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SubjectRe: [PATCH v4 3/3] serial: 8250_dw: Fix common clocks usage race condition
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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