linux-kernel - Re: [PATCH v2 2/2] serial: earlycon: allow to specify uartclk in earlycon kernel-parameter

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Date:	Wed, 21 Oct 2015 10:20:51 +0900
From:	Masahiro Yamada <yamada.masahiro@...ionext.com>
To:	Peter Hurley <peter@...leysoftware.com>,
	Rob Herring <robh@...nel.org>, Stefan Agner <stefan@...er.ch>
Cc:	linux-serial@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jslaby@...e.com>
Subject: Re: [PATCH v2 2/2] serial: earlycon: allow to specify uartclk in
 earlycon kernel-parameter

Hi Peter,
(+ Rob Herring, Stefan Agner)

2015-10-20 23:00 GMT+09:00 Peter Hurley <peter@...leysoftware.com>:
> On 10/19/2015 11:36 PM, Masahiro Yamada wrote:
>> The input clock frequency varies from device to device, but the
>> earlycon uses the fixed frequency (BASE_BAUD * 16).  It makes
>> impossible to set the correct divisor to the register.
>
> So the bootloader hasn't setup the serial port?

It does.
I use U-boot and the serial port is already set up by U-boot.

But, earlycon setup functions update hardware registers.
See  early_serial8250_setup(), ingenic_early_console_setup(), etc.

Without port->uartclk set to a valid value,
the init code in earlycon setup does not make sense.

What I want to clarify is,
what should we do in the earlycon setup function?

Currently, I see
 [1] set device->con->write callback
 [2] initialize UART port registers

For [2], we need to know baudrate and input clock frequency.
(and the latter is missing, that's why my patch is here.)

In order to be independent of a boot loader, we also need
  [3]  pinctrl (pin-muxing)

But, it is difficult to handle pinctrl in the earlycon framework.

If we depend on a boot loader, [2] is meaningless.   [1] is enough.

-- 
Best Regards
Masahiro Yamada
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