[PATCH AUTOSEL 5.15 13/18] fbdev: Disable sysfb device registration when removing conflicting FBs

[Sasha Levin <sashal@kernel.org>]

From: Javier Martinez Canillas <javierm@redhat.com>

[ Upstream commit ee7a69aa38d87a3bbced7b8245c732c05ed0c6ec ]

The platform devices registered by sysfb match with firmware-based DRM or
fbdev drivers, that are used to have early graphics using a framebuffer
provided by the system firmware.

DRM or fbdev drivers later are probed and remove conflicting framebuffers,
leading to these platform devices for generic drivers to be unregistered.

But the current solution has a race, since the sysfb_init() function could
be called after a DRM or fbdev driver is probed and request to unregister
the devices for drivers with conflicting framebuffes.

To prevent this, disable any future sysfb platform device registration by
calling sysfb_disable(), if a driver requests to remove the conflicting
framebuffers.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220607182338.344270-4-javierm@redhat.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/video/fbdev/core/fbmem.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 0371ad233fdf..ba9c66a95ada 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -19,6 +19,7 @@
 #include <linux/kernel.h>
 #include <linux/major.h>
 #include <linux/slab.h>
+#include <linux/sysfb.h>
 #include <linux/mm.h>
 #include <linux/mman.h>
 #include <linux/vt.h>
@@ -1774,6 +1775,17 @@ int remove_conflicting_framebuffers(struct apertures_struct *a,
 		do_free = true;
 	}
 
+	/*
+	 * If a driver asked to unregister a platform device registered by
+	 * sysfb, then can be assumed that this is a driver for a display
+	 * that is set up by the system firmware and has a generic driver.
+	 *
+	 * Drivers for devices that don't have a generic driver will never
+	 * ask for this, so let's assume that a real driver for the display
+	 * was already probed and prevent sysfb to register devices later.
+	 */
+	sysfb_disable();
+
 	mutex_lock(&registration_lock);
 	do_remove_conflicting_framebuffers(a, name, primary);
 	mutex_unlock(&registration_lock);
-- 
2.35.1