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Date:   Wed, 8 Jan 2020 19:55:59 +0100
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Jani Nikula <jani.nikula@linux.intel.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Chris Wilson <chris@chris-wilson.co.uk>,
        Dave Airlie <airlied@redhat.com>,
        Alexander Shiyan <shc_work@mail.ru>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Ville =?iso-8859-1?Q?Syrj=E4l=E4?= 
        <ville.syrjala@linux.intel.com>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Hans de Goede <hdegoede@redhat.com>,
        intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] i915: fix backlight configuration issue
Message-ID: <20200108185559.GK43062@phenom.ffwll.local>
Mail-Followup-To: Jani Nikula <jani.nikula@linux.intel.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        David Airlie <airlied@linux.ie>,
        Chris Wilson <chris@chris-wilson.co.uk>,
        Dave Airlie <airlied@redhat.com>,
        Alexander Shiyan <shc_work@mail.ru>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Ville =?iso-8859-1?Q?Syrj=E4l=E4?= <ville.syrjala@linux.intel.com>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Hans de Goede <hdegoede@redhat.com>,
        intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org
References: <20200108140227.3976563-1-arnd@arndb.de>
 <87o8veotf9.fsf@intel.com>
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On Wed, Jan 08, 2020 at 05:32:26PM +0200, Jani Nikula wrote:
> 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.

Yeah I'm also leaning towards that the real fix here is to convert
backlight over to be a depends on symbol, not a select symbol. It's
clearly not a simple stand-alone helper. Or someone makes select recursive
and adds a SAT solver to Kconfig :-)
-Daniel

> 
> 
> 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

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch