The efifb and simplefb drivers just render to a pre-allocated frame buffer
and rely on the display hardware being initialized before the kernel boots.

But if another driver already probed correctly and registered a fbdev, the
generic drivers shouldn't be probed since an actual driver for the display
hardware is already present.

This is more likely to occur after commit d391c5827107 ("drivers/firmware:
move x86 Generic System Framebuffers support") since the "efi-framebuffer"
and "simple-framebuffer" platform devices are registered at a later time.

Link: https://lore.kernel.org/r/20211110200253.rfudkt3edbd3nsyj@lahvuun/ (local)
Fixes: d391c5827107 ("drivers/firmware: move x86 Generic System Framebuffers support")
Reported-by: Ilya Trukhanov <redacted>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Vetter <redacted>
---

Changes in v2:
- Add a Link: tag with a reference to the bug report (Thorsten Leemhuis).
- Add a comment explaining why the probe fails earlier (Daniel Vetter).
- Add a Fixes: tag for stable to pick the fix (Daniel Vetter).
- Add Daniel Vetter's Reviewed-by: tag.
- Improve the commit message and mention the culprit commit

 drivers/video/fbdev/efifb.c    | 11 +++++++++++
 drivers/video/fbdev/simplefb.c | 11 +++++++++++
 2 files changed, 22 insertions(+)

diff --git drivers/video/fbdev/efifb.c drivers/video/fbdev/efifb.c
index edca3703b964..ea42ba6445b2 100644
--- drivers/video/fbdev/efifb.c
+++ drivers/video/fbdev/efifb.c

@@ -351,6 +351,17 @@ static int efifb_probe(struct platform_device *dev)
        char *option = NULL;
        efi_memory_desc_t md;

+       /*
+        * Generic drivers must not be registered if a framebuffer exists.
+        * If a native driver was probed, the display hardware was already
+        * taken and attempting to use the system framebuffer is dangerous.
+        */
+       if (num_registered_fb > 0) {

+               dev_err(&dev->dev,
+                       "efifb: a framebuffer is already registered\n");
+               return -EINVAL;
+       }
+
        if (screen_info.orig_video_isVGA != VIDEO_TYPE_EFI || pci_dev_disabled)
                return -ENODEV;

diff --git drivers/video/fbdev/simplefb.c drivers/video/fbdev/simplefb.c
index 62f0ded70681..b63074fd892e 100644
--- drivers/video/fbdev/simplefb.c
+++ drivers/video/fbdev/simplefb.c

@@ -407,6 +407,17 @@ static int simplefb_probe(struct platform_device *pdev)
        struct simplefb_par *par;
        struct resource *mem;

+       /*
+        * Generic drivers must not be registered if a framebuffer exists.
+        * If a native driver was probed, the display hardware was already
+        * taken and attempting to use the system framebuffer is dangerous.
+        */
+       if (num_registered_fb > 0) {