linux-kernel - Re: [PATCH] earlycon: Allow specifying a uartclk in options

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Message-ID: <CAHp75VcZSkQHFgfAgePQsfsYOZOZxsgz3-+W7X42Gw2hrxODEg@mail.gmail.com>
Date:   Thu, 1 Mar 2018 20:46:34 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Daniel Kurtz <djkurtz@...omium.org>
Cc:     adurbin@...omium.org, Brian Norris <briannorris@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] earlycon: Allow specifying a uartclk in options

On Thu, Mar 1, 2018 at 8:20 PM, Daniel Kurtz <djkurtz@...omium.org> wrote:
> Currently when an earlycon is registered, the uartclk is assumed to be
> BASE_BAUD * 16 = 1843200.  If a baud rate is specified in the earlycon
> options, then 8250_early's init_port will program the UART clock divider
> registers based on this assumed uartclk.
>
> However, not all uarts have a UART clock of 1843200.  For example, the
> 8250_dw uart in AMD's CZ/ST uses a fixed 48 MHz clock (as specified in
> cz_uart_desc in acpi_apd.c).  Thus, specifying a baud when using earlycon
> on such a device will result in incorrect divider values and a wrong UART
> clock.
>
> Fix this by extending the earlycon options parameter to allow specification
> of a uartclk, like so:
>
>  earlycon=uart,mmio32,0xfedc6000,115200,48000000
>
> If none is specified, fall-back to prior behavior - 1843200.

It needs to be discussed.

First of all, if you are going to do this you need to add a parse of
human readable formats (IIRC kernel has helpers), i.e. "48M", "38.4M"
and so on.

Next, I was under impression that purpose of earlycon (in difference
to earlyprintk) is to re-use existing drivers as fully as possible.

So, what exactly happens in your case? Are your driver lacks of
properly set clock? Or earlycon does simple not utilizing this
information?

-- 
With Best Regards,
Andy Shevchenko