| Message ID | 20120910041357.GH29537@lizard |
|---|---|
| State | New |
| Headers | show |
> serial port, the CPU receives NMI exception, and we fall into KDB shell. > So, it is our "debug console", and it is able to interrupt (and thus > debug) even IRQ handlers themselves. You seem to have an assumption of single core here. What happens if the NMI hits CPU #0 and the serial IRQ hits CPU #1 simultaneously ? Alan
On Mon, Sep 10, 2012 at 12:16:24PM +0100, Alan Cox wrote: > > serial port, the CPU receives NMI exception, and we fall into KDB > > shell. So, it is our "debug console", and it is able to interrupt > > (and thus debug) even IRQ handlers themselves. > > You seem to have an assumption of single core here. What happens if > the NMI hits CPU #0 and the serial IRQ hits CPU #1 simultaneously ? If you can't redirect all serial IRQs to NMI context, e.g. sometimes you get NMIs, sometimes IRQs, then your NMI handling is not deterministic, and surely this is not supported. The whole concept of clearing IRQs is needed if serial IRQ is routed to the NMI/FIQ (on all CPUs), which by definition guarantees that serial IRQ routine is never triggered. We "steal" all and every serial port's IRQs. Thanks!
On Mon, 10 Sep 2012 10:57:04 -0700 Anton Vorontsov <anton.vorontsov@linaro.org> wrote: > On Mon, Sep 10, 2012 at 12:16:24PM +0100, Alan Cox wrote: > > > serial port, the CPU receives NMI exception, and we fall into KDB > > > shell. So, it is our "debug console", and it is able to interrupt > > > (and thus debug) even IRQ handlers themselves. > > > > You seem to have an assumption of single core here. What happens if > > the NMI hits CPU #0 and the serial IRQ hits CPU #1 simultaneously ? > > If you can't redirect all serial IRQs to NMI context, e.g. sometimes you get > NMIs, sometimes IRQs, then your NMI handling is not deterministic, and surely > this is not supported. This seems like arch specific magic leaking into the tty code. Surely this should be buried in the depths of the platform IRQ code ? Alan
On Mon, Sep 10, 2012 at 08:19:18PM +0100, Alan Cox wrote: > On Mon, 10 Sep 2012 10:57:04 -0700 > Anton Vorontsov <anton.vorontsov@linaro.org> wrote: > > > On Mon, Sep 10, 2012 at 12:16:24PM +0100, Alan Cox wrote: > > > > serial port, the CPU receives NMI exception, and we fall into KDB > > > > shell. So, it is our "debug console", and it is able to interrupt > > > > (and thus debug) even IRQ handlers themselves. > > > > > > You seem to have an assumption of single core here. What happens if > > > the NMI hits CPU #0 and the serial IRQ hits CPU #1 simultaneously ? > > > > If you can't redirect all serial IRQs to NMI context, e.g. sometimes you get > > NMIs, sometimes IRQs, then your NMI handling is not deterministic, and surely > > this is not supported. > > This seems like arch specific magic leaking into the tty code. Surely > this should be buried in the depths of the platform IRQ code ? I'd not call this arch-specific. It's task-specific, and the task is quite peculiar, true, but still nothing to do with arch. If the arch is not able to turn serial IRQs into NMIs, then surely that arch is not able to implement serial-triggered NMI/KDB-shell, so we don't support that arch. And we don't have any other way to accoplish the same: we need to clear the serial interrupts, not some arch-specific interrupts. It is the serial device that causes NMI, not some arch's magic. So we just provide the knob to manage the device's functionality. We do have a separate arch-specific NMI-masking knob that works at CPU level, but if we mask NMI source at CPU level, we have to keep it always masked, otherwise we'll keep reentering NMI. So, we really have to clear serial's interrupt line. That's why I came with clear_irqs callback, it has defined semantics for the serial driver, and zero knowledge about any underlaying architecture or task. Thanks, Anton.
diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c index 2b42a01..0aa08c8 100644 --- a/drivers/tty/serial/kgdboc.c +++ b/drivers/tty/serial/kgdboc.c @@ -227,6 +227,15 @@ static int kgdboc_get_char(void) kgdb_tty_line); } +static void kgdboc_clear_irqs(void) +{ + if (!kgdb_tty_driver) + return; + if (kgdb_tty_driver->ops->clear_irqs) + kgdb_tty_driver->ops->clear_irqs(kgdb_tty_driver, + kgdb_tty_line); +} + static void kgdboc_put_char(u8 chr) { if (!kgdb_tty_driver) @@ -298,6 +307,7 @@ static struct kgdb_io kgdboc_io_ops = { .name = "kgdboc", .read_char = kgdboc_get_char, .write_char = kgdboc_put_char, + .clear_irqs = kgdboc_clear_irqs, .pre_exception = kgdboc_pre_exp_handler, .post_exception = kgdboc_post_exp_handler, }; diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c index cba8443..908d108 100644 --- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -2169,6 +2169,20 @@ static void uart_poll_put_char(struct tty_driver *driver, int line, char ch) port = state->uart_port; port->ops->poll_put_char(port, ch); } + +static void uart_clear_irqs(struct tty_driver *driver, int line) +{ + struct uart_driver *drv = driver->driver_state; + struct uart_state *state = drv->state + line; + struct uart_port *port; + + if (!state || !state->uart_port) + return; + + port = state->uart_port; + if (port->ops->clear_irqs) + port->ops->clear_irqs(port); +} #endif static const struct tty_operations uart_ops = { @@ -2201,6 +2215,7 @@ static const struct tty_operations uart_ops = { .poll_init = uart_poll_init, .poll_get_char = uart_poll_get_char, .poll_put_char = uart_poll_put_char, + .clear_irqs = uart_clear_irqs, #endif }; diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h index 3b111a6..1fd1cf0 100644 --- a/include/linux/kgdb.h +++ b/include/linux/kgdb.h @@ -295,6 +295,7 @@ struct kgdb_io { const char *name; int (*read_char) (void); void (*write_char) (u8); + void (*clear_irqs) (void); void (*flush) (void); int (*init) (void); void (*pre_exception) (void); diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h index 3642710..114b3f3 100644 --- a/include/linux/serial_core.h +++ b/include/linux/serial_core.h @@ -277,6 +277,7 @@ struct uart_ops { int (*poll_init)(struct uart_port *); void (*poll_put_char)(struct uart_port *, unsigned char); int (*poll_get_char)(struct uart_port *); + void (*clear_irqs)(struct uart_port *); #endif }; diff --git a/include/linux/tty_driver.h b/include/linux/tty_driver.h index 6e6dbb7..94b14cd 100644 --- a/include/linux/tty_driver.h +++ b/include/linux/tty_driver.h @@ -287,6 +287,7 @@ struct tty_operations { int (*poll_init)(struct tty_driver *driver, int line, char *options); int (*poll_get_char)(struct tty_driver *driver, int line); void (*poll_put_char)(struct tty_driver *driver, int line, char ch); + void (*clear_irqs)(struct tty_driver *driver, int line); #endif const struct file_operations *proc_fops; };
This patch implements a new callback: clear_irqs. It is used for the cases when KDB-entry (e.g. NMI) and KDB IO (e.g. serial port) shares the same interrupt. To get the idea, let's take some real example (ARM machine): we have a serial port which interrupt is routed to an NMI, and the interrupt is used to enter KDB. Once there is some activity on the serial port, the CPU receives NMI exception, and we fall into KDB shell. So, it is our "debug console", and it is able to interrupt (and thus debug) even IRQ handlers themselves. When used that way, the interrupt never reaches serial driver's IRQ handler routine, which means that serial driver will not silence the interrupt. NMIs behaviour are quite arch-specific, and we can't assume that we can use them as ordinary IRQs, e.g. on some arches (like ARM) we can't handle data aborts, the behaviour is undefined then. So we can't just handle execution to serial driver's IRQ handler from the NMI context once we're done with KDB (plus this would defeat the debugger's purpose: we want the NMI handler be as simple as possible, so it will have less chances to hang). So, given that have to deal with it somehow, we have two options: 1. Implement something that clears the interrupt; 2. Implement a whole new concept of grabbing tty for exclusive KDB use, plus implement mask/unmask callbacks, i.e.: - Since consoles might use ttys w/o opending them, we would have to make kdb respect CON_ENABLED flag (maybe a good idea to do it anyway); - Add 'bool exclusive' argument to tty_find_polling_driver(), if set to 1, the function will refuse to return an already opened tty; and will use the flag in tty_reopen() to not allow multiple users (there are already checks for pty masters, which are "open once" ttys); - Once we got the tty exclusively, we would need to call some new uart->mask_all_but_rx_interrupts call before we want to use the port for NMI/KDB, and unmask_all_but_rx_interrupts after we're done with it. The second option is obviously more complex, needlessly so, and less generic. So I went with the first one: we just consume all the interrupts. The tty becomes silently unusable for the rest of the world when we use it with KDB; but once we reroute the serial IRQ source back from NMI to an ordinary IRQ (in KDB this can be done with 'disable_nmi' command), it will behave as normal. p.s. Since the callback is so far used only by polling users, we place it under the appropriate #ifdef. Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org> --- drivers/tty/serial/kgdboc.c | 10 ++++++++++ drivers/tty/serial/serial_core.c | 15 +++++++++++++++ include/linux/kgdb.h | 1 + include/linux/serial_core.h | 1 + include/linux/tty_driver.h | 1 + 5 files changed, 28 insertions(+)