linux-kernel - Re: [PATCH] drm: i915: fix build when ACPI is disabled and BACKLIGHT=m

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Message-ID: <866940af-bbbf-a1cd-b1e3-aafe94686a22@infradead.org>
Date:   Tue, 27 Apr 2021 08:31:54 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Jani Nikula <jani.nikula@...el.com>, linux-kernel@...r.kernel.org
Cc:     Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        Damien Lespiau <damien.lespiau@...el.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm: i915: fix build when ACPI is disabled and
 BACKLIGHT=m

On 4/27/21 1:03 AM, Jani Nikula wrote:
> On Mon, 26 Apr 2021, Randy Dunlap <rdunlap@...radead.org> wrote:
>> When CONFIG_DRM_I915=y, CONFIG_ACPI is not set, and
>> CONFIG_BACKLIGHT_CLASS_DEVICE=m, not due to I915 config,
>> there are build errors trying to reference backlight_device_{un}register().
>>
>> Changing the use of IS_ENABLED() to IS_REACHABLE() in intel_panel.[ch]
>> fixes this.
> 
> I feel like a broken record...

Thanks! :)

I'll leave it b0rken as well.


> CONFIG_DRM_I915=y and CONFIG_BACKLIGHT_CLASS_DEVICE=m is an invalid
> configuration. The patch at hand just silently hides the problem,
> leaving you without backlight.
> 
> i915 should *depend* on backlight, not select it. It would express the
> dependency without chances for invalid configuration.
> 
> However, i915 alone can't depend on backlight, all users of backlight
> should depend on backlight, not select it. Otherwise, you end up with
> other configuration problems, circular dependencies and
> whatnot. Everyone should change. See also (*) why select is not a good
> idea here.
> 
> I've sent patches to this effect before, got rejected, and the same
> thing gets repeated ad infinitum.
> 
> Accepting this patch would stop the inflow of these reports and similar
> patches, but it does not fix the root cause. It just sweeps the problem
> under the rug.
> 
> 
> BR,
> Jani.
> 
> (*) 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.

Yes, I'm well aware of that.

ta.
-- 
~Randy