Re: [PATCH v2] serial: 8250_dw: Fix common clocks usage race condition

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Mon, Mar 23, 2020 at 05:46:09AM +0300, Sergey.Semin@baikalelectronics.ru wrote:
> From: Serge Semin <Sergey.Semin@baikalelectronics.ru>

The question to CLK framework maintainers, is it correct approach in general
for this case?

> There are races possible in the dw8250_set_termios() callback method
> and while the device is in PM suspend state. A race condition may
> happen if the baudrate clock source device is shared with some other
> device (in our machine 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 enable an exclusive
> reference clock rate access in case if "baudclk" device is specified.
> 
> So if some other device also acquires the rate exclusivity during the
> time of a DW UART 8250 port being opened, then DW UART 8250 driver
> won't be able to alter the baud-clock. It shall just use the available
> clock rate. Similarly another device also won't manage to change the
> rate at that time. If nothing else have the exclusive rate access
> acquired except DW UART 8250 driver, then the driver will be able to
> alter the rate as much as it needs to in accordance with the currently
> implemented logic.

Thank you for an update, my comments below.

...

> +static int dw8250_startup(struct uart_port *p)
> +{
> +	struct dw8250_data *d = to_dw8250_data(p->private_data);
> +
> +	/*
> +	 * 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 nothing will change the rate behind our back and second
> +	 * the tty/serial subsystem knows the actual reference clock rate of
> +	 * the port.
> +	 */

> +	if (clk_rate_exclusive_get(d->clk)) {
> +		dev_warn(p->dev, "Couldn't lock the clock rate\n");

So, if this fails, in ->shutdown you will disbalance reference count, or did I
miss something?

> +	} else if (d->clk) {

> +		p->uartclk = clk_get_rate(d->clk);
> +		if (!p->uartclk) {
> +			clk_rate_exclusive_put(d->clk);
> +			dev_err(p->dev, "Clock rate not defined\n");
> +			return -EINVAL;
> +		}

This operations I didn't get. If we have d->clk and suddenly get 0 as a rate
(and note, that we still update uartclk member!), we try to put (why?) the
exclusiveness of rate.

> +	}
> +
> +	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);
> +
> +	clk_rate_exclusive_put(d->clk);
> +}

-- 
With Best Regards,
Andy Shevchenko