From: Wander Lairson Costa <[email protected]>
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.
Recently I got a report of a soft lockup while loading a bunch a
scsi_debug devices (> 500).
While investigating it, I noticed that the serial console throughput
(called by the printk code) is way below the configured speed of 115200
bps in a HP Proliant DL380 Gen9 server. I was expecting something above
10KB/s, but I got 2.5KB/s. I then built a simple driver [0] to isolate
the console from the printk code. Here it is:
$ 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 artificial benchmark I could get a throughput
increase up to 100% in some cases, but in the real case described at the
beginning the gain was of about 25%.
[0] https://github.com/walac/serial-console-test
Signed-off-by: Wander Lairson Costa <[email protected]>
---
drivers/tty/serial/8250/8250.h | 3 ++
drivers/tty/serial/8250/8250_port.c | 63 +++++++++++++++++++++++++----
2 files changed, 59 insertions(+), 7 deletions(-)
diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
index 6473361525d1..c711bf118cc1 100644
--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h
@@ -83,6 +83,9 @@ struct serial8250_config {
#define UART_CAP_MINI BIT(17) /* Mini UART on BCM283X family lacks:
* STOP PARITY EPAR SPAR WLEN5 WLEN6
*/
+#define UART_CAP_CWFIFO BIT(18) /* Use the UART Fifo in
+ * serial8250_console_write
+ */
#define UART_BUG_QUOT BIT(0) /* UART has buggy quot LSB */
#define UART_BUG_TXEN BIT(1) /* UART has buggy TX IIR status */
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 5775cbff8f6e..e58938c435c9 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -82,7 +82,7 @@ static const struct serial8250_config uart_config[] = {
.tx_loadsz = 16,
.fcr = UART_FCR_ENABLE_FIFO | UART_FCR_R_TRIG_10,
.rxtrig_bytes = {1, 4, 8, 14},
- .flags = UART_CAP_FIFO,
+ .flags = UART_CAP_FIFO | UART_CAP_CWFIFO,
},
[PORT_CIRRUS] = {
.name = "Cirrus",
@@ -2063,10 +2063,7 @@ 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;
@@ -2083,6 +2080,16 @@ 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->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...
@@ -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 +3415,20 @@ 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_CWFIFO)
+ && 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 (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 Thu, Nov 04, 2021 at 02:17:31PM -0300, [email protected] wrote:
> From: Wander Lairson Costa <[email protected]>
>
> 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.
>
> Recently I got a report of a soft lockup while loading a bunch a
> scsi_debug devices (> 500).
>
> While investigating it, I noticed that the serial console throughput
> (called by the printk code) is way below the configured speed of 115200
> bps in a HP Proliant DL380 Gen9 server. I was expecting something above
> 10KB/s, but I got 2.5KB/s. I then built a simple driver [0] to isolate
> the console from the printk code. Here it is:
>
> $ 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 artificial benchmark I could get a throughput
> increase up to 100% in some cases, but in the real case described at the
> beginning the gain was of about 25%.
>
> [0] https://github.com/walac/serial-console-test
>
> Signed-off-by: Wander Lairson Costa <[email protected]>
> ---
> drivers/tty/serial/8250/8250.h | 3 ++
> drivers/tty/serial/8250/8250_port.c | 63 +++++++++++++++++++++++++----
> 2 files changed, 59 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
> index 6473361525d1..c711bf118cc1 100644
> --- a/drivers/tty/serial/8250/8250.h
> +++ b/drivers/tty/serial/8250/8250.h
> @@ -83,6 +83,9 @@ struct serial8250_config {
> #define UART_CAP_MINI BIT(17) /* Mini UART on BCM283X family lacks:
> * STOP PARITY EPAR SPAR WLEN5 WLEN6
> */
> +#define UART_CAP_CWFIFO BIT(18) /* Use the UART Fifo in
> + * serial8250_console_write
> + */
Why do you need a new bit? Why can't you just do this change for all
devices that have a fifo? Why would you _not_ want to do this for all
devices that have a fifo?
thanks,
greg k-h
On Mon, Dec 20, 2021 at 12:45 PM Greg Kroah-Hartman
<[email protected]> wrote:
>
> On Thu, Nov 04, 2021 at 02:17:31PM -0300, [email protected] wrote:
> > From: Wander Lairson Costa <[email protected]>
> >
> > 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.
> >
> > Recently I got a report of a soft lockup while loading a bunch a
> > scsi_debug devices (> 500).
> >
> > While investigating it, I noticed that the serial console throughput
> > (called by the printk code) is way below the configured speed of 115200
> > bps in a HP Proliant DL380 Gen9 server. I was expecting something above
> > 10KB/s, but I got 2.5KB/s. I then built a simple driver [0] to isolate
> > the console from the printk code. Here it is:
> >
> > $ 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 artificial benchmark I could get a throughput
> > increase up to 100% in some cases, but in the real case described at the
> > beginning the gain was of about 25%.
