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

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Date:   Wed, 27 Oct 2021 14:52:28 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Javier Martinez Canillas <javierm@...hat.com>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        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, 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'.

       Arnd