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

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Date:   Wed, 16 May 2018 13:17:36 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     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>, Tony Lindgren <tony@...mide.com>
Subject: Re: [PATCH v1 0/3] console, serial8250: Disable PM and DMA ops

On Wed, May 16, 2018 at 1:08 PM, Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
> On 2018-05-15 21:37:18 [+0300], Andy Shevchenko wrote:
>> On Tue, 2018-05-15 at 21:34 +0300, Andy Shevchenko wrote:
>> > Kernel console is sensitive to any kind of complex work needed to
>> > print
>> > out anything on it. One such case is emergency print during Oops.
>> >
>> > This series proposes to disable runtime PM and DMA operations on 8250
>> > serial console.
>> >
>> > More detailed explanation why is provided in patch 2.
>> >
>> > The series has been in our internal trees for years already with no
>> > problems observed.
>>
>> +Cc: Tony.
>>
>> You might have some thoughts / test means for this.
>
> I haven't look at the patches, just your cover letter. Disabling DMA on
> kernel-console should be fine.

Would you mind to give an Ack for it?

> The output is usually short so there
> shouldn't be much benefit from using it.

> I remember Tony wanted runtime-pm on the kernel console, too. And he
> told me explicit how to test it so that it works. Once the UART goes
> into PM (down), the whole IP block can go into power save mode. The
> board can be woken up by sending a character via the UART. The first few
> (incoming / read) characters are lost until the IP block is up again the
> frequency stable. This is known / expected.

Don't consider world the OMAP only. The things more complicated if we
go out of it. Which I tried to explain in the commit message of patch
2.

> In order to achieve the same thing you would have to disable the kernel
> console on that UART. I leave this to Tony.

Precisely the point of the series.


-- 
With Best Regards,
Andy Shevchenko