> >
> > [0] https://github.com/walac/serial-console-test
> >
> > Signed-off-by: Wander Lairson Costa <[email protected]>
> > ---
> > drivers/tty/serial/8250/8250.h | 3 ++
> > drivers/tty/serial/8250/8250_port.c | 63 +++++++++++++++++++++++++----
> > 2 files changed, 59 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
> > index 6473361525d1..c711bf118cc1 100644
> > --- a/drivers/tty/serial/8250/8250.h
> > +++ b/drivers/tty/serial/8250/8250.h
> > @@ -83,6 +83,9 @@ struct serial8250_config {
> > #define UART_CAP_MINI BIT(17) /* Mini UART on BCM283X family lacks:
> > * STOP PARITY EPAR SPAR WLEN5 WLEN6
> > */
> > +#define UART_CAP_CWFIFO BIT(18) /* Use the UART Fifo in
> > + * serial8250_console_write
> > + */
>
> Why do you need a new bit? Why can't you just do this change for all
> devices that have a fifo? Why would you _not_ want to do this for all
> devices that have a fifo?
>
The v1 patch [1] didn't have this extra bit. Andy suggested [2] to add
it so we only enabled this new code on tested controllers as a
precaution.
If it doesn't make sense to you, feel free to consider the v1 patch [1].
[1] https://lore.kernel.org/all/[email protected]/
[2] https://lore.kernel.org/all/CAHp75Vf6DjNcPWpE4Dh3SuzUMJbFQjq1UNCkrCa60uw35SpqKg@mail.gmail.com/
> thanks,
>
> greg k-h
>
On Mon, Dec 20, 2021 at 02:02:11PM -0300, Wander Costa wrote:
> On Mon, Dec 20, 2021 at 12:45 PM Greg Kroah-Hartman
> <[email protected]> wrote:
> >
> > On Thu, Nov 04, 2021 at 02:17:31PM -0300, [email protected] wrote:
> > > From: Wander Lairson Costa <[email protected]>
> > >
> > > 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.
> > >
> > > Recently I got a report of a soft lockup while loading a bunch a
> > > scsi_debug devices (> 500).
> > >
> > > While investigating it, I noticed that the serial console throughput
> > > (called by the printk code) is way below the configured speed of 115200
> > > bps in a HP Proliant DL380 Gen9 server. I was expecting something above
> > > 10KB/s, but I got 2.5KB/s. I then built a simple driver [0] to isolate
> > > the console from the printk code. Here it is:
> > >
> > > $ 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 artificial benchmark I could get a throughput
> > > increase up to 100% in some cases, but in the real case described at the
> > > beginning the gain was of about 25%.
> > >
> > > [0] https://github.com/walac/serial-console-test
> > >
> > > Signed-off-by: Wander Lairson Costa <[email protected]>
> > > ---
> > > drivers/tty/serial/8250/8250.h | 3 ++
> > > drivers/tty/serial/8250/8250_port.c | 63 +++++++++++++++++++++++++----
> > > 2 files changed, 59 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
> > > index 6473361525d1..c711bf118cc1 100644
> > > --- a/drivers/tty/serial/8250/8250.h
> > > +++ b/drivers/tty/serial/8250/8250.h
> > > @@ -83,6 +83,9 @@ struct serial8250_config {
> > > #define UART_CAP_MINI BIT(17) /* Mini UART on BCM283X family lacks:
> > > * STOP PARITY EPAR SPAR WLEN5 WLEN6
> > > */
> > > +#define UART_CAP_CWFIFO BIT(18) /* Use the UART Fifo in
> > > + * serial8250_console_write
> > > + */
> >
> > Why do you need a new bit? Why can't you just do this change for all
> > devices that have a fifo? Why would you _not_ want to do this for all
> > devices that have a fifo?
> >
> The v1 patch [1] didn't have this extra bit. Andy suggested [2] to add
> it so we only enabled this new code on tested controllers as a
> precaution.
> If it doesn't make sense to you, feel free to consider the v1 patch [1].
