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

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Date:   Wed, 29 Aug 2018 08:54:37 -0400
From:   Prarit Bhargava <prarit@...hat.com>
To:     Petr Mladek <pmladek@...e.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, 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



On 08/17/2018 08:57 AM, Petr Mladek wrote:
> On Fri 2018-08-17 07:06:56, Prarit Bhargava wrote:
>> On 08/17/2018 06:50 AM, Sergey Senozhatsky wrote:
>>>> 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.
>>
>> Thanks, I'm a user of earlycon too, but only moderately.
>>
>> peterz, to give you an overview:  Currently on x86, the SPCR information is only
>> interpreted by the early console code, and can be enabled with kernel parameter
>> "earlycon" (no arguments).  In this patch I'm proposing adding "console=auto"
>> that would enable the console based on the SPCR data.
>>
>> There are two options on the table.  One, we could go with this patch which
>> would make users do "earlycon console=auto" or, I could just change the
>> behaviour of earlycon (no arguments) to also bring up the console.  In the
>> second case the kernel parameter would just be "earlycon".  There is precedent
>> for the second option as arm64 enables both the earlycon and console by default
>> if SPCR is present.  However, on x86, I know many users do not want the console
>> enabled by default so we should keep it on demand.
>>
>> tl;dr: Pick one
>>
>> Option 1: earlycon enables both the early console and console.
> 
> I am afraid that this is not acceptable. Users are sensitive
> when a new kernel suddenly enables different consoles. For example,
> see:
> 
> + commit dac8bbbae1d0ccba9 ("Revert "printk: fix double printing with
>   earlycon")
> 
> + commit c6c7d83b9c9e6a8b3 ("Revert "console: don't prefer first
>   registered if DT specifies stdout-path")
> 
> 
> By other words, the new behavior must depend on a new option.
> 
> 
>> Option 2: earlycon only enables the early console, and console=auto enables the
>> console.
> 
> I suggest:
> 
> Option 3: "earlycon" enables early console defined by SPCR
> 	  "console" enables console defined by SPCR
> 
> I mean that "console" without extra options would enable the console
> defined by ACPI SPCR. It would work the same as "earlycon" without
> extra options for early consoles.
> 
> If you would want to make it explicit than I agree with Sergey
> and would prefer "console=spcr" instead of "console=auto".

I'm going with "console=spcr".  FYI.

P.

> 
> Note kernel tries to enable some consoles automatically even
> when "console" parameter is not defined at all. Therefore "auto"
> is somehow misleading.
> 
> Best Regards,
> Petr
> 
> PS: JFYI, I have vacation the following week...
>