mirror of
https://github.com/torvalds/linux.git
synced 2024-11-16 09:02:00 +00:00
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>
This commit is contained in:
parent
adbfddc757
commit
5021d709b3
@ -2056,10 +2056,7 @@ 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;
|
||||
|
||||
@ -2076,6 +2073,16 @@ 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) {
|
||||
@ -3325,6 +3332,35 @@ 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...
|
||||
@ -3340,7 +3376,7 @@ 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();
|
||||
@ -3372,6 +3408,19 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
|
||||
mdelay(port->rs485.delay_rts_before_send);
|
||||
}
|
||||
|
||||
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);
|
||||
|
||||
/*
|
||||
|
Loading…
Reference in New Issue
Block a user