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

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Message-ID: <76763f5e-8c37-c36a-8f64-af3efe0da254@redhat.com>
Date:   Wed, 27 Oct 2021 15:18:03 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Arnd Bergmann <arnd@...nel.org>
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 10/27/21 14:55, Jani Nikula wrote:

[snip]

>> 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
> 
> To me, this seems like the right solution. Depend on FB if
> DRM_FBDEV_EMULATION is enabled. That's exactly what the relationship is.
>

The problem as Arnd explained is that then this relationship will have to
be expressed in all the Kconfig symbols that select DRM_KMS_HELPER.

Otherwise the symbol will happily select the wrong state and even when a
warning is printed by Kconfig, it will just set an invalid configuration.

For example with CONFIG_FB=m (that led to the linker errors if the symbol
is also not CONFIG_DRM_KMS_HELPER=m) and CONFIG_SIMPLEDRM=y (that selects
CONFIG_DRM_KMS_HELPER), this would cause the following unmet dependencies:

$ make prepare modules_prepare
WARNING: unmet direct dependencies detected for DRM_KMS_HELPER
  Depends on [m]: HAS_IOMEM [=y] && DRM [=y] && (DRM_FBDEV_EMULATION [=y] && FB [=m] || !DRM_FBDEV_EMULATION [=y])
  Selected by [y]:
  - DRM_SIMPLEDRM [=y] && HAS_IOMEM [=y] && DRM [=y]
  Selected by [m]:
  - DRM_I915 [=m] && HAS_IOMEM [=y] && DRM [=y] && X86 [=y] && PCI [=y]
  - DRM_VIRTIO_GPU [=m] && HAS_IOMEM [=y] && DRM [=y] && VIRTIO_MENU [=y] && MMU [=y]

so CONFIG_DRM_KMS_HELPER will wrongly set to =y which will cause the issue.

Best regards,
-- 
Javier Martinez Canillas
Linux Engineering
Red Hat