fbdev: Split frame buffer support in FB and FB_CORE symbols

Currently the CONFIG_FB option has to be enabled even if no legacy fbdev
drivers are needed (e.g: only to have support for framebuffer consoles).

The DRM subsystem has a fbdev emulation layer, but depends on CONFIG_FB
and so it can only be enabled if that dependency is enabled as well.

That means fbdev drivers have to be explicitly disabled if users want to
enable CONFIG_FB, only to use fbcon and/or the DRM fbdev emulation layer.

This patch introduces a non-visible CONFIG_FB_CORE symbol that could be
enabled just to have core support needed for CONFIG_DRM_FBDEV_EMULATION,
allowing CONFIG_FB to be disabled (and automatically disabling all the
fbdev drivers).

Nothing from fb_backlight.o and fbmon.o is used by the DRM fbdev emulation
layer so these two objects can be compiled out when CONFIG_FB is disabled.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230719081544.741051-4-javierm@redhat.com

arch/x86/Makefile

@@ -259,7 +259,7 @@ drivers-$(CONFIG_PCI)            += arch/x86/pci/
 # suspend and hibernation support
 drivers-$(CONFIG_PM) += arch/x86/power/
 
-drivers-$(CONFIG_FB) += arch/x86/video/
+drivers-$(CONFIG_FB_CORE) += arch/x86/video/
 
 ####
 # boot loader support. Several targets are kept for legacy purposes

arch/x86/video/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
-obj-$(CONFIG_FB)               += fbdev.o
+obj-$(CONFIG_FB_CORE)		+= fbdev.o

drivers/video/backlight/backlight.c

@@ -79,8 +79,8 @@ static const char *const backlight_scale_types[] = {
 	[BACKLIGHT_SCALE_NON_LINEAR]	= "non-linear",
 };
 
-#if defined(CONFIG_FB) || (defined(CONFIG_FB_MODULE) && \
-			   defined(CONFIG_BACKLIGHT_CLASS_DEVICE_MODULE))
+#if defined(CONFIG_FB_CORE) || (defined(CONFIG_FB_CORE_MODULE) && \
+				defined(CONFIG_BACKLIGHT_CLASS_DEVICE_MODULE))
 /*
  * fb_notifier_callback
  *
@@ -155,7 +155,7 @@ static inline int backlight_register_fb(struct backlight_device *bd)
 static inline void backlight_unregister_fb(struct backlight_device *bd)
 {
 }
-#endif /* CONFIG_FB */
+#endif /* CONFIG_FB_CORE */
 
 static void backlight_generate_event(struct backlight_device *bd,
 				     enum backlight_update_reason reason)

drivers/video/console/Kconfig

@@ -72,7 +72,7 @@ config DUMMY_CONSOLE_ROWS
 
 config FRAMEBUFFER_CONSOLE
 	bool "Framebuffer Console support"
-	depends on FB && !UML
+	depends on FB_CORE && !UML
 	select VT_HW_CONSOLE_BINDING
 	select CRC32
 	select FONT_SUPPORT

drivers/video/fbdev/Kconfig

@@ -4,9 +4,9 @@
 #
 
 menuconfig FB
-	tristate "Support for frame buffer devices"
+	tristate "Support for frame buffer device drivers"
+	select FB_CORE
 	select FB_NOTIFY
-	select VIDEO_CMDLINE
 	help
 	  The frame buffer device provides an abstraction for the graphics
 	  hardware. It represents the frame buffer of some video hardware and
@@ -30,6 +30,12 @@ menuconfig FB
 	  <http://www.munted.org.uk/programming/Framebuffer-HOWTO-1.3.html> for more
 	  information.
 
+	  This enables support for native frame buffer device (fbdev) drivers.
+
+	  The DRM subsystem provides support for emulated frame buffer devices
+	  on top of KMS drivers, but this option allows legacy fbdev drivers to
+	  be enabled as well.
+
 	  Say Y here and to the driver for your graphics board below if you
 	  are compiling a kernel for a non-x86 architecture.
 

