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From: Arnd Bergmann <>
To: Laurent Pinchart <>
Cc: Tomi Valkeinen <>,
	David Airlie <>,
	dri-devel <>,
	Linux Kernel Mailing List <>
Subject: Re: [PATCH 1/3] drm/omap: displays: panel-dpi: add backlight dependency
Date: Wed, 19 Apr 2017 22:39:16 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <2979804.i6EKSW6ohY@avalon>

On Wed, Apr 19, 2017 at 10:21 PM, Laurent Pinchart
<> 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

> --- 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
>  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:

  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.


  reply	other threads:[~2017-04-19 20:39 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-04-19 17:59 Arnd Bergmann
2017-04-19 17:59 ` [PATCH 2/3] drm/panel: S6E3HA2 needs backlight code Arnd Bergmann
2017-04-19 18:03   ` [PATCH 3/3] drm/panel: add backlight dependency for sitronix-st7789v Arnd Bergmann
2017-06-14 18:13     ` Thierry Reding
2017-06-14 18:13   ` [PATCH 2/3] drm/panel: S6E3HA2 needs backlight code Thierry Reding
2017-04-19 20:21 ` [PATCH 1/3] drm/omap: displays: panel-dpi: add backlight dependency Laurent Pinchart
2017-04-19 20:39   ` Arnd Bergmann [this message]
2017-04-20  9:01     ` Jani Nikula

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