diff mbox series

[v2,1/1] tty: serial: owl: Add support for kernel debugger

Message ID 036c09732183a30eaab230884114f65ca42ca3b9.1609865007.git.cristian.ciocaltea@gmail.com
State New
Headers show
Series [v2,1/1] tty: serial: owl: Add support for kernel debugger | expand

Commit Message

Cristian Ciocaltea Jan. 5, 2021, 5:02 p.m. UTC
Implement 'poll_put_char' and 'poll_get_char' callbacks in struct
'owl_uart_ops' that enables OWL UART to be used for kernel debugging
over serial line.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
---
Changes in v2:
 - Reverted unnecessary changes per Andreas feedback
 - Optimized implementation for 'owl_uart_poll_get_char()'
   and 'owl_uart_poll_put_char()' callbacks

 drivers/tty/serial/owl-uart.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

Comments

Greg KH Jan. 7, 2021, 3:20 p.m. UTC | #1
On Tue, Jan 05, 2021 at 07:02:02PM +0200, Cristian Ciocaltea wrote:
> Implement 'poll_put_char' and 'poll_get_char' callbacks in struct

> 'owl_uart_ops' that enables OWL UART to be used for kernel debugging

> over serial line.

> 

> Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>

> ---

> Changes in v2:

>  - Reverted unnecessary changes per Andreas feedback

>  - Optimized implementation for 'owl_uart_poll_get_char()'

>    and 'owl_uart_poll_put_char()' callbacks

> 

>  drivers/tty/serial/owl-uart.c | 25 +++++++++++++++++++++++++

>  1 file changed, 25 insertions(+)

> 

> diff --git a/drivers/tty/serial/owl-uart.c b/drivers/tty/serial/owl-uart.c

> index c149f8c30007..54b24669ebc5 100644

> --- a/drivers/tty/serial/owl-uart.c

> +++ b/drivers/tty/serial/owl-uart.c

> @@ -12,6 +12,7 @@

>  #include <linux/console.h>

>  #include <linux/delay.h>

>  #include <linux/io.h>

> +#include <linux/iopoll.h>

>  #include <linux/module.h>

>  #include <linux/of.h>

>  #include <linux/platform_device.h>

> @@ -461,6 +462,26 @@ static void owl_uart_config_port(struct uart_port *port, int flags)

>  	}

>  }

>  

> +#ifdef CONFIG_CONSOLE_POLL

> +

> +static int owl_uart_poll_get_char(struct uart_port *port)

> +{

> +	if (owl_uart_read(port, OWL_UART_STAT) & OWL_UART_STAT_RFEM)

> +		return NO_POLL_CHAR;

> +

> +	return owl_uart_read(port, OWL_UART_RXDAT);

> +}

> +

> +static void owl_uart_poll_put_char(struct uart_port *port, unsigned char ch)

> +{

> +	while (owl_uart_read(port, OWL_UART_STAT) & OWL_UART_STAT_TFFU)

> +		cpu_relax();


Unbounded loops?  What could possibly go wrong?

:(

Please don't do that in the kernel, put a max bound on this.

And are you _SURE_ that cpu_relax() is what you want to call here?

thanks,

greg k-h
Cristian Ciocaltea Jan. 7, 2021, 6:16 p.m. UTC | #2
Hi Greg,

Thank you for the review!

On Thu, Jan 07, 2021 at 04:20:55PM +0100, Greg Kroah-Hartman wrote:
> On Tue, Jan 05, 2021 at 07:02:02PM +0200, Cristian Ciocaltea wrote:

> > Implement 'poll_put_char' and 'poll_get_char' callbacks in struct

> > 'owl_uart_ops' that enables OWL UART to be used for kernel debugging

> > over serial line.

> > 

> > Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>


[...]

> > +

> > +static void owl_uart_poll_put_char(struct uart_port *port, unsigned char ch)

> > +{

> > +	while (owl_uart_read(port, OWL_UART_STAT) & OWL_UART_STAT_TFFU)

> > +		cpu_relax();

> 

> Unbounded loops?  What could possibly go wrong?

> 

> :(

> 

> Please don't do that in the kernel, put a max bound on this.


I didn't realize the issue since I had encountered this pattern in many
other serial drivers, as well: altera_uart, arc_uart, atmel_serial, etc.

> And are you _SURE_ that cpu_relax() is what you want to call here?


I'm thinking of replacing the loop with 'readl_poll_timeout_atomic()',
if that would be a better approach.

Kind regards,
Cristi

> thanks,

> 

> greg k-h
Jiri Slaby Jan. 8, 2021, 7:58 a.m. UTC | #3
On 07. 01. 21, 19:16, Cristian Ciocaltea wrote:
> Hi Greg,

> 

> Thank you for the review!

> 

> On Thu, Jan 07, 2021 at 04:20:55PM +0100, Greg Kroah-Hartman wrote:

>> On Tue, Jan 05, 2021 at 07:02:02PM +0200, Cristian Ciocaltea wrote:

>>> Implement 'poll_put_char' and 'poll_get_char' callbacks in struct

>>> 'owl_uart_ops' that enables OWL UART to be used for kernel debugging

>>> over serial line.

