Re: [PATCH v5 1/1] serial/8250: Use fifo in 8250 console driver

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 31. 03. 22, 21:02, Wander Lairson Costa wrote:
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@xxxxxxxxxx>
---
  drivers/tty/serial/8250/8250_port.c | 68 ++++++++++++++++++++++++++---
  1 file changed, 62 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 318af6f13605..8f7eba5e71cf 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2077,10 +2077,7 @@ 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;
@@ -2097,6 +2094,16 @@ 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->tx_loadsz;
+	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 +3415,27 @@ 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) &&
+		/*
+		 * BCM283x requires to check the fifo
+		 * after each byte.
+		 */
+		!(up->capabilities & UART_CAP_MINI) &&
+		up->tx_loadsz > 1 &&
+		(up->fcr & UART_FCR_ENABLE_FIFO) &&
+		port-state &&

">" missing here. Doesn't a compiler warn about subtracting different types?

regards,
--
js
suse labs



[Index of Archives]     [Kernel Newbies]     [Security]     [Netfilter]     [Bugtraq]     [Linux PPP]     [Linux FS]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Samba]     [Video 4 Linux]     [Linmodem]     [Device Mapper]     [Linux Kernel for ARM]

  Powered by Linux