linux-kernel - [PATCH] drm: Make DRM_FBDEV_EMULATION deps more robust to fix linker errors

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Date:   Wed, 27 Oct 2021 09:20:44 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Dan Horák <dan@...ny.cz>,
        "Justin M . Forbes" <jforbes@...oraproject.org>,
        Peter Robinson <pbrobinson@...il.com>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        dri-devel@...ts.freedesktop.org
Subject: [PATCH] drm: Make DRM_FBDEV_EMULATION deps more robust to fix linker errors

Enabling the CONFIG_DRM_FBDEV_EMULATION Kconfig symbol can lead to linker
errors, if CONFIG_DRM_KMS_HELPER is built-in but CONFIG_FB as a module.

Because in that case the drm_kms_helper.o object will have references to
symbols that are defined in the fb.ko module, i.e:

  $ make bzImage modules
    DESCEND objtool
    CALL    scripts/atomic/check-atomics.sh
    CALL    scripts/checksyscalls.sh
    CHK     include/generated/compile.h
    GEN     .version
    CHK     include/generated/compile.h
    UPD     include/generated/compile.h
    CC      init/version.o
    AR      init/built-in.a
    LD      vmlinux.o
    MODPOST vmlinux.symvers
    MODINFO modules.builtin.modinfo
    GEN     modules.builtin
    LD      .tmp_vmlinux.kallsyms1
  ld: drivers/gpu/drm/drm_fb_helper.o: in function `drm_fb_helper_resume_worker':
  drm_fb_helper.c:(.text+0x2a0): undefined reference to `fb_set_suspend'
  ...

To prevent this, make following changes to the config option dependencies:

  * Depend on FB && !(DRM_KMS_HELPER=y && FB=m), to avoid invalid configs.
  * Depend on DRM_KMS_HELPER instead selecting it, to avoid circular deps.

Reported-by: Justin M. Forbes <jforbes@...oraproject.org>
Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
---

This fixes linker errors found when building the Fedora kernel package
for the s390x architecture:

https://kojipkgs.fedoraproject.org//work/tasks/9849/77859849/build.log

 drivers/gpu/drm/Kconfig | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
index cea777ae7fb9..9a21e57b4a0d 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -103,8 +103,8 @@ config DRM_DEBUG_DP_MST_TOPOLOGY_REFS
 config DRM_FBDEV_EMULATION
 	bool "Enable legacy fbdev support for your modesetting driver"
 	depends on DRM
-	depends on FB
-	select DRM_KMS_HELPER
+	depends on FB && !(DRM_KMS_HELPER=y && FB=m)
+	depends on DRM_KMS_HELPER
 	select FB_CFB_FILLRECT
 	select FB_CFB_COPYAREA
 	select FB_CFB_IMAGEBLIT
-- 
2.31.1