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

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Message-ID: <CAK8P3a0rQqOZ-VJjmSR_MrRmKUK2ztASK2JTMe-NwV0TStXURA@mail.gmail.com>
Date:   Wed, 19 Apr 2017 22:39:16 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Tomi Valkeinen <tomi.valkeinen@...com>,
        David Airlie <airlied@...ux.ie>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] drm/omap: displays: panel-dpi: add backlight dependency

On Wed, Apr 19, 2017 at 10:21 PM, Laurent Pinchart
<laurent.pinchart@...asonboard.com> 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.

- 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