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

[Jani Nikula]

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.

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Technology Center