linux-kernel - Re: CLK_OF_DECLARE advice required

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Message-ID: <0794f430-9761-c855-9a89-13d9871c5831@i2se.com>
Date:   Wed, 31 May 2017 17:58:08 +0200
From:   Stefan Wahren <stefan.wahren@...e.com>
To:     Stephen Warren <swarren@...dotorg.org>,
        Phil Elwell <phil@...pberrypi.org>
Cc:     Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-rpi-kernel <linux-rpi-kernel@...ts.infradead.org>,
        linux-clk@...r.kernel.org
Subject: Re: CLK_OF_DECLARE advice required

Am 31.05.2017 um 17:27 schrieb Stephen Warren:
> On 05/30/2017 06:23 AM, Phil Elwell wrote:
>> Hi,
>>
>> I've run into a problem using the fixed-factor clock on Raspberry Pi
>> and I'd
>> like some advice before I submit a patch.
>>
>> Some context: the aim is to use a standard UART and some external
>> circuitry
>> as a MIDI interface. This would be straightforward except that Linux
>> doesn't
>> recognise the required 31.25KHz as a valid UART baud rate. Rhe
>> workaround is
>> to declare the UART clock such that the reported rate differs from
>> the actual
>> rate. If one sets the reported rate to be (actual*38400)/31250 then
>> requesting a 38400 baud rate will result in an actual 31250 baud signal.
>
> This sounds like the wrong approach. Forcing the port to use a
> different clock rate than the user requests would prevent anyone from
> using that port for its standard purpose; it'd turn what should be a
> runtime decision into a compile-time decision.
>
> Are you sure there's no way to simply select the correct baud rate on
> the port? I see plenty of references to configuring custom baud rates
> under Linux when I search Google, e.g.:
>
>> https://stackoverflow.com/questions/12646324/how-to-set-a-custom-baud-rate-on-linux
>>
> "How to set a custom baud rate on Linux?"
>
>> https://sourceware.org/ml/libc-help/2009-06/msg00016.html
> "Re: Terminal interface and non-standard baudrates"
>

I remember this gist from Peter Hurley:

https://gist.github.com/peterhurley/fbace59b55d87306a5b8