linux-kernel - Re: [PATCH v2] earlycon: Allow specifying a uartclk in options

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Date:   Sat, 3 Mar 2018 17:38:43 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Aaron Durbin <adurbin@...omium.org>
Cc:     Daniel Kurtz <djkurtz@...omium.org>,
        Brian Norris <briannorris@...omium.org>,
        Jonathan Corbet <corbet@....net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoffer Dall <cdall@...aro.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Frederic Weisbecker <frederic@...nel.org>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Tom Saeger <tom.saeger@...cle.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>,
        Linux Documentation List <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>
Subject: Re: [PATCH v2] earlycon: Allow specifying a uartclk in options

On Thu, Mar 1, 2018 at 11:24 PM, Aaron Durbin <adurbin@...omium.org> wrote:
> On Thu, Mar 1, 2018 at 1:02 PM, Andy Shevchenko
> <andy.shevchenko@...il.com> wrote:
>> On Thu, Mar 1, 2018 at 9:22 PM, Daniel Kurtz <djkurtz@...omium.org> wrote:
>>> On Thu, Mar 1, 2018 at 11:47 AM Andy Shevchenko <andy.shevchenko@...il.com>
>>> wrote:
>>
>>> "earlycon simply does not utilize the information".
>>>
>>> earlycon parses iotype, mapbase and baud (from options).  However, it is
>>> hard-coded to assume that the clock used to generate the UART bitclock is
>>> always "BASE_BAUD * 16" (1843200).  While this may be true for many UARTs,
>>> it isn't true for AMD's CZ/ST which has a 8250_dw and uses a fixed 48 MHz
>>> clock.  The main 8250_dw driver uses devm_clk_get to get the "baudclk" and
>>> uses its rate to initialize uartclk.  For AMD CZ/ST, this "baudclk" is
>>> actually a set up in acpi_apd.c when there is an acpi match for "AMD0020",
>>> with a rate read from the .fixed_clk_rate param of the corresponding
>>> apd_device_desc.
>>>
>>> This patch attempts to add a way to inform earlycon about this clock.  As
>>> noted above, the information is actually already in the kernel and used by
>>> 8250_dw - I would happy be to hear recommendations for wiring this data
>>> into earlycon that doesn't require adding another command line arg.
>>
>> And it should not require that for sure!
>
> But it does require that. There's an input clock to the uart ip block.
> That is a design constraint by the hardware and is required to make
> baud calculation work.

I mean it should not be user's headache to provide this information to
the system.

> It's not a firmware problem.

If it's ACPI, then it's definitely firmware issue, since SPCR provides
a baudrate.

> Its the driver's problem in that it
> assumes an input clock to the uart block that does not reflect
> reality.

So, driver can't get this info from device tree or what?

>> Okay, configures a necessary IPs to feed UART with expected 1.8432M clock.
>
> That's only possible if there is a clock divider on the front end of
> the uart block. For this hardware that's not the case. I actually did
> this very thing on intel chromebook devices, but it was only possible
> because there was a hardware divider that could be tuned to reach the
> assumed clock that the code currently assumes.

OK.

-- 
With Best Regards,
Andy Shevchenko