CLK_OF_DECLARE advice required

Phil Elwell phil at
Wed May 31 09:28:58 PDT 2017

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?



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