linux-kernel - Re: [PATCH v2 1/2] drm/input_helper: Add new input-handling helper

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Message-ID: <YZ+q0ZlIRoq4nZMl@phenom.ffwll.local>
Date:   Thu, 25 Nov 2021 16:25:05 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Brian Norris <briannorris@...omium.org>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        "Kristian H . Kristensen" <hoegsberg@...gle.com>,
        linux-kernel@...r.kernel.org, linux-rockchip@...ts.infradead.org,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Doug Anderson <dianders@...omium.org>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Rob Clark <robdclark@...il.com>, linux-input@...r.kernel.org,
        Rob Clark <robdclark@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2 1/2] drm/input_helper: Add new input-handling helper

On Fri, Nov 19, 2021 at 11:07:16AM -0800, Brian Norris wrote:
> Hi Daniel,
> 
> On Fri, Nov 19, 2021 at 11:01:18AM +0100, Daniel Vetter wrote:
> > On Thu, Nov 18, 2021 at 11:30:43AM -0800, Brian Norris wrote:
> > > On Thu, Nov 18, 2021 at 10:05:11AM +0100, Daniel Vetter wrote:
> > > > On Wed, Nov 17, 2021 at 02:48:40PM -0800, Brian Norris wrote:
> > > > > --- a/drivers/gpu/drm/Kconfig
> > > > > +++ b/drivers/gpu/drm/Kconfig
> > > > > @@ -79,9 +79,15 @@ config DRM_DEBUG_SELFTEST
> > > > >  
> > > > >  	  If in doubt, say "N".
> > > > >  
> > > > > +config DRM_INPUT_HELPER
> > > > > +	def_bool y
> > > > > +	depends on DRM_KMS_HELPER
> > > > > +	depends on INPUT
> > > > 
> > > > Uh please no configs for each thing, it just makes everything more
> > > > complex. Do we _really_ need this?
> > > 
> > > First, it's not a configurable option (a user will never see this nor
> > > have to answer Y/N to it); it only serves as an intermediary to express
> > > the CONFIG_INPUT dependency (which is necessary) without making
> > > DRM_KMS_HELPER fully depend on CONFIG_INPUT. (We should be able to run
> > > display stacks without the input subsystem.)
> > 
> > I'm not so much worried about the user cost, but the maintenance cost.
> > Kbuild config complexity is ridiculous, anything that adds even a bit is
> > really silly.
> > 
> > > The closest alternative I can think of with fewer Kconfig symbols is to
> > > just use CONFIG_INPUT directly in the code, to decide whether to provide
> > > the helpers or else just stub them out. But that has a problem of not
> > > properly expressing the =m vs. =y necessity: if, for example,
> > > CONFIG_DRM_KMS_HELPER=y and CONFIG_INPUT=m, then we'll have linker
> > > issues.
> > 
> > Usually this is done by providing static inline dummy implementations in
> > the headers. That avoids having to sprinkle new Kconfig symbols all over.
> 
> Right, I already did that, and I'm not sprinkling
> CONFIG_DRM_INPUT_HELPER much. (I do include one around the module
> parameter, because it doesn't make much sense to have the module
> parameter even exist, if the underlying feature is stubbed out.)
> 
> But that doesn't solve the problem in my last sentence, involving
> tristates. The "stub inline" approach only works well for boolean
> features -- either built-in, or disabled. Once your feature is in a
> module, you need to ensure that no built-in code depends on it.
> 
> Do you want DRM_KMS_HELPER to unconditionally depend on CONFIG_INPUT? If
> so, I can just add a 'select' or 'depend' and drop this intermediate
> symbol.
> If not, then what do you expect to happen with DRM_KMS_HELPER=y and
> CONFIG_INPUT=m?

Yeah just add the dependency. If you still want to keep it optional the
way to do it is to add

	depends on FOO || FOO=n

And then just have #if IS_ENABLED(FOO) around your inline wrappers.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch