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

[Ilpo Järvinen]

On Tue, 15 Nov 2022, Gabriel L. Somlo wrote:

> On Tue, Nov 15, 2022 at 07:30:09PM +0200, Ilpo Järvinen wrote:
> > On Tue, 15 Nov 2022, Gabriel L. Somlo wrote:
> > 
> > > On Tue, Nov 15, 2022 at 06:14:50PM +0200, Ilpo Järvinen wrote:
> > > > On Sat, 12 Nov 2022, 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@xxxxxxxxx>
> > > > > ---
> > > > >  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
> > 
> > > > > +	if (port->irq) {
> > > > > +		u8 irq_mask = litex_read8(port->membase + OFF_EV_ENABLE);
> > > > > +		litex_write8(port->membase + OFF_EV_ENABLE, irq_mask & ~EV_TX);
> > > > 
> > > > If you put irq_mask into liteuart_port you wouldn't need to read it 
> > > > back here?
> > > 
> > > So, instead of `bool poll_tx_started` I should just keep a copy of the
> > > irq_mask there, and take `&port->lock` whenever I need to *both* update
> > > the mask *and* write it out to the actual device register?
> > 
> > I was mostly thinking of storing EV_RX there but then it could be derived 
> > from port->irq that is checked by all paths already.
> 
> So, just to clear up the best course of action here (before I rebase
> everything on top of tty-next: How about the patch below (currently
> applied separately at the end of the entire series, but I can respin
> it as part of the rx path (12/14) and tx path (13/14) as appropriate.
> 
> Let me know what you think.

I'm fine with that I think. I'd change to bits parameter name to something 
more meaningful though, I guess the irq_mask could do ok there.

-- 
 i.

> diff --git a/drivers/tty/serial/liteuart.c b/drivers/tty/serial/liteuart.c
> index eb0ae8c8bd94..185db423c65f 100644
> --- a/drivers/tty/serial/liteuart.c
> +++ b/drivers/tty/serial/liteuart.c
> @@ -47,7 +47,7 @@ struct liteuart_port {
>  	struct uart_port port;
>  	struct timer_list timer;
>  	u32 id;
> -	bool poll_tx_started;
> +	u8 irq_reg;
>  };
>  
>  #define to_liteuart_port(port)	container_of(port, struct liteuart_port, port)
> @@ -70,26 +70,27 @@ static struct uart_driver liteuart_driver = {
>  #endif
>  };
>  
> +static void liteuart_update_irq_reg(struct uart_port *port, bool set, u8 bits)
> +{
> +	struct liteuart_port *uart = to_liteuart_port(port);
> +
> +	if (set)
> +		uart->irq_reg |= bits;
> +	else
> +		uart->irq_reg &= ~bits;
> +
> +	if (port->irq)
> +		litex_write8(port->membase + OFF_EV_ENABLE, uart->irq_reg);
> +}
> +
>  static void liteuart_stop_tx(struct uart_port *port)
>  {
> -	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;
> -	}
> +	liteuart_update_irq_reg(port, false, EV_TX);
>  }
>  
>  static void liteuart_start_tx(struct uart_port *port)
>  {
> -	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;
> -	}
> +	liteuart_update_irq_reg(port, true, EV_TX);
>  }
>  
>  static void liteuart_stop_rx(struct uart_port *port)
> @@ -149,12 +150,10 @@ 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);
> -
> -	if (!(port->irq || uart->poll_tx_started))
> -		isr &= ~EV_TX;	/* polling mode with tx stopped */
> +	u8 isr;
>  
>  	spin_lock(&port->lock);
> +	isr = litex_read8(port->membase + OFF_EV_PENDING) & uart->irq_reg;
>  	if (isr & EV_RX)
>  		liteuart_rx_chars(port);
>  	if (isr & EV_TX)
> @@ -195,39 +194,40 @@ static unsigned int liteuart_get_mctrl(struct uart_port *port)
>  static int liteuart_startup(struct uart_port *port)
>  {
>  	struct liteuart_port *uart = to_liteuart_port(port);
> +	unsigned long flags;
>  	int ret;
> -	u8 irq_mask = 0;
>  
>  	if (port->irq) {
>  		ret = request_irq(port->irq, liteuart_interrupt, 0,
>  				  KBUILD_MODNAME, uart);
> -		if (ret == 0) {
> -			/* only enabling rx interrupts at startup */
> -			irq_mask = EV_RX;
> -		} else {
> +		if (ret) {
>  			pr_err(pr_fmt("line %d irq %d failed: using polling\n"),
>  				port->line, port->irq);
>  			port->irq = 0;
>  		}
>  	}
>  
> +	spin_lock_irqsave(&port->lock, flags);
> +	/* only enabling rx irqs during startup */
> +	liteuart_update_irq_reg(port, true, EV_RX);
> +	spin_unlock_irqrestore(&port->lock, flags);
> +
>  	if (!port->irq) {
> -		uart->poll_tx_started = false;
>  		timer_setup(&uart->timer, liteuart_timer, 0);
>  		mod_timer(&uart->timer, jiffies + uart_poll_timeout(port));
>  	}
>  
> -	litex_write8(port->membase + OFF_EV_ENABLE, irq_mask);
> -
>  	return 0;
>  }
>  
>  static void liteuart_shutdown(struct uart_port *port)
>  {
>  	struct liteuart_port *uart = to_liteuart_port(port);
> +	unsigned long flags;
>  
> -	litex_write8(port->membase + OFF_EV_ENABLE, 0);
> -	uart->poll_tx_started = false;
> +	spin_lock_irqsave(&port->lock, flags);
> +	liteuart_update_irq_reg(port, false, EV_RX | EV_TX);
> +	spin_unlock_irqrestore(&port->lock, flags);
>  
>  	if (port->irq)
>  		free_irq(port->irq, port);
>