On Sat, Jul 04, 2020 at 06:43:26PM +0300, Andy Shevchenko wrote: > On Fri, Jul 03, 2020 at 01:31:24PM +0200, Geert Uytterhoeven wrote: > > On Thu, Jul 2, 2020 at 4:48 PM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote: > > > On Sun, May 24, 2020 at 7:11 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote: > > > > On Mon, Feb 17, 2020 at 01:40:12PM +0200, Andy Shevchenko wrote: > > > > > In the future we would like to disable power management on the serial devices > > > > > used as kernel consoles to avoid weird behaviour in some cases. However, > > > > > disabling PM may prevent system to go to deep sleep states, which in its turn > > > > > leads to the higher power consumption. > > > > > > > > > > Tony Lindgren proposed a work around, i.e. allow user to detach such consoles > > > > > to make PM working again. In case user wants to see what's going on, it also > > > > > provides a mechanism to attach console back. > > > > > > > > > > Link: https://lists.openwall.net/linux-kernel/2018/09/29/65 > > > > > Suggested-by: Tony Lindgren <tony@xxxxxxxxxxx> > > > > > Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> > > > > > > > > --- a/drivers/tty/serial/serial_core.c > > > > > +++ b/drivers/tty/serial/serial_core.c > > > > > @@ -1919,7 +1919,7 @@ static inline bool uart_console_enabled(struct uart_port *port) > > > > > */ > > > > > static inline void uart_port_spin_lock_init(struct uart_port *port) > > > > > { > > > > > - if (uart_console_enabled(port)) > > > > > + if (uart_console(port)) > > > > > > > > This results in lockdep splashes such as the one attached below. Is there > > > > > > Or "BUG: spinlock bad magic on CPU#3, swapper/0/1", cfr. [1]. > > > So far I hadn't noticed that, as the issue only shows up when using the > > > legacy way of passing a "console=ttyS*" kernel command line parameter, > > > and not when relying on the modern "chosen/stdout-path" DT property. > > > > > > > any special reason for this change ? It is not really explained in the > > > > commit description. > > > > > > Indeed. Why this change? > > > > > > I also don't agree with your typical fix for drivers, which is like: > > > > > > @@ -567,6 +567,9 @@ static int hv_probe(struct platform_device *op) > > > sunserial_console_match(&sunhv_console, op->dev.of_node, > > > &sunhv_reg, port->line, false); > > > > > > + /* We need to initialize lock even for non-registered console */ > > > + spin_lock_init(&port->lock); > > > + > > > err = uart_add_one_port(&sunhv_reg, port); > > > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > > > calls uart_port_spin_lock_init() > > > > > > if (err) > > > goto out_unregister_driver; > > > > > > as this initializes the spinlock twice for non-console= ports. > > > > I had a deeper look... > > > > /* > > * Ensure that the serial console lock is initialised early. > > * If this port is a console, then the spinlock is already initialised. > > */ > > static inline void uart_port_spin_lock_init(struct uart_port *port) > > { > > if (uart_console(port)) > > return; > > > > spin_lock_init(&port->lock); > > lockdep_set_class(&port->lock, &port_lock_key); > > } > > > > So according to the comment, the spinlock is assumed to be already > > initialized, as the port is already in use as a console. Makes sense. > > Thanks, Geert! Yes, the change makes code aligned with a comment. I did it due > to some issues with attaching / detaching consoles (I can try to reproduce > later, perhaps next week, I'm a bit limited now to fulfil kernel work / > testing). > > > Now, where should it be initialized? > > 1. For modern DT systems, chosen/stdout-path is used, and the spinlock > > is initialized in register_earlycon(), just before calling > > register_console(). And everything's fine. > > > > 2. With "console=" (even on DT systems with chosen/stdout-path), > > the serial console must gets registered differently. > > Naively, I assumed that's done in the serial driver, but apparently > > that is no longer the case: the single register_console() call in > > drivers/tty/serial/sh-sci.c is used on legacy SuperH only. > > So we're back to drivers/tty/serial/serial_core.c, which calls > > register_console(), but does so _after_ taking the spinlock: > > > > uart_add_one_port() > > uart_port_spin_lock_init() /* skips spin_lock_init()! */ > > uart_configure_port() > > spin_lock_irqsave(&port->lock, flags); /* BUG! */ > > register_console()) > > > > So who's to blame for _not_ initializing the spinlock? > > This is a very good question. Code is so old and I don't know why we have such > interesting implementation among serial drivers. The 8250 does this > initialisation at console_initcall() when it *properly* calls > register_console() before adding port (yet). > > /* > * If this driver supports console, and it hasn't been > * successfully registered yet, try to re-register it. > * It may be that the port was not available. > */ > if (port->cons && !(port->cons->flags & CON_ENABLED)) > register_console(port->cons); > > Seems like a chicken-egg problem. Any advice? This comment even more interesting... /* * Ensure that the modem control lines are de-activated. * keep the DTR setting that is set in uart_set_options() * We probably don't need a spinlock around this, but */ Investigation shows that this comes from (see history.git from history group) commit 33c0d1b0c3ebb61243d9b19ce70d9063acff2aac Author: Russell King <rmk@xxxxxxxxxxxxxxxx> Date: Sun Jul 21 02:39:46 2002 -0700 [PATCH] Serial driver stuff According to documentation ->set_mctrl() is called with lock taken. Hmm... So, removing that spin lock and moving spin lock initialisation call after uart_configure_port should fix this for all. But I'm not sure we don't break anything. >From d361b1b35f4e8d318c584f224d67240b775fbe7f Mon Sep 17 00:00:00 2001 From: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> Date: Sat, 4 Jul 2020 19:30:39 +0300 Subject: [PATCH 1/1] serial: core: Drop ambiguous spin lock in uart_configure_port() Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> --- drivers/tty/serial/serial_core.c | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-) diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c index 3cc183acf7ba..b4ed1e20dd5c 100644 --- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -2374,11 +2374,8 @@ uart_configure_port(struct uart_driver *drv, struct uart_state *state, /* * Ensure that the modem control lines are de-activated. * keep the DTR setting that is set in uart_set_options() - * We probably don't need a spinlock around this, but */ - spin_lock_irqsave(&port->lock, flags); port->ops->set_mctrl(port, port->mctrl & TIOCM_DTR); - spin_unlock_irqrestore(&port->lock, flags); /* * If this driver supports console, and it hasn't been @@ -2886,8 +2883,6 @@ int uart_add_one_port(struct uart_driver *drv, struct uart_port *uport) goto out; } - uart_port_spin_lock_init(uport); - if (uport->cons && uport->dev) of_console_check(uport->dev->of_node, uport->cons->name, uport->line); @@ -2896,6 +2891,8 @@ int uart_add_one_port(struct uart_driver *drv, struct uart_port *uport) port->console = uart_console(uport); + uart_port_spin_lock_init(uport); + num_groups = 2; if (uport->attr_group) num_groups++; -- With Best Regards, Andy Shevchenko
