linux-kernel - Re: [PATCH v3) posix-timers: make it configurable

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Date:   Sun, 18 Sep 2016 12:54:58 -0400 (EDT)
From:   Nicolas Pitre <nicolas.pitre@...aro.org>
To:     Richard Cochran <richardcochran@...il.com>
cc:     Josh Triplett <josh@...htriplett.org>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3) posix-timers: make it configurable

On Sun, 18 Sep 2016, Richard Cochran wrote:

> On Fri, Sep 16, 2016 at 10:57:58PM -0400, Nicolas Pitre wrote:
> > Subject: [PATCH] ptp_clock: allow for it to be optional
> > 
> > In order to break the hard dependency between the PTP clock subsystem and
> > ethernet drivers capable of being clock providers, this patch provides
> > simple PTP stub functions to allow linkage of those drivers into the
> > kernel even when the PTP subsystem is configured out.
> > 
> > And to make it possible for PTP to be configured out, the select statement
> > in the Kconfig entry for those ethernet drivers is changed from selecting
> > PTP_1588_CLOCK to PTP_1588_CLOCK_DEFAULT whose purpose is to indicate the
> > default Kconfig value for the PTP subsystem.
> > 
> > This way the PTP subsystem may have Kconfig dependencies of its own, such
> > as POSIX_TIMERS, without making those ethernet drivers unavailable if
> > POSIX timers are cconfigured out. And when support for POSIX timers is
> > selected again then PTP clock support will also be selected accordingly.
> > 
> > Drivers must be ready to accept NULL from ptp_clock_register().
> > The pch_gbe driver is a bit special as it relies on extra code in
> > drivers/ptp/ptp_pch.c. Therefore we let the make process descend into
> > drivers/ptp/ even if PTP_1588_CLOCK is unselected.
> 
> Thanks for the detailed description.
> 
> > diff --git a/drivers/ptp/Kconfig b/drivers/ptp/Kconfig
> > index 00e6098e9a..c36c018f0f 100644
> > --- a/drivers/ptp/Kconfig
> > +++ b/drivers/ptp/Kconfig
> > @@ -4,8 +4,12 @@
> >  
> >  menu "PTP clock support"
> >  
> > +config PTP_1588_CLOCK_DEFAULT
> > +	tristate
> 
> I see what this option is doing, but I wonder about the name
> "DEFAULT".  In what sense is this a default?

It provides the default answer for when the PTP_1588_CLOCK config option 
is available.  I could have called it HAVE_PTP_1588_CLOCK or 
PTP_1588_CLOCK_WANTED but none of the alternatives struck me as any 
better.  Any idea?

> > +/**
> > + * ptp_clock_register() - register a PTP hardware clock driver
> > + *
> > + * @info:   Structure describing the new clock.
> > + * @parent: Pointer to the parent device of the new clock.
> 
> Here we should finally explain the return value for authors of new
> drivers.  Something like this:
> 
>  * Returns a valid pointer on success or PTR_ERR on failure.  If PHC
>  * support is missing at the configuration level, this function
>  * returns NULL, and drivers are expected to gracefully handle that
>  * case separately.

Good point.


Nicolas