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tty: serial: Use fifo in 8250 console driver
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@redhat.com> Link: https://lore.kernel.org/r/20211222112831.1968392-2-wander@redhat.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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1 changed files with 55 additions and 6 deletions
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@ -2056,10 +2056,7 @@ static void serial8250_break_ctl(struct uart_port *port, int break_state)
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serial8250_rpm_put(up);
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}
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/*
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* Wait for transmitter & holding register to empty
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*/
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static void wait_for_xmitr(struct uart_8250_port *up, int bits)
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static void wait_for_lsr(struct uart_8250_port *up, int bits)
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{
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unsigned int status, tmout = 10000;
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@ -2076,6 +2073,16 @@ static void wait_for_xmitr(struct uart_8250_port *up, int bits)
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udelay(1);
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touch_nmi_watchdog();
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}
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}
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/*
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* Wait for transmitter & holding register to empty
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*/
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static void wait_for_xmitr(struct uart_8250_port *up, int bits)
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{
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unsigned int tmout;
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wait_for_lsr(up, bits);
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/* Wait up to 1s for flow control if necessary */
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if (up->port.flags & UPF_CONS_FLOW) {
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@ -3325,6 +3332,35 @@ static void serial8250_console_restore(struct uart_8250_port *up)
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serial8250_out_MCR(up, UART_MCR_DTR | UART_MCR_RTS);
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}
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/*
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* Print a string to the serial port using the device FIFO
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*
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* It sends fifosize bytes and then waits for the fifo
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* to get empty.
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*/
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static void serial8250_console_fifo_write(struct uart_8250_port *up,
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const char *s, unsigned int count)
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{
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int i;
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const char *end = s + count;
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unsigned int fifosize = up->port.fifosize;
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bool cr_sent = false;
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while (s != end) {
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wait_for_lsr(up, UART_LSR_THRE);
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for (i = 0; i < fifosize && s != end; ++i) {
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if (*s == '\n' && !cr_sent) {
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serial_out(up, UART_TX, '\r');
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cr_sent = true;
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} else {
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serial_out(up, UART_TX, *s++);
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cr_sent = false;
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}
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}
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}
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}
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/*
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* Print a string to the serial port trying not to disturb
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* any possible real use of the port...
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@ -3340,7 +3376,7 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
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struct uart_8250_em485 *em485 = up->em485;
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struct uart_port *port = &up->port;
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unsigned long flags;
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unsigned int ier;
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unsigned int ier, use_fifo;
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int locked = 1;
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touch_nmi_watchdog();
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@ -3372,7 +3408,20 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
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mdelay(port->rs485.delay_rts_before_send);
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}
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uart_console_write(port, s, count, serial8250_console_putchar);
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use_fifo = (up->capabilities & UART_CAP_FIFO) &&
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port->fifosize > 1 &&
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(serial_port_in(port, UART_FCR) & UART_FCR_ENABLE_FIFO) &&
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/*
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* After we put a data in the fifo, the controller will send
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* it regardless of the CTS state. Therefore, only use fifo
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* if we don't use control flow.
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*/
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!(up->port.flags & UPF_CONS_FLOW);
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if (likely(use_fifo))
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serial8250_console_fifo_write(up, s, count);
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else
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uart_console_write(port, s, count, serial8250_console_putchar);
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/*
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* Finally, wait for transmitter to become empty
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