[PATCH v6 0/1] serial/8250: Use fifo in 8250 console driver

[PATCH v6 0/1] serial/8250: Use fifo in 8250 console driver

[Wander Lairson Costa]

This is v6 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 v5
--------------------

* Fixed a typo in patch patch "port-state" becomes "port->state".

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 v6] 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..8113e6c73407 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 v6] serial/8250: Use fifo in 8250 console driver

[Andy Shevchenko]

On Fri, Apr 01, 2022 at 04:46:42PM -0300, 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.

...

> +	use_fifo = (up->capabilities & UART_CAP_FIFO) &&
> +		/*
> +		 * BCM283x requires to check the fifo
> +		 * after each byte.
> +		 */
> +		!(up->capabilities & UART_CAP_MINI) &&

Perhaps you need to also comment why we are using tx_loadsz and not fifosize.

> +		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);

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH v6] serial/8250: Use fifo in 8250 console driver

[Wander Costa]

On Mon, Apr 4, 2022 at 6:32 AM Andy Shevchenko
<andriy.shevchenko@linux.intel.com> wrote:
>
> On Fri, Apr 01, 2022 at 04:46:42PM -0300, 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.
>
> ...
>
> > +     use_fifo = (up->capabilities & UART_CAP_FIFO) &&
> > +             /*
> > +              * BCM283x requires to check the fifo
> > +              * after each byte.
> > +              */
> > +             !(up->capabilities & UART_CAP_MINI) &&
>
> Perhaps you need to also comment why we are using tx_loadsz and not fifosize.
>

Maybe it is better to document their difference in the struct
declaration and not in a random usage.


[snip]

Re: [PATCH v6] serial/8250: Use fifo in 8250 console driver

[Andy Shevchenko]

On Mon, Apr 04, 2022 at 10:27:30AM -0300, Wander Costa wrote:
> On Mon, Apr 4, 2022 at 6:32 AM Andy Shevchenko
> <andriy.shevchenko@linux.intel.com> wrote:
> > On Fri, Apr 01, 2022 at 04:46:42PM -0300, Wander Lairson Costa wrote:

...

> > > +     use_fifo = (up->capabilities & UART_CAP_FIFO) &&
> > > +             /*
> > > +              * BCM283x requires to check the fifo
> > > +              * after each byte.
> > > +              */
> > > +             !(up->capabilities & UART_CAP_MINI) &&
> >
> > Perhaps you need to also comment why we are using tx_loadsz and not fifosize.
> 
> Maybe it is better to document their difference in the struct
> declaration and not in a random usage.

Here, when one reads a code (as a non-familiar with the area), the use of
tx_loadsz confuses if one saw previously fifosize used somewhere. So, I agree
that it's good to document in the structure, but here it's also good to have
a comment to briefly hint the reader why this and not the other one is used.

-- 
With Best Regards,
Andy Shevchenko