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

From: Andy Shevchenko <andy.shevchenko@gmail.com>
To: Tony Lindgren <tony@atomide.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jslaby@suse.com>,
	"open list:SERIAL DRIVERS" <linux-serial@vger.kernel.org>,
	Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH v1 0/3] console, serial8250: Disable PM and DMA ops
Date: Wed, 23 May 2018 00:52:01 +0300	[thread overview]
Message-ID: <CAHp75Vf7L3zsSgaj1vfoyNKdPo0w6Nu+mW5HuM97SKAK-1y2Pg@mail.gmail.com> (raw)
In-Reply-To: <20180517194834.GW25808@atomide.com>

On Thu, May 17, 2018 at 10:48 PM, Tony Lindgren <tony@atomide.com> wrote:
> * Andy Shevchenko <andy.shevchenko@gmail.com> [180517 16:38]:
>> On Thu, May 17, 2018 at 4:48 PM, Tony Lindgren <tony@atomide.com> wrote:
>> > * Sebastian Andrzej Siewior <bigeasy@linutronix.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