Re: [PATCH 1/3] drm/omap: displays: panel-dpi: add backlight dependency

[Jani Nikula]

On Wed, 19 Apr 2017, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Wed, Apr 19, 2017 at 10:21 PM, Laurent Pinchart
> <laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:
>>>
>>> This adds a dependency like we have for the other panel drivers.
>>
>> I believe the dependency should be made optional. DPI panels that don't need
>> backlight control should be supported by a kernel that has backlight support
>> compiled out.
>
> That would be nice in principle, but I fear this would cause additional
> problems.
>
>> --- a/include/linux/backlight.h
>> +++ b/include/linux/backlight.h
>> @@ -162,7 +162,7 @@ struct generic_bl_info {
>>         void (*kick_battery)(void);
>>  };
>>
>> -#ifdef CONFIG_OF
>> +#if IS_ENABLED(CONFIG_OF) && IS_ENABLED(CONFIG_BACKLIGHT_CLASS_DEVICE)
>>  struct backlight_device *of_find_backlight_by_node(struct device_node *node);
>>  #else
>>  static inline struct backlight_device *
>>
>>
>> We might need to create stubs for backlight_force_update() and
>> backlight_device_set_brightness() too.
>>
>
> With BACKLIGHT_CLASS_DEVICE=m, you still get a link error when the user is
> in a built-in driver. Using 'depends on' usually solves this (except for drivers
> that cannot be modules).
>
> There are three possible workarounds for this that I can think of:
>
> - Use 'depends on BACKLIGHT_CLASS_DEVICE || BACKLIGHT_CLASS_DEVICE=n'
>   in each driver that implements optional backlight support. We do
> this elsewhere, but
>   it's confusing and easy to get wrong.

FWIW I think this is the fix, and not a workaround.

"depends on FOO || FOO=n" is an expression used throughout the kernel,
and it accurately describes the dependency here. Of course, all drivers
implementing this must still wrap backlight class usage around
IS_ENABLED().

BR,
Jani.

>
> - use IS_REACHABLE() instead of IS_ENABLED() when testing for
>   backlight support. This will always result in a kernel that builds cleanly,
>   but can be surprising for users when backlight support is a module that
>   gets loaded at boot, but it is still not used.
>
> - Make BACKLIGHT_CLASS_DEVICE a 'bool' symbol instead, and force the
>   core API code to always be built-in or completely disabled. This makes
>   it really easy to use, at the expense of a larger kernel image for those that
>   currently use a loadable module.
>
>        Arnd
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-- 
Jani Nikula, Intel Open Source Technology Center