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

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Date:	Fri, 23 Oct 2015 20:15:33 +0900
From:	Masahiro Yamada <yamada.masahiro@...ionext.com>
To:	Rob Herring <robh@...nel.org>
Cc:	Peter Hurley <peter@...leysoftware.com>,
	Stefan Agner <stefan@...er.ch>,
	"linux-serial@...r.kernel.org" <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

2015-10-23 0:34 GMT+09:00 Rob Herring <robh@...nel.org>:
> On Tue, Oct 20, 2015 at 8:20 PM, Masahiro Yamada
> <yamada.masahiro@...ionext.com> wrote:
>> 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.
>
> This problem is now moving into u-boot which is using DT for configuration.
>
>> 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.)
>
> I'm missing context of what you did, but it needs to be parse-able
> from a flattened tree. My suggestion would be use clock-frequency
> property in the uart node. We should be able to parse that. This came
> up with u-boot as well[1].

Right.

But, I think things have been going wrong
since SPL started to parse a device tree.




-- 
Best Regards
Masahiro Yamada
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