>>>

>>> Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>

> 

> [...]

> 

>>> +

>>> +static void owl_uart_poll_put_char(struct uart_port *port, unsigned char ch)

>>> +{

>>> +	while (owl_uart_read(port, OWL_UART_STAT) & OWL_UART_STAT_TFFU)

>>> +		cpu_relax();

>>

>> Unbounded loops?  What could possibly go wrong?

>>

>> :(

>>

>> Please don't do that in the kernel, put a max bound on this.

> 

> I didn't realize the issue since I had encountered this pattern in many

> other serial drivers, as well: altera_uart, arc_uart, atmel_serial, etc.

> 

>> And are you _SURE_ that cpu_relax() is what you want to call here?

> 

> I'm thinking of replacing the loop with 'readl_poll_timeout_atomic()',

> if that would be a better approach.


It might be better, yes. Either way, if you add a bound to the loop, you 
definitely need a more precise timing, so ndelay/udelay instead of 
cpu_relax.

thanks,
-- 
js
Cristian Ciocaltea Jan. 8, 2021, 2:10 p.m. UTC | #4
On Fri, Jan 08, 2021 at 08:58:38AM +0100, Jiri Slaby wrote:
> On 07. 01. 21, 19:16, Cristian Ciocaltea wrote:

> > Hi Greg,

> > 

> > Thank you for the review!

> > 

> > On Thu, Jan 07, 2021 at 04:20:55PM +0100, Greg Kroah-Hartman wrote:

> > > On Tue, Jan 05, 2021 at 07:02:02PM +0200, Cristian Ciocaltea wrote:

> > > > Implement 'poll_put_char' and 'poll_get_char' callbacks in struct

> > > > 'owl_uart_ops' that enables OWL UART to be used for kernel debugging

> > > > over serial line.

> > > > 

> > > > Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>

> > 

> > [...]

> > 

> > > > +

> > > > +static void owl_uart_poll_put_char(struct uart_port *port, unsigned char ch)

> > > > +{

> > > > +	while (owl_uart_read(port, OWL_UART_STAT) & OWL_UART_STAT_TFFU)

> > > > +		cpu_relax();

> > > 

> > > Unbounded loops?  What could possibly go wrong?

> > > 

> > > :(

> > > 

> > > Please don't do that in the kernel, put a max bound on this.

> > 

> > I didn't realize the issue since I had encountered this pattern in many

> > other serial drivers, as well: altera_uart, arc_uart, atmel_serial, etc.

> > 

> > > And are you _SURE_ that cpu_relax() is what you want to call here?

> > 

> > I'm thinking of replacing the loop with 'readl_poll_timeout_atomic()',

> > if that would be a better approach.

> 

> It might be better, yes. Either way, if you add a bound to the loop, you

> definitely need a more precise timing, so ndelay/udelay instead of

> cpu_relax.


I will use 1-5 us for the timing, but I'm not quite sure about the
overall timeout - 10 ms would suffice?

Thanks,
Cristi

> thanks,

> -- 

> js
diff mbox series

Patch

diff --git a/drivers/tty/serial/owl-uart.c b/drivers/tty/serial/owl-uart.c
index c149f8c30007..54b24669ebc5 100644
--- a/drivers/tty/serial/owl-uart.c
+++ b/drivers/tty/serial/owl-uart.c
@@ -12,6 +12,7 @@ 
 #include <linux/console.h>
 #include <linux/delay.h>
 #include <linux/io.h>
+#include <linux/iopoll.h>
 #include <linux/module.h>
 #include <linux/of.h>
 #include <linux/platform_device.h>
@@ -461,6 +462,26 @@  static void owl_uart_config_port(struct uart_port *port, int flags)
 	}
 }
 
+#ifdef CONFIG_CONSOLE_POLL
+
+static int owl_uart_poll_get_char(struct uart_port *port)
+{
+	if (owl_uart_read(port, OWL_UART_STAT) & OWL_UART_STAT_RFEM)
+		return NO_POLL_CHAR;
+
+	return owl_uart_read(port, OWL_UART_RXDAT);
+}
+
+static void owl_uart_poll_put_char(struct uart_port *port, unsigned char ch)
+{
+	while (owl_uart_read(port, OWL_UART_STAT) & OWL_UART_STAT_TFFU)
+		cpu_relax();
+
+	owl_uart_write(port, ch, OWL_UART_TXDAT);
+}
+
+#endif /* CONFIG_CONSOLE_POLL */
+
 static const struct uart_ops owl_uart_ops = {
 	.set_mctrl = owl_uart_set_mctrl,
 	.get_mctrl = owl_uart_get_mctrl,
@@ -476,6 +497,10 @@  static const struct uart_ops owl_uart_ops = {
 	.request_port = owl_uart_request_port,
 	.release_port = owl_uart_release_port,
 	.verify_port = owl_uart_verify_port,
+#ifdef CONFIG_CONSOLE_POLL
+	.poll_get_char = owl_uart_poll_get_char,
+	.poll_put_char = owl_uart_poll_put_char,
+#endif
 };
 
 #ifdef CONFIG_SERIAL_OWL_CONSOLE