linux-kernel - Re: [PATCH v3 4/4] serial: 8250_dw: Fix common clocks usage race condition

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Date:   Fri, 15 May 2020 17:10:46 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Serge Semin <fancer.lancer@...il.com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Paul Burton <paulburton@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Arnd Bergmann <arnd@...db.de>,
        Long Cheng <long.cheng@...iatek.com>,
        Maxime Ripard <mripard@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        linux-mips@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/4] serial: 8250_dw: Fix common clocks usage race
 condition

On Thu, May 07, 2020 at 02:31:35AM +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.

I'm wondering if clock framework itself can provide such a notifier?

> 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.

So, and how you propose to update the clock when ->set_termios() is called?

> 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.

...

> +	struct notifier_block	clk_notifier;
> +	struct work_struct	clk_work;

Oh, this seems too much.
Perhaps, the compatible based quirk with your initial approach is much better for time being.

-- 
With Best Regards,
Andy Shevchenko