linux-kernel - Re: [PATCH v2] console: Add console=auto option

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Date:   Fri, 17 Aug 2018 19:50:46 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Prarit Bhargava <prarit@...hat.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org, Mark Salter <msalter@...hat.com>,
        Al Stone <ahs3@...hat.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        x86@...nel.org, Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-pm@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] console: Add console=auto option

Hello,

Cc-ing Peter Zijlstra

  lkml.kernel.org/r/728a8e68-ea4b-4040-a0fc-217df4f1928d@...hat.com
  lkml.kernel.org/r/20180817081947.m425gok2ugt7tglp@...hway.suse.cz
  lkml.kernel.org/r/00c60dca-60bc-8568-eaa3-d4b0c326cab4@...hat.com

On (08/17/18 06:36), Prarit Bhargava wrote:
> On 08/17/2018 05:38 AM, Sergey Senozhatsky wrote:
> > On (08/16/18 13:39), Prarit Bhargava wrote:
> >>
> >> +		auto	[X86] Enable ACPI SPCR console
> > 			^^^^
> > 			And arm64?
> 
> Hi Sergey, on arm64 if an SPCR is present the early console and console are
> initialized by default.  IOW no kernel parameter is necessary to initialize the
> console in that case.

OK, thanks.

> > Any chance we can rename param to "spcr" or something more clear?
> > To explicitly state what exactly it's going to do. `auto' sounds
> > too general and doesn't tell me that much. I'm probably the only
> > here who can't see a connection between "auto" and "SPCR", but
> > still.
> 
> I came up with "auto" because I think it is generic.  I also thought about
> "console=fw", or just "console".  If in the future another arch wants to
> optionally bring up a firmware or hardware defined console then they could use
> auto too.

Hmm, I see your point.
My [sort of a] problem with "auto" is that it tells me as much as "magic"
[and "magic" tells me almost nothing]. By the way, would be fun if we had
"magic" instead of "auto" all over the kernel

	echo "magic" > /sys/bus/usb/...../power/control

> > 	void arch_console_setup(void)
> > 	{
> > 		if (acpi_parse_spcr(false, true))
> > 			pr_err(.........);
> > 	}
> > 
> > There can be other consoles in the system, logging an error is not
> > such a useless thing.
> 
> I can make the second change.  The problem (IIRC) with returning an error in an
> setup fn is that the rest of the setup functions will not execute.  I don't want
> to fail the setup callbacks because of an incorrect SPCR table.

OK, fair enough.
Letting users know that SPCR is incorrect also makes sense, so option #2
I guess is what we want after all.

> Like I mentioned to Petr, I'd like to know if you (or anyone else) has strong
> feelings about changing the behaviour of earlycon on x86?  I could make it so
> that specifying just earlycon would also initialize the console.

x86 people and/or scheduler people might have strong opinions on this.
I Cc-ed Peter Zijlstra; he represents both groups and is known to be
a hardcore earlycon user.

	-ss