Subject: [PATCH v4 0/4] ACPI: parse the SPCR table

From: [email-masked] (Aleksey Makarov)
Date: Fri, 4 Mar 2016 14:53:15 +0300

On 03/03/2016 06:35 PM, Peter Hurley wrote:
> On 03/03/2016 03:59 AM, Aleksey Makarov wrote:
>>
>>
>> On 03/01/2016 06:31 PM, Peter Hurley wrote:
>>> On 02/29/2016 04:02 AM, Aleksey Makarov wrote:
>>>> 'ARM Server Base Boot Requirements' [1] mentions SPCR (Serial Port
>>>> Console Redirection Table) [2] as a mandatory ACPI table that
>>>> specifies the configuration of serial console.
>>>>
>>>> Introduce a new function acpi_console_check().  At the uart port
>>>> registration, this function checks if the ACPI SPCR table specifies
>>>> its argument of type struct uart_port to be a console
>>>> and if so calls add_preferred_console().
>>>
>>> How will a user enable an earlycon on the same console as the SPCR
>>> console if there is no DBG2 table?
>>
>> ...
>> [    0.000000] earlycon: pl11 at MMIO 0x0000000009000000 (options '')
>> [    0.000000] bootconsole [pl11] enabled
>> ...
>> [    0.000000] Kernel command line:  root=/dev/vda1 rw systemd.show_status=no acpi=force earlycon=pl011,0x9000000
>> ...
>> [    0.318248] ACPI: SPCR: adding preferred console [ttyAMA0]
>> [    0.318736] ARMH0011:00: ttyAMA0 at MMIO 0x9000000 (irq = 5, base_baud = 0) is a SBSA
>> [    0.319502] console [ttyAMA0] enabled
>> [    0.319502] console [ttyAMA0] enabled
>> [    0.319933] bootconsole [pl11] disabled
>> [    0.319933] bootconsole [pl11] disabled
>> ...
>>
>> Why?
> 
> That's pretty disingenuous; via command line?
> 
> By that measure, none of your patches are required because a user
> can already start both console and earlycon without them.
> 
> With the console location specified in the SPCR, earlycon should
> be opt-in on the command-line simply with "earlycon" command-line
> parameter.

Yes.  That's why we have SPCR *and* DBG2. 
DBG2 specifies where we should run earlycon.

>>> How will a user enable an earlycon on the same console as the SPCR
>>> console if there is no DBG2 table?

In no way.  You need DBG2 to run earlycon.

(If you don't want to specify it's address etc explicitly)