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