linux-kernel - [PATCH] fbdev: simplefb: fix Kconfig dependencies

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Date:   Wed, 21 Jul 2021 17:17:46 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Maxime Ripard <maxime@...no.tech>
Cc:     Arnd Bergmann <arnd@...db.de>, Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org
Subject: [PATCH] fbdev: simplefb: fix Kconfig dependencies

From: Arnd Bergmann <arnd@...db.de>

Configurations with both CONFIG_FB_SIMPLE=y and CONFIG_DRM_SIMPLEDRM=m
are allowed by Kconfig because the 'depends on !DRM_SIMPLEDRM' dependency
does not disallow FB_SIMPLE as long as SIMPLEDRM is not built-in. This
can however result in a build failure when cfb_fillrect() etc are then
also in loadable modules:

x86_64-linux-ld: drivers/video/fbdev/simplefb.o:(.rodata+0x1f8): undefined reference to `cfb_fillrect'
x86_64-linux-ld: drivers/video/fbdev/simplefb.o:(.rodata+0x200): undefined reference to `cfb_copyarea'
x86_64-linux-ld: drivers/video/fbdev/simplefb.o:(.rodata+0x208): undefined reference to `cfb_imageblit'

To work around this, change FB_SIMPLE to be a 'tristate' symbol,
which still allows both to be =m together, but not one of them to
be =y if the other one is =m. If a distro kernel picks this
configuration, it can be determined by local policy which of
the two modules gets loaded. The 'of_chosen' export is needed
as this is the first loadable module referencing it.

Alternatively, the Kconfig dependency could be changed to
'depends on DRM_SIMPLEDRM=n', which would forbid the configuration
with both drivers.

Fixes: 11e8f5fd223b ("drm: Add simpledrm driver")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 drivers/of/base.c           | 1 +
 drivers/video/fbdev/Kconfig | 5 +++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/of/base.c b/drivers/of/base.c
index 48e941f99558..073ea7cd007b 100644
--- a/drivers/of/base.c
+++ b/drivers/of/base.c
@@ -36,6 +36,7 @@ LIST_HEAD(aliases_lookup);
 struct device_node *of_root;
 EXPORT_SYMBOL(of_root);
 struct device_node *of_chosen;
+EXPORT_SYMBOL(of_chosen);
 struct device_node *of_aliases;
 struct device_node *of_stdout;
 static const char *of_stdout_options;
diff --git a/drivers/video/fbdev/Kconfig b/drivers/video/fbdev/Kconfig
index 8a98539ec490..5fbcb3cc2e8e 100644
--- a/drivers/video/fbdev/Kconfig
+++ b/drivers/video/fbdev/Kconfig
@@ -2207,8 +2207,9 @@ config FB_HYPERV
 	  This framebuffer driver supports Microsoft Hyper-V Synthetic Video.
 
 config FB_SIMPLE
-	bool "Simple framebuffer support"
-	depends on (FB = y) && !DRM_SIMPLEDRM
+	tristate "Simple framebuffer support"
+	depends on FB
+	depends on !DRM_SIMPLEDRM
 	select FB_CFB_FILLRECT
 	select FB_CFB_COPYAREA
 	select FB_CFB_IMAGEBLIT
-- 
2.29.2