linux-kernel - Re: [PATCH] [RESEND] drm: fb_helper: fix CONFIG

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Message-ID: <87tuh2hb17.fsf@intel.com>
Date:   Wed, 27 Oct 2021 16:06:44 +0300
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Arnd Bergmann <arnd@...nel.org>,
        Javier Martinez Canillas <javierm@...hat.com>
Cc:     Daniel Vetter <daniel@...ll.ch>, Kees Cook <keescook@...omium.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>, Arnd Bergmann <arnd@...db.de>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [RESEND] drm: fb_helper: fix CONFIG_FB dependency

On Wed, 27 Oct 2021, Arnd Bergmann <arnd@...nel.org> wrote:
> On Wed, Oct 27, 2021 at 2:38 PM Javier Martinez Canillas
> <javierm@...hat.com> wrote:
>> >
>> > This is something we can't easily express in Kconfig, as we can't add the
>> > dependency to a symbol that only gets selected by other drivers, which
>> > is why the dependency has to be in the user-visible symbol,
>> > in this case DRM_FBDEV_EMULATION.
>> >
>>
>> Why the dependency has to be in a user-visible symbol? What could be the
>> problem with having something like:
>>
>> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
>> index cea777ae7fb9..f80b404946ca 100644
>> --- a/drivers/gpu/drm/Kconfig
>> +++ b/drivers/gpu/drm/Kconfig
>> @@ -82,6 +82,7 @@ config DRM_DEBUG_SELFTEST
>>  config DRM_KMS_HELPER
>>         tristate
>>         depends on DRM
>> +       depends on (DRM_FBDEV_EMULATION && FB) || !DRM_FBDEV_EMULATION
>>         help
>>           CRTC helpers for KMS drivers.
>>
>> @@ -104,7 +105,6 @@ config DRM_FBDEV_EMULATION
>>         bool "Enable legacy fbdev support for your modesetting driver"
>>         depends on DRM
>>         depends on FB
>> -       select DRM_KMS_HELPER
>>         select FB_CFB_FILLRECT
>>         select FB_CFB_COPYAREA
>>         select FB_CFB_IMAGEBLIT
>
> This fails because of all the other drivers that try to 'select DRM_KMS_HELPER'.
> Kconfig will now complain about a symbol that gets selected while its
> dependencies
> are not met.
>
> To work around that, every single driver that has 'selects DRM_KMS_HELPER' would
> now have to also list 'depends on (DRM_FBDEV_EMULATION && FB) ||
> !DRM_FBDEV_EMULATION'.

So the fix would be that nobody selects DRM_KMS_HELPER...

BR,
Jani.

>
>        Arnd

-- 
Jani Nikula, Intel Open Source Graphics Center