[PATCH 5/5] serial/aspeed-vuart: Implement quick throttle mechanism

[Eddie James <eajames@linux.vnet.ibm.com>]

On 03/20/2018 09:52 PM, Jeremy Kerr wrote:
> Although we populate the ->throttle and ->unthrottle UART operations,
> these may not be called until the ldisc has had a chance to schedule and
> check buffer space. This means that we may overflow the flip buffers
> without ever hitting the ldisc's throttle threshold.
>
> This change implements an interrupt-based throttle, where we check for
> space in the flip buffers before reading characters from the UART's
> FIFO. If there's no space available, we disable the RX interrupt and
> schedule a timer to check for space later.
>
> This prevents a case where we drop characters under heavy RX load.

Tested-by: Eddie James <eajames@linux.vnet.ibm.com>

>
> Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
> ---
>   drivers/tty/serial/8250/8250_aspeed_vuart.c | 103 ++++++++++++++++++++++++++--
>   1 file changed, 99 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/tty/serial/8250/8250_aspeed_vuart.c b/drivers/tty/serial/8250/8250_aspeed_vuart.c
> index 50c082b4a1ac..bc2b63578e58 100644
> --- a/drivers/tty/serial/8250/8250_aspeed_vuart.c
> +++ b/drivers/tty/serial/8250/8250_aspeed_vuart.c
> @@ -14,6 +14,8 @@
>   #include <linux/of_address.h>
>   #include <linux/of_irq.h>
>   #include <linux/of_platform.h>
> +#include <linux/tty.h>
> +#include <linux/tty_flip.h>
>   #include <linux/clk.h>
>
>   #include "8250.h"
> @@ -32,8 +34,16 @@ struct aspeed_vuart {
>   	void __iomem		*regs;
>   	struct clk		*clk;
>   	int			line;
> +	struct timer_list	unthrottle_timer;
>   };
>
> +/*
> + * If we fill the tty flip buffers, we throttle the data ready interrupt
> + * to prevent dropped characters. This timeout defines how long we wait
> + * to (conditionally, depending on buffer state) unthrottle.
> + */
> +static const int unthrottle_timeout = HZ/10;
> +
>   /*
>    * The VUART is basically two UART 'front ends' connected by their FIFO
>    * (no actual serial line in between). One is on the BMC side (management
> @@ -183,17 +193,23 @@ static void aspeed_vuart_shutdown(struct uart_port *uart_port)
>   	serial8250_do_shutdown(uart_port);
>   }
>
> -static void aspeed_vuart_set_throttle(struct uart_port *port, bool throttle)
> +static void __aspeed_vuart_set_throttle(struct uart_8250_port *up,
> +		bool throttle)
>   {
>   	unsigned char irqs = UART_IER_RLSI | UART_IER_RDI;
> -	struct uart_8250_port *up = up_to_u8250p(port);
> -	unsigned long flags;
>
> -	spin_lock_irqsave(&port->lock, flags);
>   	up->ier &= ~irqs;
>   	if (!throttle)
>   		up->ier |= irqs;
>   	serial_out(up, UART_IER, up->ier);
> +}
> +static void aspeed_vuart_set_throttle(struct uart_port *port, bool throttle)
> +{
> +	struct uart_8250_port *up = up_to_u8250p(port);
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&port->lock, flags);
> +	__aspeed_vuart_set_throttle(up, throttle);
>   	spin_unlock_irqrestore(&port->lock, flags);
>   }
>
> @@ -207,6 +223,81 @@ static void aspeed_vuart_unthrottle(struct uart_port *port)
>   	aspeed_vuart_set_throttle(port, false);
>   }
>
> +static void aspeed_vuart_unthrottle_exp(unsigned long data)
> +{
> +	struct uart_8250_port *up = (void *)data;
