From cc816969d7b5973d2720d9afd381f06f0eac6885 Mon Sep 17 00:00:00 2001 From: Serge Semin Date: Thu, 23 Jul 2020 03:33:57 +0300 Subject: serial: 8250_dw: Fix common clocks usage race condition The race condition may happen if the UART reference clock is shared with some other device (on Baikal-T1 SoC it's another DW UART port). In this case if that device changes the clock rate while serial console is using it the DW 8250 UART port might not only end up with an invalid uartclk value saved, but may also experience a distorted output data since baud-clock could have been changed. In order to fix this lets at least try to adjust the 8250 port setting like UART clock rate in case if the reference clock rate change is discovered. The driver will call the new method to update 8250 UART port clock rate settings. It's done by means of the clock event notifier registered at the port startup and unregistered in the shutdown callback method. Note 1. In order to avoid deadlocks we had to execute the UART port update method in a dedicated deferred work. This is due to (in my opinion redundant) the clock update implemented in the dw8250_set_termios() method. Note 2. Before the ref clock is manually changed by the custom set_termios() function we swap the port uartclk value with new rate adjusted to be suitable for the requested baud. It is necessary in order to effectively disable a functionality of the ref clock events handler for the current UART port, since uartclk update will be done a bit further in the generic serial8250_do_set_termios() function. Signed-off-by: Serge Semin Link: https://lore.kernel.org/r/20200723003357.26897-5-Sergey.Semin@baikalelectronics.ru Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/8250/8250_dw.c | 103 +++++++++++++++++++++++++++++++++++++- 1 file changed, 101 insertions(+), 2 deletions(-) diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c index c1511f9244bb..87f450b7c177 100644 --- a/drivers/tty/serial/8250/8250_dw.c +++ b/drivers/tty/serial/8250/8250_dw.c @@ -19,6 +19,8 @@ #include #include #include +#include +#include #include #include #include @@ -43,6 +45,8 @@ struct dw8250_data { int msr_mask_off; struct clk *clk; struct clk *pclk; + struct notifier_block clk_notifier; + struct work_struct clk_work; struct reset_control *rst; unsigned int skip_autocfg:1; @@ -54,6 +58,16 @@ static inline struct dw8250_data *to_dw8250_data(struct dw8250_port_data *data) return container_of(data, struct dw8250_data, data); } +static inline struct dw8250_data *clk_to_dw8250_data(struct notifier_block *nb) +{ + return container_of(nb, struct dw8250_data, clk_notifier); +} + +static inline struct dw8250_data *work_to_dw8250_data(struct work_struct *work) +{ + return container_of(work, struct dw8250_data, clk_work); +} + static inline int dw8250_modify_msr(struct uart_port *p, int offset, int value) { struct dw8250_data *d = to_dw8250_data(p->private_data); @@ -260,6 +274,46 @@ static int dw8250_handle_irq(struct uart_port *p) return 0; } +static void dw8250_clk_work_cb(struct work_struct *work) +{ + struct dw8250_data *d = work_to_dw8250_data(work); + struct uart_8250_port *up; + unsigned long rate; + + rate = clk_get_rate(d->clk); + if (rate <= 0) + return; + + up = serial8250_get_port(d->data.line); + + serial8250_update_uartclk(&up->port, rate); +} + +static int dw8250_clk_notifier_cb(struct notifier_block *nb, + unsigned long event, void *data) +{ + struct dw8250_data *d = clk_to_dw8250_data(nb); + + /* + * We have no choice but to defer the uartclk update due to two + * deadlocks. First one is caused by a recursive mutex lock which + * happens when clk_set_rate() is called from dw8250_set_termios(). + * Second deadlock is more tricky and is caused by an inverted order of + * the clk and tty-port mutexes lock. It happens if clock rate change + * is requested asynchronously while set_termios() is executed between + * tty-port mutex lock and clk_set_rate() function invocation and + * vise-versa. Anyway if we didn't have the reference clock alteration + * in the dw8250_set_termios() method we wouldn't have needed this + * deferred event handling complication. + */ + if (event == POST_RATE_CHANGE) { + queue_work(system_unbound_wq, &d->clk_work); + return NOTIFY_OK; + } + + return NOTIFY_DONE; +} + static void dw8250_do_pm(struct uart_port *port, unsigned int state, unsigned int old) { @@ -283,9 +337,16 @@ static void dw8250_set_termios(struct uart_port *p, struct ktermios *termios, clk_disable_unprepare(d->clk); rate = clk_round_rate(d->clk, newrate); if (rate > 0) { + /* + * Premilinary set the uartclk to the new clock rate so the + * clock update event handler caused by the clk_set_rate() + * calling wouldn't actually update the UART divisor since + * we about to do this anyway. + */ + swap(p->uartclk, rate); ret = clk_set_rate(d->clk, newrate); - if (!ret) - p->uartclk = rate; + if (ret) + swap(p->uartclk, rate); } clk_prepare_enable(d->clk); @@ -312,6 +373,39 @@ static void dw8250_set_ldisc(struct uart_port *p, struct ktermios *termios) serial8250_do_set_ldisc(p, termios); } +static int dw8250_startup(struct uart_port *p) +{ + struct dw8250_data *d = to_dw8250_data(p->private_data); + int ret; + + /* + * Some platforms may provide a reference clock shared between several + * devices. In this case before using the serial port first we have to + * make sure that any clock state change is known to the UART port at + * least post factum. + */ + if (d->clk) { + ret = clk_notifier_register(d->clk, &d->clk_notifier); + if (ret) + dev_warn(p->dev, "Failed to set the clock notifier\n"); + } + + return serial8250_do_startup(p); +} + +static void dw8250_shutdown(struct uart_port *p) +{ + struct dw8250_data *d = to_dw8250_data(p->private_data); + + serial8250_do_shutdown(p); + + if (d->clk) { + clk_notifier_unregister(d->clk, &d->clk_notifier); + + flush_work(&d->clk_work); + } +} + /* * dw8250_fallback_dma_filter will prevent the UART from getting just any free * channel on platforms that have DMA engines, but don't have any channels @@ -407,6 +501,8 @@ static int dw8250_probe(struct platform_device *pdev) p->serial_out = dw8250_serial_out; p->set_ldisc = dw8250_set_ldisc; p->set_termios = dw8250_set_termios; + p->startup = dw8250_startup; + p->shutdown = dw8250_shutdown; p->membase = devm_ioremap(dev, regs->start, resource_size(regs)); if (!p->membase) @@ -468,6 +564,9 @@ static int dw8250_probe(struct platform_device *pdev) if (IS_ERR(data->clk)) return PTR_ERR(data->clk); + INIT_WORK(&data->clk_work, dw8250_clk_work_cb); + data->clk_notifier.notifier_call = dw8250_clk_notifier_cb; + err = clk_prepare_enable(data->clk); if (err) dev_warn(dev, "could not enable optional baudclk: %d\n", err); -- cgit v1.2.3