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

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Date:   Wed, 16 May 2018 12:08:42 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, 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 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. 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.
In order to achieve the same thing you would have to disable the kernel
console on that UART. I leave this to Tony.

Sebastian