This version restores the v1 version of the patch (without the UART_CAP_CWFIFO flag),
but fixes the "checkpatch -strict" warnings caught in review.
Wander Lairson Costa (1):
tty: serial: Use fifo in 8250 console driver
drivers/tty/serial/8250/8250_port.c | 61 ++++++++++++++++++++++++++---
1 file changed, 55 insertions(+), 6 deletions(-)
--
2.27.0
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 <[email protected]>
---
drivers/tty/serial/8250/8250_port.c | 61 ++++++++++++++++++++++++++---
1 file changed, 55 insertions(+), 6 deletions(-)
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 46e2079ad1aa..5805f18520dd 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -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
--
2.27.0
On 2021-12-22 08:28:30 [-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:
was this the HW access or did this include the wait-for-fifo empty?
> $ 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 | }
So this includes waiting for empty slot. It is wait_for_xmitr() only.
> 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.
Either the HW is slow on starting to work, or…
What I noticed now in -rc1 is this output during boot:
|[ 6.370196] ACPI: button: Power Button [PWRF]
|[ 6.443501] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
|[0I 15
| [0I 15
| [No
| [ld
| [a2
| [a20tm
| [a2nct
| [s
|[s
|[s
|[s
…
|[sk65,
| [rt
| [Pe
| [a
| [ 6.873611] ata1: SATA link down (SStatus 0 SControl 300)
|[ 6.879680] ata3: SATA link down (SStatus 0 SControl 300)
The kernel buffer reports here:
|[ 6.370196] ACPI: button: Power Button [PWRF]
|[ 6.443501] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
|[ 6.450643] 00:03: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A
|[ 6.451625] 00:04: ttyS1 at I/O 0x2f8 (irq = 3, base_baud = 115200) is a 16550A
|[ 6.453808] Non-volatile memory driver v1.3
|[ 6.475688] loop: module loaded
|[ 6.476401] ahci 0000:00:1f.2: version 3.0
|[ 6.487238] ahci 0000:00:1f.2: AHCI 0001.0300 32 slots 6 ports 6 Gbps 0x3f impl SATA mode
I did remove the last few lines but it appears that since the
initialisation of the port some of the lines got lost.
Do you see the same?
> Signed-off-by: Wander Lairson Costa <[email protected]>
Sebastian
On Tue, Jan 25, 2022 at 10:07 AM Sebastian Andrzej Siewior
<[email protected]> wrote:
>
> On 2021-12-22 08:28:30 [-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:
>
> was this the HW access or did this include the wait-for-fifo empty?
>
> > $ 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 | }
>
> So this includes waiting for empty slot. It is wait_for_xmitr() only.
>
> > 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.
>
> Either the HW is slow on starting to work, or…
>
> What I noticed now in -rc1 is this output during boot:
>
> |[ 6.370196] ACPI: button: Power Button [PWRF]
> |[ 6.443501] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
> |[0I 15
> | [0I 15
> | [No
> | [ld
> | [a2
> | [a20tm
> | [a2nct
> | [s
> |[s
> |[s
> |[s
> …
> |[sk65,
> | [rt
> | [Pe
> | [a
> | [ 6.873611] ata1: SATA link down (SStatus 0 SControl 300)
> |[ 6.879680] ata3: SATA link down (SStatus 0 SControl 300)
>
> The kernel buffer reports here:
>
> |[ 6.370196] ACPI: button: Power Button [PWRF]
> |[ 6.443501] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
> |[ 6.450643] 00:03: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A
> |[ 6.451625] 00:04: ttyS1 at I/O 0x2f8 (irq = 3, base_baud = 115200) is a 16550A
> |[ 6.453808] Non-volatile memory driver v1.3
> |[ 6.475688] loop: module loaded
> |[ 6.476401] ahci 0000:00:1f.2: version 3.0
> |[ 6.487238] ahci 0000:00:1f.2: AHCI 0001.0300 32 slots 6 ports 6 Gbps 0x3f impl SATA mode
>
> I did remove the last few lines but it appears that since the
> initialisation of the port some of the lines got lost.
>
> Do you see the same?
There is another thread[1] reporting some issues with this patch.
There, this diff seems to fix the problems, could you please try and
report if it works for you too?
diff --git a/drivers/tty/serial/8250/8250_port.c
b/drivers/tty/serial/8250/8250_port.c
index 2abb3de11a48..d3a93e5d55f7 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -3343,7 +3343,7 @@ static void serial8250_console_fifo_write(struct
uart_8250_port *up,
{
int i;
const char *end = s + count;
- unsigned int fifosize = up->port.fifosize;
+ unsigned int fifosize = up->tx_loadsz;
bool cr_sent = false;
while (s != end) {
@@ -3409,8 +3409,8 @@ void serial8250_console_write(struct
uart_8250_port *up, const char *s,
}
use_fifo = (up->capabilities & UART_CAP_FIFO) &&
- port->fifosize > 1 &&
- (serial_port_in(port, UART_FCR) & UART_FCR_ENABLE_FIFO) &&
+ up->tx_loadsz > 1 &&
+ (up->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
[1] https://lore.kernel.org/all/[email protected]/
>
> > Signed-off-by: Wander Lairson Costa <[email protected]>
>
> Sebastian
>
On 2022-01-25 10:15:04 [-0300], Wander Costa wrote:
> There is another thread[1] reporting some issues with this patch.
> There, this diff seems to fix the problems, could you please try and
> report if it works for you too?
Nope. Still there.
Sebastian
On Tue, Jan 25, 2022 at 11:11 AM Sebastian Andrzej Siewior
<[email protected]> wrote:
>
> On 2022-01-25 10:15:04 [-0300], Wander Costa wrote:
> > There is another thread[1] reporting some issues with this patch.
> > There, this diff seems to fix the problems, could you please try and
> > report if it works for you too?
>
> Nope. Still there.
I will check on my side if I see something similar. Do you still get
lines lost as well?
On 2022-01-25 11:26:03 [-0300], Wander Costa wrote:
> I will check on my side if I see something similar. Do you still get
> lines lost as well?
Yes. The output is garbage for a while and then it fixes itself. Setting
"use_fifo = 0" works.
Thanks.
Sebastian