linux-kernel - Re: [PATCH v3 0/4] serial: 8250

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Message-ID: <20200507182906.GC185537@smile.fi.intel.com>
Date:   Thu, 7 May 2020 21:29:06 +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>,
        Maxim Kaurkin <Maxim.Kaurkin@...kalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>,
        Alexey Kolotnikov <Alexey.Kolotnikov@...kalelectronics.ru>,
        Ramil Zaripov <Ramil.Zaripov@...kalelectronics.ru>,
        Ekaterina Skachko <Ekaterina.Skachko@...kalelectronics.ru>,
        Vadim Vlasov <V.Vlasov@...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, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/4] serial: 8250_dw: Fix ref clock usage

On Thu, May 07, 2020 at 02:31:31AM +0300, Serge Semin wrote:
> It might be dangerous if an UART port reference clock rate is suddenly
> changed. In particular the 8250 port drivers (and AFAICS most of the tty
> drivers using common clock framework clocks) rely either on the
> exclusive reference clock utilization or on the ref clock rate being
> always constant. Needless to say that it turns out not true and if some
> other service suddenly changes the clock rate behind an UART port driver
> back it's no good. So the port might not only end up with an invalid
> uartclk value saved, but may also experience a distorted output/input
> data since such action will effectively update the programmed baud-clock.
> We discovered such problem on Baikal-T1 SoC where two DW 8250 ports have
> got a shared reference clock. Allwinner SoC is equipped with an UART,
> which clock is derived from the CPU PLL clock source, so the CPU frequency
> change might be propagated down up to the serial port reference clock.
> This patchset provides a way to fix the problem to the 8250 serial port
> controllers and mostly fixes it for the DW 8250-compatible UART. I say
> mostly because due to not having a facility to pause/stop and resume/
> restart on-going transfers we implemented the UART clock rate update
> procedure executed post factum of the actual reference clock rate change.
> 
> In addition the patchset includes a few fixes we discovered when were
> working the issue. First one concerns the maximum baud rate setting used
> to determine a serial port baud based on the current UART port clock rate.
> Another one simplifies the ref clock rate setting procedure a bit.
> 
> This patchset is rebased and tested on the mainline Linux kernel 5.7-rc4:
> 0e698dfa2822 ("Linux 5.7-rc4")
> tag: v5.7-rc4

Thanks!

I will look at them later, but first impression that the first approach narrowed
to the certain SoC (by matching compatible string) looks better solution for
time being.

> Changelog v3:
> - Refactor the original patch to adjust the UART port divisor instead of
>   requesting an exclusive ref clock utilization.
> 
> Signed-off-by: Serge Semin <Sergey.Semin@...kalelectronics.ru>
> Cc: Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>
> Cc: Maxim Kaurkin <Maxim.Kaurkin@...kalelectronics.ru>
> Cc: Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>
> Cc: Alexey Kolotnikov <Alexey.Kolotnikov@...kalelectronics.ru>
> Cc: Ramil Zaripov <Ramil.Zaripov@...kalelectronics.ru>
> Cc: Ekaterina Skachko <Ekaterina.Skachko@...kalelectronics.ru>
> Cc: Vadim Vlasov <V.Vlasov@...kalelectronics.ru>
> Cc: Alexey Kolotnikov <Alexey.Kolotnikov@...kalelectronics.ru>
> Cc: Paul Burton <paulburton@...nel.org>
> Cc: Ralf Baechle <ralf@...ux-mips.org>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Long Cheng <long.cheng@...iatek.com>
> Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> Cc: Maxime Ripard <mripard@...nel.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Russell King <linux@...linux.org.uk>
> Cc: linux-mips@...r.kernel.org
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: linux-mediatek@...ts.infradead.org
> Cc: linux-serial@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> 
> Serge Semin (4):
>   serial: 8250: Fix max baud limit in generic 8250 port
>   serial: 8250: Add 8250 port clock update method
>   serial: 8250_dw: Simplify the ref clock rate setting procedure
>   serial: 8250_dw: Fix common clocks usage race condition
> 
>  drivers/tty/serial/8250/8250_dw.c   | 125 +++++++++++++++++++++++++---
>  drivers/tty/serial/8250/8250_port.c |  42 +++++++++-
>  include/linux/serial_8250.h         |   2 +
>  3 files changed, 156 insertions(+), 13 deletions(-)
> 
> -- 
> 2.25.1
> 

-- 
With Best Regards,
Andy Shevchenko