Subject: Re: [PATCH 1/4] kconfig: introduce the "imply" keyword
To: Nicolas Pitre <nicolas.pitre@linaro.org>
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CC: John Stultz <john.stultz@linaro.org>,
        Richard Cochran <richardcochran@gmail.com>,
        Yann E MORIN <yann.morin.1998@free.fr>,
        "Thomas Gleixner" <tglx@linutronix.de>,
        Josh Triplett <josh@joshtriplett.org>, <netdev@vger.kernel.org>,
        <linux-kbuild@vger.kernel.org>, <linux-kernel@vger.kernel.org>
From: Edward Cree <ecree@solarflare.com>
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Date: Thu, 20 Oct 2016 18:41:02 +0100
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On 20/10/16 18:04, Nicolas Pitre wrote:
> On Thu, 20 Oct 2016, Edward Cree wrote:
>> Also, I don't think having any FOO=y should preclude BAZ=m.  Suppose both
>> FOO and FOO2 imply BAZ, FOO=y and FOO2=m.
> Some people didn't like the fact that you could turn a driver from m to 
> y and silently lose some features if they were provided by a subsystem 
> that also used to be m, which arguably is not the same as being 
> explicitly disabled.  With "select" this is not a problem as the target 
> symbol is also promoted to y in that case, so I wanted to preserve that 
> property.
Right, but that's an argument for pushing the subsystem's default to y,
not for preventing changing the subsystem back to m afterwards.
>> Then if BAZ-features are only
>> desired for driver FOO2, BAz=m makes sense.
> In that case it would make more sense to add a config option related to 
> FOO asking if BAZ features are desired for that driver (there is already 
> one occurrence of that with PTP).  Or you could simply drop the "imply" 
> statement from the FOO config entry.
But the desire is a property of the user, not of the driver.  If you're
willing to add CONFIG_FOO_BAZ to every combination of (driver, subsystem)
then "imply" becomes unnecessary, doesn't it?  Conversely, if you *don't*
want to have to do that, then "imply" needs to only ever deal in defaults,
not in limitations.
>> There is also the case of drivers with the ability to detect at runtime
>> whether BAZ is present, rather than making the decision at build time, but
>> I'm not sure how common that is.
> Right now that's how PTP support is done.  Drivers can optimize things 
> at build time, but most of them simply cope with a NULL return from 
> ptp_clock_register().  Hence the imply statement becomes a big 
> configuration hint rather than some hard build dependency.
Right, so those drivers can use PTP if they're y and PTP is m, as long as
the PTP module is loaded when they probe.  But current "imply" semantics
won't allow that...

I think that Josh's suggestion (have the UI warn you if you set BAZ to m
while FOO=y) is the right approach, but I also think it should be done
now rather than at some unspecified future time.  Otherwise you forbid
potentially valid configs.

-Ed