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

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();
+
+	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

-- 
2.30.0


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

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

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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:

quoted

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>

[...]

quoted

+
+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

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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:

quoted

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

quoted

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>

[...]

quoted

quoted

+
+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.

quoted

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

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On Fri, Jan 08, 2021 at 08:58:38AM +0100, Jiri Slaby wrote:

On 07. 01. 21, 19:16, Cristian Ciocaltea wrote:

quoted

Hi Greg,

Thank you for the review!

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

quoted

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

quoted

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>

[...]

quoted

quoted

+
+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.

quoted

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

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