>
> [1] https://lore.kernel.org/all/[email protected]/
> [2] https://lore.kernel.org/all/CAHp75Vf6DjNcPWpE4Dh3SuzUMJbFQjq1UNCkrCa60uw35SpqKg@mail.gmail.com/
I do like [1] better, but can you fix up the checkpatch issues and
resend it:
CHECK: Alignment should match open parenthesis
#116: FILE: drivers/tty/serial/8250/8250_port.c:3342:
+static void serial8250_console_fifo_write(struct uart_8250_port *up,
+ const char *s, unsigned int count)
CHECK: Logical continuations should be on the previous line
#156: FILE: drivers/tty/serial/8250/8250_port.c:3412:
+ use_fifo = (up->capabilities & UART_CAP_FIFO)
+ && port->fifosize > 1
CHECK: Logical continuations should be on the previous line
#157: FILE: drivers/tty/serial/8250/8250_port.c:3413:
+ && port->fifosize > 1
+ && (serial_port_in(port, UART_FCR) & UART_FCR_ENABLE_FIFO)
CHECK: Logical continuations should be on the previous line
#163: FILE: drivers/tty/serial/8250/8250_port.c:3419:
+ */
+ && !(up->port.flags & UPF_CONS_FLOW);
thanks,
greg k-h
On Tue, Dec 21, 2021 at 5:12 AM Greg Kroah-Hartman
<[email protected]> wrote:
>
> On Mon, Dec 20, 2021 at 02:02:11PM -0300, Wander Costa wrote:
> > On Mon, Dec 20, 2021 at 12:45 PM Greg Kroah-Hartman
> > <[email protected]> wrote:
> > >
> > > On Thu, Nov 04, 2021 at 02:17:31PM -0300, [email protected] wrote:
> > > > From: Wander Lairson Costa <[email protected]>
> > > >
> > > > 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.
> > > >
> > > > Recently I got a report of a soft lockup while loading a bunch a
> > > > scsi_debug devices (> 500).
> > > >
> > > > While investigating it, I noticed that the serial console throughput
> > > > (called by the printk code) is way below the configured speed of 115200
> > > > bps in a HP Proliant DL380 Gen9 server. I was expecting something above
> > > > 10KB/s, but I got 2.5KB/s. I then built a simple driver [0] to isolate
> > > > the console from the printk code. Here it is:
> > > >
> > > > $ 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 artificial benchmark I could get a throughput
> > > > increase up to 100% in some cases, but in the real case described at the
> > > > beginning the gain was of about 25%.
> > > >
> > > > [0] https://github.com/walac/serial-console-test
> > > >
> > > > Signed-off-by: Wander Lairson Costa <[email protected]>
> > > > ---
> > > > drivers/tty/serial/8250/8250.h | 3 ++
> > > > drivers/tty/serial/8250/8250_port.c | 63 +++++++++++++++++++++++++----
> > > > 2 files changed, 59 insertions(+), 7 deletions(-)
> > > >
> > > > diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
> > > > index 6473361525d1..c711bf118cc1 100644
> > > > --- a/drivers/tty/serial/8250/8250.h
> > > > +++ b/drivers/tty/serial/8250/8250.h
> > > > @@ -83,6 +83,9 @@ struct serial8250_config {
> > > > #define UART_CAP_MINI BIT(17) /* Mini UART on BCM283X family lacks:
> > > > * STOP PARITY EPAR SPAR WLEN5 WLEN6
> > > > */
> > > > +#define UART_CAP_CWFIFO BIT(18) /* Use the UART Fifo in
> > > > + * serial8250_console_write
> > > > + */
> > >
> > > Why do you need a new bit? Why can't you just do this change for all
> > > devices that have a fifo? Why would you _not_ want to do this for all
> > > devices that have a fifo?
> > >
> > The v1 patch [1] didn't have this extra bit. Andy suggested [2] to add
> > it so we only enabled this new code on tested controllers as a
> > precaution.
> > If it doesn't make sense to you, feel free to consider the v1 patch [1].
> >
> > [1] https://lore.kernel.org/all/[email protected]/
> > [2] https://lore.kernel.org/all/CAHp75Vf6DjNcPWpE4Dh3SuzUMJbFQjq1UNCkrCa60uw35SpqKg@mail.gmail.com/
>
> I do like [1] better, but can you fix up the checkpatch issues and
> resend it:
>
Sure. No problem.
> CHECK: Alignment should match open parenthesis
> #116: FILE: drivers/tty/serial/8250/8250_port.c:3342:
> +static void serial8250_console_fifo_write(struct uart_8250_port *up,
> + const char *s, unsigned int count)
>
> CHECK: Logical continuations should be on the previous line
> #156: FILE: drivers/tty/serial/8250/8250_port.c:3412:
> + use_fifo = (up->capabilities & UART_CAP_FIFO)
> + && port->fifosize > 1
>
> CHECK: Logical continuations should be on the previous line
> #157: FILE: drivers/tty/serial/8250/8250_port.c:3413:
> + && port->fifosize > 1
> + && (serial_port_in(port, UART_FCR) & UART_FCR_ENABLE_FIFO)
>
> CHECK: Logical continuations should be on the previous line
> #163: FILE: drivers/tty/serial/8250/8250_port.c:3419:
> + */
> + && !(up->port.flags & UPF_CONS_FLOW);
>
>
>
> thanks,
>
> greg k-h
>