Messages in this thread | | | Date | Tue, 26 May 2020 19:57:01 +0300 | From | Andy Shevchenko <> | Subject | Re: [PATCH v4 3/3] serial: 8250_dw: Fix common clocks usage race condition |
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On Tue, May 26, 2020 at 07:03:16PM +0300, Serge Semin wrote: > 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.
...
> +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) {
if (rate <= 0) return;
?
> + up = serial8250_get_port(d->data.line); > + > + serial8250_update_uartclk(&up->port, rate); > + } > +}
...
> +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) {
Do you need this?
> + ret = clk_notifier_register(d->clk, &d->clk_notifier);
Okay, seems clk_notifier_register() and its counterpart should be fixed for optional clocks.
> + if (ret) > + dev_warn(p->dev, "Failed to set the clock notifier\n");
So, what does this warning mean on the platforms which does not need notifier at all (i.o.w. all but baikal)?
> + /* > + * Get current reference clock rate to make sure the UART port > + * is equipped with an up-to-date value before it's started up. > + */
Why? We call ->set_termios() for it, no?
> + p->uartclk = clk_get_rate(d->clk); > + if (!p->uartclk) { > + dev_err(p->dev, "Clock rate not defined\n"); > + return -EINVAL; > + } > + } > + > + 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) {
Ditto.
> + clk_notifier_unregister(d->clk, &d->clk_notifier); > + > + flush_work(&d->clk_work); > + } > +}
-- With Best Regards, Andy Shevchenko
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