Re: [PATCH 1/2] drm/i915/display: fix randconfig build

[Jani Nikula <jani.nikula@linux.intel.com>]

On Tue, 04 Oct 2022, "Jiri Slaby (SUSE)" <jirislaby@kernel.org> wrote:
> When DRM_I915=y and BACKLIGHT_CLASS_DEVICE=m, the build fails:
> ld: drivers/gpu/drm/i915/display/intel_backlight.o: in function `intel_backlight_device_register':
> intel_backlight.c:(.text+0x5587): undefined reference to `backlight_device_get_by_name'
>
> ld: drivers/gpu/drm/i915/display/intel_backlight.o: in function `intel_backlight_device_unregister':
> intel_backlight.c:(.text+0x576e): undefined reference to `backlight_device_unregister'
>
> To fix this, use IS_REACHABLE(), not IS_ENABLED() in backlight. That is,
> with the above config, backlight support is disabled.

So I don't want this. I'll take a patch that fixes the dependencies to
block DRM_I915=y and BACKLIGHT_CLASS_DEVICE=m. Nobody wants that combo,
and IMO using IS_REACHABLE() is a workaround to hide a broken config
under the carpet.

The right thing to do is

config DRM_I915
	depends on BACKLIGHT_CLASS_DEVICE || BACKLIGHT_CLASS_DEVICE=n.

We're selecting BACKLIGHT_CLASS_DEVICE because almost everyone else is
too, and a combo of selecting and depending leads to circular
dependencies. But depending is the right fix.

Documentation/kbuild/kconfig-language.rst:

  Note:
	select should be used with care. select will force
	a symbol to a value without visiting the dependencies.
	By abusing select you are able to select a symbol FOO even
	if FOO depends on BAR that is not set.
	In general use select only for non-visible symbols
	(no prompts anywhere) and for symbols with no dependencies.
	That will limit the usefulness but on the other hand avoid
	the illegal configurations all over.


BR,
Jani.

>
> Cc: Jani Nikula <jani.nikula@linux.intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> Cc: David Airlie <airlied@gmail.com>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Cc: intel-gfx@lists.freedesktop.org
> Cc: dri-devel@lists.freedesktop.org
> Reported-by: Martin Liška <mliska@suse.cz>
> Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
> ---
>  drivers/gpu/drm/i915/display/intel_backlight.c | 2 +-
>  drivers/gpu/drm/i915/display/intel_backlight.h | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_backlight.c b/drivers/gpu/drm/i915/display/intel_backlight.c
> index beba39a38c87..c1ba68796b6d 100644
> --- a/drivers/gpu/drm/i915/display/intel_backlight.c
> +++ b/drivers/gpu/drm/i915/display/intel_backlight.c
> @@ -825,7 +825,7 @@ void intel_backlight_enable(const struct intel_crtc_state *crtc_state,
>  	mutex_unlock(&dev_priv->display.backlight.lock);
>  }
>  
> -#if IS_ENABLED(CONFIG_BACKLIGHT_CLASS_DEVICE)
> +#if IS_REACHABLE(CONFIG_BACKLIGHT_CLASS_DEVICE)
>  static u32 intel_panel_get_backlight(struct intel_connector *connector)
>  {
>  	struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
> diff --git a/drivers/gpu/drm/i915/display/intel_backlight.h b/drivers/gpu/drm/i915/display/intel_backlight.h
> index 339643f63897..207fe1c613d8 100644
> --- a/drivers/gpu/drm/i915/display/intel_backlight.h
> +++ b/drivers/gpu/drm/i915/display/intel_backlight.h
> @@ -36,7 +36,7 @@ u32 intel_backlight_invert_pwm_level(struct intel_connector *connector, u32 leve
>  u32 intel_backlight_level_to_pwm(struct intel_connector *connector, u32 level);
>  u32 intel_backlight_level_from_pwm(struct intel_connector *connector, u32 val);
>  
> -#if IS_ENABLED(CONFIG_BACKLIGHT_CLASS_DEVICE)
> +#if IS_REACHABLE(CONFIG_BACKLIGHT_CLASS_DEVICE)
>  int intel_backlight_device_register(struct intel_connector *connector);
>  void intel_backlight_device_unregister(struct intel_connector *connector);
>  #else /* CONFIG_BACKLIGHT_CLASS_DEVICE */

-- 
Jani Nikula, Intel Open Source Graphics Center