Re: [PATCH] i915: fix backlight configuration issue

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

On Wed, 08 Jan 2020, Arnd Bergmann <arnd@arndb.de> wrote:
> The i915 driver can use the backlight subsystem as an option, and usually
> selects it when CONFIG_ACPI is set. However it is possible to configure
> a kernel with modular backlight classdev support and a built-in i915
> driver, which leads to a linker error:
>
> drivers/gpu/drm/i915/display/intel_panel.o: In function `intel_backlight_device_register':
> intel_panel.c:(.text+0x2f58): undefined reference to `backlight_device_register'
> drivers/gpu/drm/i915/display/intel_panel.o: In function `intel_backlight_device_unregister':
> intel_panel.c:(.text+0x2fe4): undefined reference to `backlight_device_unregister'
>
> Add another Kconfig option to ensure the driver only tries to use
> the backlight support when it can in fact be linked that way. The
> new option is on by default to keep the existing behavior.
>
> This is roughly what other drivers like nouveau do as well.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> I've had this one lying around for a long time, it is still needed
> but I am not sure which solution is best here. This version is
> probably the least invasive, but it does not solve the bigger
> problem around too many 'select' statements in drm

This is just another hack that's only required because backlight is
selected instead of depended on throughout the kernel. (*)

i915 (and most drm drivers, with some variations) could easily handle
this with:

	depends on (ACPI && ACPI_VIDEO) || ACPI=n
	depends on BACKLIGHT_CLASS_DEVICE || BACKLIGHT_CLASS_DEVICE=n

Those two lines express the allowed configurations. It's just that we
can't do that in i915 *alone*. The combinations of depends and selects
lead to impossible configurations. It's all or nothing.

I am not amused by adding more hacks, and I am really *not* interested
in adding another useless i915 config option to "solve" this issue.

So thanks, but no thanks. I'm not taking this patch.


BR,
Jani.


(*) The deeper issue is that people as well as the kconfig tools ignore
the warnings in Documentation/kbuild/kconfig-language.rst:

	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.

I don't think we can, uh, fix the people, but it might be possible to
warn about selecting visible symbols or symbols with dependencies.

> ---
>  drivers/gpu/drm/i915/Kconfig               | 11 ++++++++++-
>  drivers/gpu/drm/i915/display/intel_panel.c |  4 ++--
>  drivers/gpu/drm/i915/display/intel_panel.h |  6 +++---
>  3 files changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/Kconfig b/drivers/gpu/drm/i915/Kconfig
> index ba9595960bbe..81d956040d18 100644
> --- a/drivers/gpu/drm/i915/Kconfig
> +++ b/drivers/gpu/drm/i915/Kconfig
> @@ -16,7 +16,7 @@ config DRM_I915
>  	select IRQ_WORK
>  	# i915 depends on ACPI_VIDEO when ACPI is enabled
>  	# but for select to work, need to select ACPI_VIDEO's dependencies, ick
> -	select BACKLIGHT_CLASS_DEVICE if ACPI
> +	select DRM_I915_BACKLIGHT if ACPI
>  	select INPUT if ACPI
>  	select ACPI_VIDEO if ACPI
>  	select ACPI_BUTTON if ACPI
> @@ -68,6 +68,15 @@ config DRM_I915_FORCE_PROBE
>  
>  	  Use "*" to force probe the driver for all known devices.
>  
> +config DRM_I915_BACKLIGHT
> +	tristate "Control backlight support"
> +	depends on DRM_I915
> +	default DRM_I915
> +	select BACKLIGHT_CLASS_DEVICE
> +	help
> +          Say Y here if you want to control the backlight of your display
> +          (e.g. a laptop panel).
> +
>  config DRM_I915_CAPTURE_ERROR
>  	bool "Enable capturing GPU state following a hang"
>  	depends on DRM_I915
> diff --git a/drivers/gpu/drm/i915/display/intel_panel.c b/drivers/gpu/drm/i915/display/intel_panel.c
> index 7b3ec6eb3382..e2fe7a50dcbf 100644
> --- a/drivers/gpu/drm/i915/display/intel_panel.c
> +++ b/drivers/gpu/drm/i915/display/intel_panel.c
> @@ -1203,7 +1203,7 @@ void intel_panel_enable_backlight(const struct intel_crtc_state *crtc_state,
>  	mutex_unlock(&dev_priv->backlight_lock);
>  }
>  
> -#if IS_ENABLED(CONFIG_BACKLIGHT_CLASS_DEVICE)
> +#if IS_ENABLED(CONFIG_DRM_I915_BACKLIGHT)
>  static u32 intel_panel_get_backlight(struct intel_connector *connector)
>  {
>  	struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
> @@ -1370,7 +1370,7 @@ void intel_backlight_device_unregister(struct intel_connector *connector)
>  		panel->backlight.device = NULL;
>  	}
>  }
> -#endif /* CONFIG_BACKLIGHT_CLASS_DEVICE */
> +#endif /* CONFIG_DRM_I915_BACKLIGHT */
>  
>  /*
>   * CNP: PWM clock frequency is 19.2 MHz or 24 MHz.
> diff --git a/drivers/gpu/drm/i915/display/intel_panel.h b/drivers/gpu/drm/i915/display/intel_panel.h
> index cedeea443336..e6e81268b7ed 100644
> --- a/drivers/gpu/drm/i915/display/intel_panel.h
> +++ b/drivers/gpu/drm/i915/display/intel_panel.h
> @@ -49,10 +49,10 @@ intel_panel_edid_fixed_mode(struct intel_connector *connector);
>  struct drm_display_mode *
>  intel_panel_vbt_fixed_mode(struct intel_connector *connector);
>  
> -#if IS_ENABLED(CONFIG_BACKLIGHT_CLASS_DEVICE)
> +#if IS_ENABLED(CONFIG_DRM_I915_BACKLIGHT)
>  int intel_backlight_device_register(struct intel_connector *connector);
>  void intel_backlight_device_unregister(struct intel_connector *connector);
> -#else /* CONFIG_BACKLIGHT_CLASS_DEVICE */
> +#else /* CONFIG_DRM_I915_BACKLIGHT */
>  static inline int intel_backlight_device_register(struct intel_connector *connector)
>  {
>  	return 0;
> @@ -60,6 +60,6 @@ static inline int intel_backlight_device_register(struct intel_connector *connec
>  static inline void intel_backlight_device_unregister(struct intel_connector *connector)
>  {
>  }
> -#endif /* CONFIG_BACKLIGHT_CLASS_DEVICE */
> +#endif /* CONFIG_DRM_I915_BACKLIGHT */
>  
>  #endif /* __INTEL_PANEL_H__ */

-- 
Jani Nikula, Intel Open Source Graphics Center