drivers/video/fbdev/core/Kconfig

@@ -3,6 +3,10 @@
 # fbdev core configuration
 #
 
+config FB_CORE
+	select VIDEO_CMDLINE
+	tristate
+
 config FB_NOTIFY
 	bool
 
@@ -23,7 +27,7 @@ config FIRMWARE_EDID
 
 config FB_DEVICE
 	bool "Provide legacy /dev/fb* device"
-	depends on FB
+	depends on FB_CORE
 	default y
 	help
 	  Say Y here if you want the legacy /dev/fb* device file and
@@ -39,7 +43,7 @@ config FB_DDC
 
 config FB_CFB_FILLRECT
 	tristate
-	depends on FB
+	depends on FB_CORE
 	help
 	  Include the cfb_fillrect function for generic software rectangle
 	  filling. This is used by drivers that don't provide their own
@@ -47,7 +51,7 @@ config FB_CFB_FILLRECT
 
 config FB_CFB_COPYAREA
 	tristate
-	depends on FB
+	depends on FB_CORE
 	help
 	  Include the cfb_copyarea function for generic software area copying.
 	  This is used by drivers that don't provide their own (accelerated)
@@ -55,7 +59,7 @@ config FB_CFB_COPYAREA
 
 config FB_CFB_IMAGEBLIT
 	tristate
-	depends on FB
+	depends on FB_CORE
 	help
 	  Include the cfb_imageblit function for generic software image
 	  blitting. This is used by drivers that don't provide their own
@@ -63,7 +67,7 @@ config FB_CFB_IMAGEBLIT
 
 config FB_CFB_REV_PIXELS_IN_BYTE
 	bool
-	depends on FB
+	depends on FB_CORE
 	help
 	  Allow generic frame-buffer functions to work on displays with 1, 2
 	  and 4 bits per pixel depths which has opposite order of pixels in
@@ -71,7 +75,7 @@ config FB_CFB_REV_PIXELS_IN_BYTE
 
 config FB_SYS_FILLRECT
 	tristate
-	depends on FB
+	depends on FB_CORE
 	help
 	  Include the sys_fillrect function for generic software rectangle
 	  filling. This is used by drivers that don't provide their own
@@ -79,7 +83,7 @@ config FB_SYS_FILLRECT
 
 config FB_SYS_COPYAREA
 	tristate
-	depends on FB
+	depends on FB_CORE
 	help
 	  Include the sys_copyarea function for generic software area copying.
 	  This is used by drivers that don't provide their own (accelerated)
@@ -87,7 +91,7 @@ config FB_SYS_COPYAREA
 
 config FB_SYS_IMAGEBLIT
 	tristate
-	depends on FB
+	depends on FB_CORE
 	help
 	  Include the sys_imageblit function for generic software image
 	  blitting. This is used by drivers that don't provide their own
@@ -126,22 +130,22 @@ endchoice
 
 config FB_SYS_FOPS
 	tristate
-	depends on FB
+	depends on FB_CORE
 
 config FB_DEFERRED_IO
 	bool
-	depends on FB
+	depends on FB_CORE
 
 config FB_IO_HELPERS
 	bool
-	depends on FB
+	depends on FB_CORE
 	select FB_CFB_COPYAREA
 	select FB_CFB_FILLRECT
 	select FB_CFB_IMAGEBLIT
 
 config FB_SYS_HELPERS
 	bool
-	depends on FB
+	depends on FB_CORE
 	select FB_SYS_COPYAREA
 	select FB_SYS_FILLRECT
 	select FB_SYS_FOPS
@@ -149,7 +153,7 @@ config FB_SYS_HELPERS
 
 config FB_SYS_HELPERS_DEFERRED
 	bool
-	depends on FB
+	depends on FB_CORE
 	select FB_DEFERRED_IO
 	select FB_SYS_HELPERS
 

drivers/video/fbdev/core/Makefile

 # SPDX-License-Identifier: GPL-2.0
 obj-$(CONFIG_FB_NOTIFY)           += fb_notify.o
-obj-$(CONFIG_FB)                  += fb.o
-fb-y                              := fb_backlight.o \
-                                     fb_info.o \
-                                     fbmem.o fbmon.o fbcmap.o \
+obj-$(CONFIG_FB_CORE)             += fb.o
+fb-y                              := fb_info.o \
+                                     fbmem.o fbcmap.o \
                                      modedb.o fbcvt.o fb_cmdline.o fb_io_fops.o
+ifdef CONFIG_FB
+fb-y                              += fb_backlight.o fbmon.o
+endif
 fb-$(CONFIG_FB_DEFERRED_IO)       += fb_defio.o
 fb-$(CONFIG_FB_DEVICE)            += fb_chrdev.o \
                                      fb_procfs.o \