[PATCH v5 0/1] serial/8250: Use fifo in 8250 console driver

[PATCH v5 0/1] serial/8250: Use fifo in 8250 console driver

[Wander Lairson Costa]

This is v5 of the serial fifo patch. In relation to the previous
reverted patch, I describe the main changes in the "What changed from
v3" section.

What changed from v4
--------------------

* It squashes all the patches in a single patch
* It adds `port-state &&` check in the `use_fifo` condition as a
* preventive measure.

What changed from v3
--------------------

* Reads the FCR value from the port struct. The earlier patch
erroneously read the value from the controller, but FCR is a write-only
register. Thanks to Jiri Slaby for point this out.

* Use tx_loadsz as the transmitter fifo size. We previously used the
port->fifosize field, which caused data loss in some controllers. Thanks
Jon Hunter for the bug report.

* Exclude the BCM283x from fifo write. This is based on Phil Elwell's
original patch [1].

* Check if the port is initialized before write through fifo.
The serial driver set the value of uart_8250_port.fcr in the function
serial8250_config_port, but only writes the value to the controller
register later in the initalization code. That opens a small window in
which is not safe to use the fifo for console write. Unfortunately, I
lost track of who originally reported the issue. If s/he is reading
this, please speak up so I can give you the due credit.

[1] https://lore.kernel.org/all/20220126141124.4086065-1-phil@raspberrypi.com/

Wander Lairson Costa (1):
  serial/8250: Use fifo in 8250 console driver

 drivers/tty/serial/8250/8250_port.c | 68 ++++++++++++++++++++++++++---
 1 file changed, 62 insertions(+), 6 deletions(-)

-- 
2.35.1

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

[Wander Lairson Costa]

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 &&
+		test_bit(TTY_PORT_INITIALIZED, &port->state->port.iflags) &&
+		/*
+		 * 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
-- 
2.35.1

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

[Jiri Slaby]

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

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

[Wander Costa]

On Fri, Apr 1, 2022 at 1:36 AM Jiri Slaby <jirislaby@kernel.org> wrote:
>

>
> ">" missing here. Doesn't a compiler warn about subtracting different types?
>

My local machine doesn't generate a warning; it causes a build error.
So my question was, how was I able to build and test yesterday in a
remote server? It turns out I built it with make -j40, so I missed the
error message because of the parallel build, and I didn't pay
attention that the output didn't display the modules_install typical
output. Usually, I do my tests in an RHEL9 machine, which ships with
kernel 5.14, but this time I tested in a Fedora machine, which ships
kernel 5.16. And for a moment, I forgot that. So when I booted with
kernel 5.16, I believed I was running my custom kernel. Luckily, v5
mainly was a squash of v4 with this port->state check added. Anyway,
sorry about the typo. v6 is on the way.

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

[Steven Rostedt]

On Fri, 1 Apr 2022 06:35:58 +0200
Jiri Slaby <jirislaby@kernel.org> wrote:

> > 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
> > 

Note, the function graph tracer can add a noticeable amount of overhead to
these timings. If you want a more accurate time for a function, just trace
that one function:

	trace-cmd record -p function_graph -l serial8250_console_write ...

As that will only trace the serial8250_console_write() function (think of
-l as "limit"), and the overhead of function graph for tracing a single
function becomes negligible, and then you can see the true time of that
function.

-- Steve

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

[Wander Costa]

On Fri, Apr 1, 2022 at 1:12 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Fri, 1 Apr 2022 06:35:58 +0200
> Jiri Slaby <jirislaby@kernel.org> wrote:
>
> > > 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
> > >
>
> Note, the function graph tracer can add a noticeable amount of overhead to
> these timings. If you want a more accurate time for a function, just trace
> that one function:
>
>         trace-cmd record -p function_graph -l serial8250_console_write ...
>
> As that will only trace the serial8250_console_write() function (think of
> -l as "limit"), and the overhead of function graph for tracing a single
> function becomes negligible, and then you can see the true time of that
> function.
>

Thanks for the tip. I used bpftrace as a low overhead tracer. Then I
used the function-graph tracer to get the details.

> -- Steve
>