@@ -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();
@@ -3372,7 +3408,20 @@ 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
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> --- drivers/tty/serial/8250/8250_port.c | 61 ++++++++++++++++++++++++++--- 1 file changed, 55 insertions(+), 6 deletions(-)