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Message-Id: <20211029201402.428284-1-wander@redhat.com>
Date: Fri, 29 Oct 2021 17:14:01 -0300
From: wander@...hat.com
To: unlisted-recipients:; (no To-header on input)
Cc: Wander Lairson Costa <wander@...hat.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby <jirislaby@...nel.org>,
"Maciej W. Rozycki" <macro@...am.me.uk>,
Johan Hovold <johan@...nel.org>,
Andrew Jeffery <andrew@...id.au>,
linux-serial@...r.kernel.org (open list:SERIAL DRIVERS),
linux-kernel@...r.kernel.org (open list)
Subject: [PATCH] tty: serial: Use fifo in 8250 console driver
From: Wander Lairson Costa <wander@...hat.com>
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.
While investigating a bug in the RHEL kernel, I noticed that the serial
console throughput is way below the configured speed of 115200 bps in
a HP Proliant DL380 Gen9. I was expecting something above 10KB/s, but
I got 2.5KB/s.
$ 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 benchmarks I got around 25% improvement in the slow
machine, and no performance penalty in the fast machine.
Signed-off-by: Wander Lairson Costa <wander@...hat.com>
---
drivers/tty/serial/8250/8250_port.c | 61 ++++++++++++++++++++++++++---
1 file changed, 55 insertions(+), 6 deletions(-)
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 66374704747e..edf88a8338a2 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -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) {
@@ -3319,6 +3326,35 @@ 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...
@@ -3334,7 +3370,7 @@ 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();
@@ -3366,7 +3402,20 @@ 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_FIFO)
+ && 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 (likely(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
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