> +	struct aspeed_vuart *vuart = up->port.private_data;
> +
> +	if (!tty_buffer_space_avail(&up->port.state->port)) {
> +		mod_timer(&vuart->unthrottle_timer, unthrottle_timeout);
> +		return;
> +	}
> +
> +	aspeed_vuart_unthrottle(&up->port);
> +}
> +
> +/*
> + * Custom interrupt handler to manage finer-grained flow control. Although we
> + * have throttle/unthrottle callbacks, we've seen that the VUART device can
> + * deliver characters faster than the ldisc has a chance to check buffer space
> + * against the throttle threshold. This results in dropped characters before
> + * the throttle.
> + *
> + * We do this by checking for flip buffer space before RX. If we have no space,
> + * throttle now and schedule an unthrottle for later, once the ldisc has had
> + * a chance to drain the buffers.
> + */
> +static int aspeed_vuart_handle_irq(struct uart_port *port) { struct
> +	uart_8250_port *up = up_to_u8250p(port); unsigned int iir, lsr;
> +	unsigned long flags; int space, count;
> +
> +	iir = serial_port_in(port, UART_IIR);
> +
> +	if (iir & UART_IIR_NO_INT)
> +		return 0;
> +
> +	spin_lock_irqsave(&port->lock, flags);
> +
> +	lsr = serial_port_in(port, UART_LSR);
> +
> +	if (lsr & (UART_LSR_DR | UART_LSR_BI)) {
> +		space = tty_buffer_space_avail(&port->state->port);
> +
> +		if (!space) {
> +			/* throttle and schedule an unthrottle later */
> +			struct aspeed_vuart *vuart = port->private_data;
> +			__aspeed_vuart_set_throttle(up, true);
> +
> +			if (!timer_pending(&vuart->unthrottle_timer)) {
> +				vuart->unthrottle_timer.data =
> +					(unsigned long)up;
> +				mod_timer(&vuart->unthrottle_timer,
> +						unthrottle_timeout);
> +			}
> +
> +		} else {
> +			count = min(space, 256);
> +
> +			do {
> +				serial8250_read_char(up, lsr);
> +				lsr = serial_in(up, UART_LSR);
> +				if (--count == 0)
> +					break;
> +			} while (lsr & (UART_LSR_DR | UART_LSR_BI));
> +
> +			tty_flip_buffer_push(&port->state->port);
> +		}
> +	}
> +
> +	serial8250_modem_status(up);
> +	if (lsr & UART_LSR_THRE)
> +		serial8250_tx_chars(up);
> +
> +	spin_unlock_irqrestore(&port->lock, flags);
> +
> +	return 1;
> +}
> +
>   static int aspeed_vuart_probe(struct platform_device *pdev)
>   {
>   	struct uart_8250_port port;
> @@ -223,6 +314,8 @@ static int aspeed_vuart_probe(struct platform_device *pdev)
>   		return -ENOMEM;
>
>   	vuart->dev = &pdev->dev;
> +	setup_timer(&vuart->unthrottle_timer,
> +			aspeed_vuart_unthrottle_exp, (unsigned long)vuart);
>
>   	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
>   	vuart->regs = devm_ioremap_resource(&pdev->dev, res);
> @@ -284,6 +377,7 @@ static int aspeed_vuart_probe(struct platform_device *pdev)
>
>   	port.port.irq = irq_of_parse_and_map(np, 0);
>   	port.port.irqflags = IRQF_SHARED;
> +	port.port.handle_irq = aspeed_vuart_handle_irq;
>   	port.port.iotype = UPIO_MEM;
>   	port.port.type = PORT_16550A;
>   	port.port.uartclk = clk;
> @@ -323,6 +417,7 @@ static int aspeed_vuart_remove(struct platform_device *pdev)
>   {
>   	struct aspeed_vuart *vuart = platform_get_drvdata(pdev);
>
> +	del_timer_sync(&vuart->unthrottle_timer);
>   	aspeed_vuart_set_enabled(vuart, false);
>   	serial8250_unregister_port(vuart->line);
>   	sysfs_remove_group(&vuart->dev->kobj, &aspeed_vuart_attr_group);