[PATCH v7 1/3] backlight: Add IS_ENABLED(CONFIG_BACKLIGHT_CLASS_DEVICE)

[Daniel Vetter]

On Mon, Oct 02, 2017 at 12:00:54PM +0300, Jani Nikula wrote:
> On Mon, 02 Oct 2017, Daniel Thompson <daniel.thompson at linaro.org> wrote:
> > So I'm coming to this patchset cold but can you explain *why* something 
> > wants to call of_find_backlight_by_node() when there is no backlight 
> > support enabled. Why isn't the code that called is conditional on 
> > BACKLIGHT_CLASS_DEVICE?
> >
> > The undefined symbol issue is a pain but to be honest I'd rather solve 
> > the use of undefined symbols by avoiding declaring them; this making 
> > them into compile errors rather than link errors.
> 
> Typically the kernel header files define static inline stubs of the
> functions when the actual functions aren't configured/built. The code
> using the functions looks the same regardless of the config option, and
> handles the -ENODEV or NULL or whatever returns from the stubs
> gracefully. With the inlines, the compiler can usually throw out much of
> the code anyway.
> 
> In this regard, the backlight interface is an exception, forcing the
> callers to wrap the code around IS_ENABLED(BACKLIGHT_CLASS_DEVICE), not
> the rule.

fwiw, I think it'd be great if we can move backlight over to the common
pattern, like everyone else. It's pretty big pain as-is ...
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch