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

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Date:   Fri, 17 Aug 2018 06:26:38 -0400
From:   Prarit Bhargava <prarit@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     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
Subject: Re: [PATCH v2] console: Add console=auto option



On 08/17/2018 04:19 AM, Petr Mladek wrote:
> On Thu 2018-08-16 13:39:01, Prarit Bhargava wrote:
>> ACPI may contain an SPCR table that defines a default system console.
>> On ARM if the table is present then the SPCR console is enabled by default.
>> On x86 the SPCR console is used if 'earlycon' (no parameters) is
>> specified as a kernel parameter and is used only as the early console.
>> To use the SPCR data as a console a user must boot with 'earlycon',
>> grep logs & specify a console= kernel parameter, and then reboot again.
>>
>> Add 'console=auto' that enables a firmware or hardware console, and on
>> x86 enable the SPCR console if 'console=auto' is specified.
> 
> I basically like the idea. Just one or two nits below.
> 
> 
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index a32f2a126791..dd057224f33b 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -635,6 +635,7 @@
>>  
>>  		hvc<n>	Use the hypervisor console device <n>. This is for
>>  			both Xen and PowerPC hypervisors.
>> +		auto	[X86] Enable ACPI SPCR console
> 
> The "auto" option sounds reasonable. But earlycon does exactly this
> when used without no extra options. I prefer to stay consistent
> with the existing earlycon behavior.

Hi Petr, on x86 earlycon still only enables the early console but does not
enable the console.

> 
> 
>>  		If the device connected to the port is not a TTY but a braille
>>  		device, prepend "brl," before the device type, for instance
>> diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
>> index 3b20607d581b..fb2616ba3b21 100644
>> --- a/arch/x86/kernel/acpi/boot.c
>> +++ b/arch/x86/kernel/acpi/boot.c
>> @@ -1771,3 +1771,8 @@ void __init arch_reserve_mem_area(acpi_physical_address addr, size_t size)
>>  	e820__range_add(addr, size, E820_TYPE_ACPI);
>>  	e820__update_table_print();
>>  }
>> +
>> +void __init arch_console_setup(void)
>> +{
>> +	acpi_parse_spcr(false, true);
> 
> Just for record. I was curious that this might be called twice
> (also from acpi_boot_init(). But it looks safe after all.
> 

Yes, the console code prevents the same console from being registered twice.

> The trick is in the two bool parameters. One call allows to
> register/enable earlycon and ignores normal console. This other
> call does exactly the opposite. I do not see any unwanted side
> effects.

Thanks.  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.

P.

> 
> Best Regards,
> Petr
>