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Message-ID: <4767809d-5818-ad40-a0e7-b3af40aa071e@kernel.org>
Date: Fri, 1 Apr 2022 06:35:58 +0200
From: Jiri Slaby <jirislaby@...nel.org>
To: Wander Lairson Costa <wander@...hat.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Johan Hovold <johan@...nel.org>,
"Maciej W. Rozycki" <macro@...am.me.uk>,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Lukas Wunner <lukas@...ner.de>,
Pali Rohár <pali@...nel.org>,
"open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
open list <linux-kernel@...r.kernel.org>
Cc: rostedt@...dmis.org, senozhatsky@...omium.org,
andre.goddard@...il.com, sudipm.mukherjee@...il.com,
andy.shevchenko@...il.com, David.Laight@...lab.com,
jonathanh@...dia.com, phil@...pberrypi.com
Subject: Re: [PATCH v5 1/1] serial/8250: Use fifo in 8250 console driver
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@...hat.com>
> ---
> 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
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