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

From: Peter Hurley <peter@hurleysoftware.com>
To: Aleksey Makarov <aleksey.makarov@linaro.org>,
	Aleksey Makarov <amakarov.linux@gmail.com>,
	linux-acpi@vger.kernel.org
Cc: linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Russell King <linux@arm.linux.org.uk>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Leif Lindholm <leif.lindholm@linaro.org>,
	Graeme Gregory <graeme.gregory@linaro.org>,
	Al Stone <ahs3@redhat.com>,
	Christopher Covington <cov@codeaurora.org>,
	Yury Norov <ynorov@caviumnetworks.com>,
	"Zheng, Lv" <lv.zheng@intel.com>
Subject: Re: [PATCH v4 0/4] ACPI: parse the SPCR table
Date: Fri, 4 Mar 2016 07:47:33 -0800	[thread overview]
Message-ID: <56D9AE15.106@hurleysoftware.com> (raw)
In-Reply-To: <56D9772B.5000402@linaro.org>

On 03/04/2016 03:53 AM, Aleksey Makarov wrote:
> 
> 
> 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.

And that's an entirely arbitrary decision being made by you.
Which I think is unnecessarily limited.

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