[PATCH v2] tty: serial: Use fifo in 8250 console driver
From: <hidden>
Date: 2021-11-04 17:18:28
Also in:
lkml
Subsystem:
8250/16?50 (and clone uarts) serial driver, the rest, tty layer and serial drivers · Maintainers:
Greg Kroah-Hartman, Linus Torvalds, Jiri Slaby
From: Wander Lairson Costa <redacted>
Note: I am using a small test app + driver located at [0] for the
problem description. serco is a driver whose write function dispatches
to the serial controller. sertest is a user-mode app that writes n bytes
to the serial console using the serco driver.
Recently I got a report of a soft lockup while loading a bunch a
scsi_debug devices (> 500).
While investigating it, I noticed that the serial console throughput
(called by the printk code) is way below the configured speed of 115200
bps in a HP Proliant DL380 Gen9 server. I was expecting something above
10KB/s, but I got 2.5KB/s. I then built a simple driver [0] to isolate
the console from the printk code. Here it is:
$ time ./sertest -n 2500 /tmp/serco
real 0m0.997s
user 0m0.000s
sys 0m0.997s
With the help of the function tracer, I then noticed the serial
controller was taking around 410us seconds to dispatch one single byte:
$ trace-cmd record -p function_graph -g serial8250_console_write \
./sertest -n 1 /tmp/serco
$ trace-cmd report
| serial8250_console_write() {
0.384 us | _raw_spin_lock_irqsave();
1.836 us | io_serial_in();
1.667 us | io_serial_out();
| uart_console_write() {
| serial8250_console_putchar() {
| wait_for_xmitr() {
1.870 us | io_serial_in();
2.238 us | }
1.737 us | io_serial_out();
4.318 us | }
4.675 us | }
| wait_for_xmitr() {
1.635 us | io_serial_in();
| __const_udelay() {
1.125 us | delay_tsc();
1.429 us | }
...
...
...
1.683 us | io_serial_in();
| __const_udelay() {
1.248 us | delay_tsc();
1.486 us | }
1.671 us | io_serial_in();
411.342 us | }
In another machine, I measured a throughput of 11.5KB/s, with the serial
controller taking between 80-90us to send each byte. That matches the
expected throughput for a configuration of 115200 bps.
This patch changes the serial8250_console_write to use the 16550 fifo
if available. In my artificial benchmark I could get a throughput
increase up to 100% in some cases, but in the real case described at the
beginning the gain was of about 25%.
[0] https://github.com/walac/serial-console-test
Signed-off-by: Wander Lairson Costa <redacted>
---
drivers/tty/serial/8250/8250.h | 3 ++
drivers/tty/serial/8250/8250_port.c | 63 +++++++++++++++++++++++++----
2 files changed, 59 insertions(+), 7 deletions(-)
diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
index 6473361525d1..c711bf118cc1 100644
--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h@@ -83,6 +83,9 @@ struct serial8250_config { #define UART_CAP_MINI BIT(17) /* Mini UART on BCM283X family lacks: * STOP PARITY EPAR SPAR WLEN5 WLEN6 */ +#define UART_CAP_CWFIFO BIT(18) /* Use the UART Fifo in + * serial8250_console_write + */ #define UART_BUG_QUOT BIT(0) /* UART has buggy quot LSB */ #define UART_BUG_TXEN BIT(1) /* UART has buggy TX IIR status */
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 5775cbff8f6e..e58938c435c9 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c@@ -82,7 +82,7 @@ static const struct serial8250_config uart_config[] = { .tx_loadsz = 16, .fcr = UART_FCR_ENABLE_FIFO | UART_FCR_R_TRIG_10, .rxtrig_bytes = {1, 4, 8, 14}, - .flags = UART_CAP_FIFO, + .flags = UART_CAP_FIFO | UART_CAP_CWFIFO, }, [PORT_CIRRUS] = { .name = "Cirrus",
@@ -2063,10 +2063,7 @@ static void serial8250_break_ctl(struct uart_port *port, int break_state) serial8250_rpm_put(up); } -/* - * Wait for transmitter & holding register to empty - */ -static void wait_for_xmitr(struct uart_8250_port *up, int bits) +static void wait_for_lsr(struct uart_8250_port *up, int bits) { unsigned int status, tmout = 10000;
@@ -2083,6 +2080,16 @@ static void wait_for_xmitr(struct uart_8250_port *up, int bits) udelay(1); touch_nmi_watchdog(); } +} + +/* + * Wait for transmitter & holding register to empty + */ +static void wait_for_xmitr(struct uart_8250_port *up, int bits) +{ + unsigned int tmout; + + wait_for_lsr(up, bits); /* Wait up to 1s for flow control if necessary */ if (up->port.flags & UPF_CONS_FLOW) {
@@ -3332,6 +3339,35 @@ static void serial8250_console_restore(struct uart_8250_port *up) serial8250_out_MCR(up, UART_MCR_DTR | UART_MCR_RTS); } +/* + * Print a string to the serial port using the device FIFO + * + * It sends fifosize bytes and then waits for the fifo + * to get empty. + */ +static void serial8250_console_fifo_write(struct uart_8250_port *up, + const char *s, unsigned int count) +{ + int i; + const char *end = s + count; + unsigned int fifosize = up->port.fifosize; + bool cr_sent = false; + + while (s != end) { + wait_for_lsr(up, UART_LSR_THRE); + + for (i = 0; i < fifosize && s != end; ++i) { + if (*s == '\n' && !cr_sent) { + serial_out(up, UART_TX, '\r'); + cr_sent = true; + } else { + serial_out(up, UART_TX, *s++); + cr_sent = false; + } + } + } +} + /* * Print a string to the serial port trying not to disturb * any possible real use of the port...
@@ -3347,7 +3383,7 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s, struct uart_8250_em485 *em485 = up->em485; struct uart_port *port = &up->port; unsigned long flags; - unsigned int ier; + unsigned int ier, use_fifo; int locked = 1; touch_nmi_watchdog();
@@ -3379,7 +3415,20 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s, mdelay(port->rs485.delay_rts_before_send); } - uart_console_write(port, s, count, serial8250_console_putchar); + use_fifo = (up->capabilities & UART_CAP_CWFIFO) + && port->fifosize > 1 + && (serial_port_in(port, UART_FCR) & UART_FCR_ENABLE_FIFO) + /* + * After we put a data in the fifo, the controller will send + * it regardless of the CTS state. Therefore, only use fifo + * if we don't use control flow. + */ + && !(up->port.flags & UPF_CONS_FLOW); + + if (use_fifo) + serial8250_console_fifo_write(up, s, count); + else + uart_console_write(port, s, count, serial8250_console_putchar); /* * Finally, wait for transmitter to become empty
--
2.27.0