Message-ID: <256cf892-cf3f-155e-05d7-eaed7728000a@wwwdotorg.org>
Date:   Wed, 31 May 2017 10:47:50 -0600
From:   Stephen Warren <swarren@...dotorg.org>
To:     Phil Elwell <phil@...pberrypi.org>,
        Stefan Wahren <stefan.wahren@...e.com>
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

On 05/31/2017 10:28 AM, Phil Elwell wrote:
> On 31/05/2017 16:58, Stefan Wahren wrote:
>> 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
> 
> Thank you, Stephen and Stephan. Stephen - the clock scaling is applied by a DT overlay so
> it effectively a runtime setting, but I take your point about the elegance of the solution.
> Stefan - anybaud looks promising, although I would have preferred for users to continue to
> use the existing user-space tools - kernel changes can be deployed more easily.
> 
> For my edification, can you pretend for a moment that the application was a valid one and
> answer any of my original questions?:
> 
> 1. Should all system clock drivers use OF_CLK_DECLARE? Doing so would probably
> avoid this problem, but further initialisation order dependencies may
> require more drivers to be initialised early.
> 
> 2. Why does the clock initialisation hook registered by OF_CLK_DECLARE not
> return any indication of success? If it did, and if the OF_POPULATED flag
> was only set after successful initialisation then the normal retrying of
> a deferred probe would also avoid this problem.
> 
> 3. Would adding the OF_CLK_DECLARE hook prevent the use of the devm_ managed
> functions like devm_kzalloc? If not, why doesn't fixed-factor-clock use it?

Sorry, I don't know the answers to these questions; I expect the clock 
subsystem maintainers will have to chime in. My only general comment is 
that probe deferral is the typical mechanism to handle 
driver/device/object dependencies, but I have no idea how that would 
interact with static initialization hooks like OF_CLK_DECLARE.

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