On 2024-02-22, Peter Collingbourne <pcc@xxxxxxxxxx> wrote: > During the handoff from earlycon to the real console driver, we have > two separate drivers operating on the same device concurrently. In the > case of the 8250 driver these concurrent accesses cause problems due > to the driver's use of banked registers, controlled by LCR.DLAB. It is > possible for the setup(), config_port(), pm() and set_mctrl() callbacks > to set DLAB, which can cause the earlycon code that intends to access > TX to instead access DLL, leading to missed output and corruption on > the serial line due to unintended modifications to the baud rate. > > In particular, for setup() we have: > > univ8250_console_setup() > -> serial8250_console_setup() > -> uart_set_options() > -> serial8250_set_termios() > -> serial8250_do_set_termios() > -> serial8250_do_set_divisor() > > For config_port() we have: > > serial8250_config_port() > -> autoconfig() > > For pm() we have: > > serial8250_pm() > -> serial8250_do_pm() > -> serial8250_set_sleep() > > For set_mctrl() we have (for some devices): > > serial8250_set_mctrl() > -> omap8250_set_mctrl() > -> __omap8250_set_mctrl() > > To avoid such problems, let's make it so that the console is locked > during pre-registration calls to these callbacks, which will prevent > the earlycon driver from running concurrently. Even after the current atomic/threaded print rework is completed, the console lock will be used to synchronize boot consoles with non-boot consoles. I am fine with this solution. Comments below... > Remove the partial solution to this problem in the 8250 driver > that locked the console only during autoconfig_irq(), as this would > result in a deadlock with the new approach. The console continues > to be locked during autoconfig_irq() because it can only be called > through uart_configure_port(). > > Although this patch introduces more locking than strictly necessary > (and in particular it also locks during the call to rs485_config() > which is not affected by this issue as far as I can tell), it follows > the principle that it is the responsibility of the generic console > code to manage the earlycon handoff by ensuring that earlycon and real > console driver code cannot run concurrently, and not the individual > drivers. > > Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx> > Link: https://linux-review.googlesource.com/id/I7cf8124dcebf8618e6b2ee543fa5b25532de55d8 > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") > Cc: stable@xxxxxxxxxxxxxxx > --- > drivers/tty/serial/8250/8250_port.c | 6 ------ > drivers/tty/serial/serial_core.c | 10 ++++++++++ > kernel/printk/printk.c | 20 +++++++++++++++++--- > 3 files changed, 27 insertions(+), 9 deletions(-) > > diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c > index 8ca061d3bbb9..1d65055dde27 100644 > --- a/drivers/tty/serial/8250/8250_port.c > +++ b/drivers/tty/serial/8250/8250_port.c > @@ -1329,9 +1329,6 @@ static void autoconfig_irq(struct uart_8250_port *up) > inb_p(ICP); > } > > - if (uart_console(port)) > - console_lock(); > - > /* forget possible initially masked and pending IRQ */ > probe_irq_off(probe_irq_on()); > save_mcr = serial8250_in_MCR(up); > @@ -1371,9 +1368,6 @@ static void autoconfig_irq(struct uart_8250_port *up) > if (port->flags & UPF_FOURPORT) > outb_p(save_ICP, ICP); > > - if (uart_console(port)) > - console_unlock(); > - > port->irq = (irq > 0) ? irq : 0; > } > > diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c > index d6a58a9e072a..128aa0e0ae24 100644 > --- a/drivers/tty/serial/serial_core.c > +++ b/drivers/tty/serial/serial_core.c > @@ -2608,7 +2608,11 @@ uart_configure_port(struct uart_driver *drv, struct uart_state *state, > port->type = PORT_UNKNOWN; > flags |= UART_CONFIG_TYPE; > } It would be nice to add a comment here mentioning why the console_lock is taken. Even if it is something brief like: /* Sychronize with possible boot console. */ > + if (uart_console(port)) > + console_lock(); > port->ops->config_port(port, flags); > + if (uart_console(port)) > + console_unlock(); > } > > if (port->type != PORT_UNKNOWN) { > @@ -2616,6 +2620,9 @@ uart_configure_port(struct uart_driver *drv, struct uart_state *state, > > uart_report_port(drv, port); > Also, here a brief comment. > + if (uart_console(port)) > + console_lock(); > + > /* Power up port for set_mctrl() */ > uart_change_pm(state, UART_PM_STATE_ON); > > @@ -2632,6 +2639,9 @@ uart_configure_port(struct uart_driver *drv, struct uart_state *state, > > uart_rs485_config(port); > > + if (uart_console(port)) > + console_unlock(); > + > /* > * If this driver supports console, and it hasn't been > * successfully registered yet, try to re-register it. > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index f2444b581e16..db69545e6250 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -3263,6 +3263,20 @@ static int __init keep_bootcon_setup(char *str) > > early_param("keep_bootcon", keep_bootcon_setup); > And here. Maybe slightly more verbose since there is an entire wrapper function created for the purpose. > +static int console_call_setup(struct console *newcon, char *options) > +{ > + int err; > + > + if (!newcon->setup) > + return 0; > + > + console_lock(); > + err = newcon->setup(newcon, options); > + console_unlock(); > + > + return err; > +} > + > /* > * This is called by register_console() to try to match > * the newly registered console with any of the ones selected > @@ -3298,8 +3312,8 @@ static int try_enable_preferred_console(struct console *newcon, > if (_braille_register_console(newcon, c)) > return 0; > > - if (newcon->setup && > - (err = newcon->setup(newcon, c->options)) != 0) > + err = console_call_setup(newcon, c->options); > + if (err != 0) > return err; > } > newcon->flags |= CON_ENABLED; > @@ -3325,7 +3339,7 @@ static void try_enable_default_console(struct console *newcon) > if (newcon->index < 0) > newcon->index = 0; > > - if (newcon->setup && newcon->setup(newcon, NULL) != 0) > + if (console_call_setup(newcon, NULL) != 0) > return; > > newcon->flags |= CON_ENABLED; > -- > 2.44.0.rc1.240.g4c46232300-goog With comments added: Reviewed-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
