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

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Date:	Thu, 22 Oct 2015 00:31:05 +0900
From:	Masahiro Yamada <yamada.masahiro@...ionext.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Rob Herring <robh@...nel.org>, Stefan Agner <stefan@...er.ch>,
	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.


2015-10-21 21:27 GMT+09:00 Peter Hurley <peter@...leysoftware.com>:
> On 10/20/2015 09:20 PM, Masahiro Yamada 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.
>>
>>
>> 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.
>
> The 8250 earlycon doesn't try to initialize the hardware (other than
> masking interrupts) if the baud rate is uninitialized
> (!device->baud in early_serial8250_setup()).
>

Right.

If it is always correct to preserve the initialization done by boot-loader,
the following code in 8250_early.c does not make sense.

Delete?


        divisor = DIV_ROUND_CLOSEST(port->uartclk, 16 * device->baud);
        c = serial8250_early_in(port, UART_LCR);
        serial8250_early_out(port, UART_LCR, c | UART_LCR_DLAB);
        serial8250_early_out(port, UART_DLL, divisor & 0xff);
        serial8250_early_out(port, UART_DLM, (divisor >> 8) & 0xff);
        serial8250_early_out(port, UART_LCR, c & ~UART_LCR_DLAB);








-- 
Best Regards
Masahiro Yamada
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