_port *port)
> @@ -131,18 +127,47 @@ static void liteuart_rx_chars(struct uart_port *port)
> tty_flip_buffer_push(&port->state->port);
> }
>
> +static void liteuart_tx_chars(struct uart_port *port)
> +{
> + struct circ_buf *xmit = &port->state->xmit;
> +
> + if (unlikely(port->x_char)) {
> + litex_write8(port->membase + OFF_RXTX, port->x_char);
> + port->x_char = 0;
> + port->icount.tx++;
> + return;
> + }
> +
> + while (!litex_read8(port->membase + OFF_TXFULL)) {
> + if (xmit->head == xmit->tail)
> + break;
> + litex_write8(port->membase + OFF_RXTX, xmit->buf[xmit->tail]);

Also, this:

> + xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
> + port->icount.tx++;

should now be `uart_xmit_advance(port, 1);` instead.
I'm going to take that into account in v4, in addition to any extra
feedback I'll receive.

Thanks!

> + }
> +
> + if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
> + uart_write_wakeup(port);
> +
> + if (uart_circ_empty(xmit))
> + liteuart_stop_tx(port);
> +}
> +
> static irqreturn_t liteuart_interrupt(int irq, void *data)
> {
> struct liteuart_port *uart = data;
> struct uart_port *port = &uart->port;
> u8 isr = litex_read8(port->membase + OFF_EV_PENDING);
>
> - /* for now, only rx path triggers interrupts */
> - isr &= EV_RX;
> + if (!(port->irq || uart->poll_tx_started))
> + isr &= ~EV_TX; /* polling mode with tx stopped */
>
> spin_lock(&port->lock);
> if (isr & EV_RX)
> liteuart_rx_chars(port);
> + if (isr & EV_TX) {
> + liteuart_tx_chars(port);
> + }
> spin_unlock(&port->lock);
>
> return IRQ_RETVAL(isr);
> @@ -196,6 +221,7 @@ static int liteuart_startup(struct uart_port *port)
> }
>
> if (!port->irq) {
> + uart->poll_tx_started = false;
> timer_setup(&uart->timer, liteuart_timer, 0);
> mod_timer(&uart->timer, jiffies + uart_poll_timeout(port));
> }
> @@ -210,6 +236,7 @@ static void liteuart_shutdown(struct uart_port *port)
> struct liteuart_port *uart = to_liteuart_port(port);
>
> litex_write8(port->membase + OFF_EV_ENABLE, 0);
> + uart->poll_tx_started = false;
>
> if (port->irq)
> free_irq(port->irq, port);
> --
> 2.37.3
>