linux-kernel - Re: [PATCH] drm/panel-edp: modify Kconfig to prevent build error

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Message-ID: <34aeb1de-7edf-77bd-e9d9-8cf203e73433@infradead.org>
Date:   Wed, 17 Nov 2021 12:04:33 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kernel test robot <lkp@...el.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Sam Ravnborg <sam@...nborg.org>,
        Douglas Anderson <dianders@...omium.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH] drm/panel-edp: modify Kconfig to prevent build error

On 11/16/21 11:58 PM, Arnd Bergmann wrote:
> On Wed, Nov 17, 2021 at 7:27 AM Randy Dunlap <rdunlap@...radead.org> wrote:
>>
>> When CONFIG_DRM_KMS_HELPER=m and CONFIG_DRM_PANEL_EDP=y,
>> there is a build error in gpu/drm/panel/panel-edp.o:
>>
>> arm-linux-gnueabi-ld: drivers/gpu/drm/panel/panel-edp.o: in function `panel_edp_probe':
>> panel-edp.c:(.text+0xf38): undefined reference to `drm_panel_dp_aux_backlight'
>>
>> Fix this by limiting DRM_PANEL_DEP by the value of the DRM_KMS_HELPER
>> symbol.
> 
> I think the analysis is correct, but this is not the correct fix since
> DRM_KMS_HELPER
> is not user-selectable. (Almost) all other drivers that rely on DRM_KMS_HELPER
> use 'select' for this, and mixing the two risks running into circular
> dependencies.

Oops. Thanks for catching that.

My first (local) version of the patch used select, but that got me
into kconfig circular dependency chain land (ugly).
Maybe your all-at-once patch can take care of that problem.

> 
> I see that there are already some 'depends on DRM_KMS_HELPER' in bridge
> and panel drivers, so it's possible that we have to fix them all at the same to
> do this right. I ran into another problem like this the other day and
> I'm currently
> testing with the patch below, but I have not posted that yet since I am not
> fully convinced that this is the correct fix either.

I'll test some with it also.

thanks.

-- 
~Randy