CLK_OF_DECLARE advice required

Stephen Warren swarren at
Wed May 31 09:47:50 PDT 2017

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.:
>>> "How to set a custom baud rate on Linux?"
>>> "Re: Terminal interface and non-standard baudrates"
>> I remember this gist from Peter Hurley:
> 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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