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

[Jiri Slaby <jirislaby@kernel.org>]

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@redhat.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