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

[Andy Shevchenko]

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