fbdev: Make registered_fb[] private to fbmem.c

Well except when the olpc dcon fbdev driver is enabled, that thing
digs around in there in rather unfixable ways.

Cc oldc_dcon maintainers as fyi.

v2: I typoed the config name (0day)

Cc: kernel test robot <lkp@intel.com>
Cc: Jens Frederich <jfrederich@gmail.com>
Cc: Jon Nettleton <jon.nettleton@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-staging@lists.linux.dev
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Helge Deller <deller@gmx.de>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Zhen Lei <thunder.leizhen@huawei.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Cc: linux-fbdev@vger.kernel.org
Cc: Zheyu Ma <zheyuma97@gmail.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Series-cc: Thomas Zimmermann <tzimmermann@suse.de>
Series-cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Series-cc: dri-devel@lists.freedesktop.org
Series-cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Series-version: 4
Series-changes: 4
- Add patch to make registered_fb[] private.
Cover-letter:
Fix some race between sysfb device registration and drivers probe
Hello,

The patches in this series contain patches changes suggested by Daniel Vetter
to fix a race condition that exists between sysfb platform device registration
and fbdev or DRM drivers probing and registering devices.

For example, it is currently possible for sysfb to register a platform
device after a real DRM driver was registered and requested to remove the
conflicting framebuffers.

A symptom of this issue, was worked around with by commit fb561bf
("fbdev: Prevent probing generic drivers if a FB is already registered")
but that's really a hack and should be reverted.

This series attempt to fix it more properly and revert the mentioned hack.
That will also allow to make the num_registered_fb variable not visible to
drivers anymore, since that's internal to fbdev core.

This is a v4 that addresses issues pointed out in v3 by Thomas Zimmermann.

Patch #1 is just a trivial preparatory change.

Patch #2 add a sysfb_disable() helper for fbdev and DRM to use it.

Patch #3 fixes the mentioned race condition by disabling sysfb if a driver
probes and registers a device correctly.

Patch #4 is the revert patch that was posted by Daniel before but dropped
from his set and finally patch #5 is the one that makes num_registered_fb
private to fbmem.c and not allow drivers to use it anymore.

The patches were tested on a rpi4 using the following configurations:

* simpledrm and vc4 both built-in
* simpledrm built-in, vc4 as a module
* simpledrm as module, vc4 built-in
* simplefb and vc4 both built-in
* simplefb built-in, vc4 as a module
* simplefb as a module, vc4 built-in

Best regards,
Javier
END

drivers/video/fbdev/core/fbmem.c

@@ -50,10 +50,14 @@
 static DEFINE_MUTEX(registration_lock);
 
 struct fb_info *registered_fb[FB_MAX] __read_mostly;
-EXPORT_SYMBOL(registered_fb);
-
 int num_registered_fb __read_mostly;
+#if IS_ENABLED(CONFIG_FB_OLPC_DCON)
+EXPORT_SYMBOL(registered_fb);
 EXPORT_SYMBOL(num_registered_fb);
+#endif
+#define for_each_registered_fb(i)		\
+	for (i = 0; i < FB_MAX; i++)		\
+		if (!registered_fb[i]) {} else
 
 bool fb_center_logo __read_mostly;
 

include/linux/fb.h

@@ -623,16 +623,15 @@ extern int fb_get_color_depth(struct fb_var_screeninfo *var,
 extern int fb_get_options(const char *name, char **option);
 extern int fb_new_modelist(struct fb_info *info);
 
+#if IS_ENABLED(CONFIG_FB_OLPC_DCON)
 extern struct fb_info *registered_fb[FB_MAX];
+
 extern int num_registered_fb;
+#endif
 extern bool fb_center_logo;
 extern int fb_logo_count;
 extern struct class *fb_class;
 
-#define for_each_registered_fb(i)		\
-	for (i = 0; i < FB_MAX; i++)		\
-		if (!registered_fb[i]) {} else
-
 static inline void lock_fb_info(struct fb_info *info)
 {
 	mutex_lock(&info->lock);