[PATCH v2 4/4] serial/aspeed-vuart: Implement quick throttle mechanism
Jeremy Kerr
jk at ozlabs.org
Tue Mar 27 14:48:27 AEDT 2018
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.
For this, we need an unlocked version of the set_throttle function to be
able to change throttle state from the irq_handler, which already holds
port->lock.
This prevents a case where we drop characters under heavy RX load.
Signed-off-by: Jeremy Kerr <jk at ozlabs.org>
Tested-by: Eddie James <eajames at linux.vnet.ibm.com>
---
v2: Use updated timer API, fix text wrap
---
drivers/tty/serial/8250/8250_aspeed_vuart.c | 105 ++++++++++++++++++++++++++--
1 file changed, 101 insertions(+), 4 deletions(-)
diff --git a/drivers/tty/serial/8250/8250_aspeed_vuart.c b/drivers/tty/serial/8250/8250_aspeed_vuart.c
index cd1bb49dadfe..023db3266757 100644
--- a/drivers/tty/serial/8250/8250_aspeed_vuart.c
+++ b/drivers/tty/serial/8250/8250_aspeed_vuart.c
@@ -10,6 +10,8 @@
#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"
@@ -28,8 +30,17 @@ struct aspeed_vuart {
void __iomem *regs;
struct clk *clk;
int line;
+ struct timer_list unthrottle_timer;
+ struct uart_8250_port *port;
};
+/*
+ * 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
@@ -179,17 +190,23 @@ 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);
}
@@ -203,6 +220,83 @@ static void aspeed_vuart_unthrottle(struct uart_port *port)
aspeed_vuart_set_throttle(port, false);
}
+static void aspeed_vuart_unthrottle_exp(struct timer_list *timer)
+{
+ struct aspeed_vuart *vuart = from_timer(vuart, timer, unthrottle_timer);
+ struct uart_8250_port *up = vuart->port;
+
+ 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->port = 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;
@@ -219,6 +313,7 @@ static int aspeed_vuart_probe(struct platform_device *pdev)
return -ENOMEM;
vuart->dev = &pdev->dev;
+ timer_setup(&vuart->unthrottle_timer, aspeed_vuart_unthrottle_exp, 0);
res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
vuart->regs = devm_ioremap_resource(&pdev->dev, res);
@@ -280,6 +375,7 @@ 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;
@@ -319,6 +415,7 @@ 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);
--
2.14.1
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