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serial: sh-sci: use hrtimer for receive timeout
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High latencies of classic timers cause performance issues for high-
speed serial transmissions. This patch transforms rx_timer into an
hrtimer to reduce the minimum latency.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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@gregkh
Ulrich Hecht authored and gregkh committed Feb 28, 2018
1 parent 08fb00c commit b96408b
Showing 1 changed file with 30 additions and 17 deletions.
47 changes: 30 additions & 17 deletions drivers/tty/serial/sh-sci.c
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Original file line number Diff line number Diff line change
Expand Up @@ -33,6 +33,7 @@
#include <linux/init.h>
#include <linux/interrupt.h>
#include <linux/ioport.h>
#include <linux/ktime.h>
#include <linux/major.h>
#include <linux/module.h>
#include <linux/mm.h>
Expand Down Expand Up @@ -143,8 +144,8 @@ struct sci_port {
void *rx_buf[2];
size_t buf_len_rx;
struct work_struct work_tx;
struct timer_list rx_timer;
unsigned int rx_timeout;
struct hrtimer rx_timer;
unsigned int rx_timeout; /* microseconds */
#endif
unsigned int rx_frame;
int rx_trigger;
Expand Down Expand Up @@ -1229,6 +1230,15 @@ static void sci_rx_dma_release(struct sci_port *s, bool enable_pio)
}
}

static void start_hrtimer_us(struct hrtimer *hrt, unsigned long usec)
{
long sec = usec / 1000000;
long nsec = (usec % 1000000) * 1000;
ktime_t t = ktime_set(sec, nsec);

hrtimer_start(hrt, t, HRTIMER_MODE_REL);
}

static void sci_dma_rx_complete(void *arg)
{
struct sci_port *s = arg;
Expand All @@ -1247,7 +1257,7 @@ static void sci_dma_rx_complete(void *arg)
if (active >= 0)
count = sci_dma_rx_push(s, s->rx_buf[active], s->buf_len_rx);

mod_timer(&s->rx_timer, jiffies + s->rx_timeout);
start_hrtimer_us(&s->rx_timer, s->rx_timeout);

if (count)
tty_flip_buffer_push(&port->state->port);
Expand Down Expand Up @@ -1391,9 +1401,9 @@ static void work_fn_tx(struct work_struct *work)
dma_async_issue_pending(chan);
}

static void rx_timer_fn(struct timer_list *t)
static enum hrtimer_restart rx_timer_fn(struct hrtimer *t)
{
struct sci_port *s = from_timer(s, t, rx_timer);
struct sci_port *s = container_of(t, struct sci_port, rx_timer);
struct dma_chan *chan = s->chan_rx;
struct uart_port *port = &s->port;
struct dma_tx_state state;
Expand All @@ -1410,7 +1420,7 @@ static void rx_timer_fn(struct timer_list *t)
active = sci_dma_rx_find_active(s);
if (active < 0) {
spin_unlock_irqrestore(&port->lock, flags);
return;
return HRTIMER_NORESTART;
}

status = dmaengine_tx_status(s->chan_rx, s->active_rx, &state);
Expand All @@ -1420,7 +1430,7 @@ static void rx_timer_fn(struct timer_list *t)
s->active_rx, active);

/* Let packet complete handler take care of the packet */
return;
return HRTIMER_NORESTART;
}

dmaengine_pause(chan);
Expand All @@ -1435,7 +1445,7 @@ static void rx_timer_fn(struct timer_list *t)
if (status == DMA_COMPLETE) {
spin_unlock_irqrestore(&port->lock, flags);
dev_dbg(port->dev, "Transaction complete after DMA engine was stopped");
return;
return HRTIMER_NORESTART;
}

/* Handle incomplete DMA receive */
Expand All @@ -1460,6 +1470,8 @@ static void rx_timer_fn(struct timer_list *t)
serial_port_out(port, SCSCR, scr | SCSCR_RIE);

spin_unlock_irqrestore(&port->lock, flags);

return HRTIMER_NORESTART;
}

static struct dma_chan *sci_request_dma_chan(struct uart_port *port,
Expand Down Expand Up @@ -1571,7 +1583,8 @@ static void sci_request_dma(struct uart_port *port)
dma += s->buf_len_rx;
}

timer_setup(&s->rx_timer, rx_timer_fn, 0);
hrtimer_init(&s->rx_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
s->rx_timer.function = rx_timer_fn;

if (port->type == PORT_SCIFA || port->type == PORT_SCIFB)
sci_submit_rx(s);
Expand Down Expand Up @@ -1630,9 +1643,9 @@ static irqreturn_t sci_rx_interrupt(int irq, void *ptr)
/* Clear current interrupt */
serial_port_out(port, SCxSR,
ssr & ~(SCIF_DR | SCxSR_RDxF(port)));
dev_dbg(port->dev, "Rx IRQ %lu: setup t-out in %u jiffies\n",
dev_dbg(port->dev, "Rx IRQ %lu: setup t-out in %u us\n",
jiffies, s->rx_timeout);
mod_timer(&s->rx_timer, jiffies + s->rx_timeout);
start_hrtimer_us(&s->rx_timer, s->rx_timeout);

return IRQ_HANDLED;
}
Expand All @@ -1643,7 +1656,7 @@ static irqreturn_t sci_rx_interrupt(int irq, void *ptr)
scif_set_rtrg(port, s->rx_trigger);

mod_timer(&s->rx_fifo_timer, jiffies + DIV_ROUND_UP(
s->rx_frame * s->rx_fifo_timeout, 1000));
s->rx_frame * HZ * s->rx_fifo_timeout, 1000000));
}

/* I think sci_receive_chars has to be called irrespective
Expand Down Expand Up @@ -2079,7 +2092,7 @@ static void sci_shutdown(struct uart_port *port)
if (s->chan_rx) {
dev_dbg(port->dev, "%s(%d) deleting rx_timer\n", __func__,
port->line);
del_timer_sync(&s->rx_timer);
hrtimer_cancel(&s->rx_timer);
}
#endif

Expand Down Expand Up @@ -2480,11 +2493,11 @@ static void sci_set_termios(struct uart_port *port, struct ktermios *termios,
if (termios->c_cflag & PARENB)
bits++;

s->rx_frame = (100 * bits * HZ) / (baud / 10);
s->rx_frame = (10000 * bits) / (baud / 100);
#ifdef CONFIG_SERIAL_SH_SCI_DMA
s->rx_timeout = DIV_ROUND_UP(s->buf_len_rx * 2 * s->rx_frame, 1000);
if (s->rx_timeout < msecs_to_jiffies(20))
s->rx_timeout = msecs_to_jiffies(20);
s->rx_timeout = s->buf_len_rx * 2 * s->rx_frame;
if (s->rx_timeout < 20)
s->rx_timeout = 20;
#endif

if ((termios->c_cflag & CREAD) != 0)
Expand Down

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