linux-kernel - Re: [PATCH v6 3/3] serial: 8250_dw: Fix common clocks usage race condition

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Message-ID: <CAHp75VcoV+aC9H5TYAxQX2O9HLz==xnts9bcKKQBcdtvohpi6g@mail.gmail.com>
Date:   Thu, 18 Jun 2020 11:29:36 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>,
        Tony Lindgren <tony@...mide.com>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Serge Semin <fancer.lancer@...il.com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Arnd Bergmann <arnd@...db.de>,
        Maxime Ripard <mripard@...nel.org>,
        Will Deacon <will@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        linux-mips@...r.kernel.org,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 3/3] serial: 8250_dw: Fix common clocks usage race condition

On Thu, Jun 18, 2020 at 1:50 AM Serge Semin
<Sergey.Semin@...kalelectronics.ru> 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.

So, regarding runtime PM...

> +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);

(Btw, this can be done directly in the definition block above.)

> +       serial8250_update_uartclk(&up->port, rate);

This I think should require a device to be powered on. What in your
opinion is a better place to have it done?
To me it looks like serial8250_update_uartclk() misses it.

> +}

--
With Best Regards,
Andy Shevchenko