Subject: [PATCH 2/3] ACPI: parse SPCR and enable matching console

From: [email-masked] (Aleksey Makarov)
Date: Wed, 27 Jan 2016 16:57:34 +0300

On 01/25/2016 07:32 PM, Peter Hurley wrote:
> On 01/25/2016 03:45 AM, Aleksey Makarov wrote:
>> 'ARM Server Base Boot Requiremets' [1] mention SPCR (Serial Port
>> Console Redirection Table) [2] as a mandatory ACPI table that
>> specifies the configuration of serial console.
>> 
>> Parse this table and check if any registered console match the
>> description.  If it does, enable that console.
>> 
>> To implement that, introduce a new member int (*acpi_match)(struct
>> console *, struct acpi_table_spcr *) of struct console.  It allows
>> drivers to check if they provide a matching console device.
> 
> Many, many platform proms with all sorts of binary table layout are
> already supported by the existing console infrastructure. Why is ACPI
> different, that requires extensive (and messy) changes to console
> initialization?

Without this patch, when linux calls register_console(), that function
checks if any console has been enabled so far. 1) If not, it enables the
console being registered. 2) If there exists any enabled console, it
looks at the console_cmdline array. That array holds a list of
consoles that user wishes to enable. There are two ways to append
an item to that list: first is to pass "console=..." option in command
line and second is to call add_preferred_console(char *name, int idx,
char *options). As it is clear from the signature, the function
requires the name of the driver (like "ttyS") and the line id. On the
other hand, the SPCR ACPI table describes console by specifying the
address of it's registers or PCI Device ID / PCI Vendor ID or PCI Bus
Number / PCI Device Number. So to use this function we would need to
have a method to translate this info to the name of terminal and line
index. I could not figure out any way to do that.

In the initial version of the patch after getting the reference to the
SPCR ACPI table the full tree of ACPI devices was searched to find any
device with the same address.  When uart_add_one_port() was called
to register a new serial port, the ACPI companion of this port was
compared to the found device. If it was the same device, the code
called add_preferred_console() (the terminal name and line index are
known in uart_add_one_port()).

This original approach had two problems: 

    1) It works with the SPCR tables that describe consoles only by
the address of the registers. I do not think that consoles that are
described by PCI info will appear in the near future, but decided to
implement this in a generic way. I would like to discuss if this
decision was good.

    2) Wrong order of initialization.  Many console drivers have already
been registered by the time uart_add_one_port() adds an item to the
console_cmdline array.  There is a similar problem with my
implementation,  but having a dedicated acpi_match() callback I
believe made it simpler to circumwent.

That's why I believe we need to add a new funcion pointer to struct
console. On the other hand, I do not understand which existing
structure you are referring.

> How is this going to work with earlycon?

If an earlycon that matches SPCR is being registered, the code will enable it.
While it is harmless. Even so I will check for earlycon in the next version
of the patch set, thank you.

> This commit log is missing the reasoning behind adding locks,
> refactoring into delete_from_console_list(), and retry loops.

I will add this to the next verion of the series.

Thank you
Aleksey Makarov

>> [1]
>> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.den0044a/index.html
>>
>> [2] http://msdn.microsoft.com/en-us/library/windows/hardware/dn639131(v=vs.85).aspx
>> 
>> Signed-off-by: Aleksey Makarov <[email-masked]>

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