linux-kernel - Re: [PATCH v1 0/3] console, serial8250: Disable PM and DMA ops

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Date:   Wed, 23 May 2018 00:52:01 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Tony Lindgren <tony@...mide.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v1 0/3] console, serial8250: Disable PM and DMA ops

On Thu, May 17, 2018 at 10:48 PM, Tony Lindgren <tony@...mide.com> wrote:
> * Andy Shevchenko <andy.shevchenko@...il.com> [180517 16:38]:
>> On Thu, May 17, 2018 at 4:48 PM, Tony Lindgren <tony@...mide.com> wrote:
>> > * Sebastian Andrzej Siewior <bigeasy@...utronix.de> [180516 10:49]:

>> > The idea breaking PM seems silly to me considering that we've had
>> > it working for years already.
>>
>> Same question  / note. World is much more complex than just being OMAP.
>
> Sorry but you are making assumptions about hardware being powered on
> all the time.

Nope, other way around. The so called "support" _prevents_ our
hardware to go to sleep.

So it's not breaking, it's fixing. It's pity that OMAP solution is just a hack.

> It may have been a safe assumption up to mid-90's and might still be
> valid assumption for hardware providing on MS-DOS boot floppy
> compability.
>
> But that is not a safe assumption for Linux kernel at all. Especially
> for console TX where there's no need to keep it powered unless there
> is actually something to write.
>
> If there are runtime PM issues for consoles, let's just fix them.

Yes, there is an issue to begin with, i.e. irq_safe hack.
Removal of that hack reveals the real issues with the code.

> Then for really seeing console messages on next reboot from a hung
> system, CONFIG_PSTORE_CONSOLE option seems to do a very good job.

How this helps to prevent a more serious system crashes during an
attempt to power UART on?

> With a warm reset after triggered by watchdog the console messages
> are readable most of the time even when using DDR as the back end.

Ditto.

-- 
With Best Regards,
Andy Shevchenko