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? -- With Best Regards, Andy Shevchenko
