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

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Date:   Sun, 13 Nov 2022 07:06:14 -0500
From:   "Gabriel L. Somlo" <gsomlo@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-serial@...r.kernel.org, gregkh@...uxfoundation.org,
        jirislaby@...nel.org, kgugala@...micro.com, mholenko@...micro.com,
        joel@....id.au, david.abdurachmanov@...il.com,
        florent@...oy-digital.fr, geert@...ux-m68k.org,
        ilpo.jarvinen@...ux.intel.com
Subject: Re: [PATCH v3 13/14] serial: liteuart: add IRQ support for the TX
 path

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@...il.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
>