fbdev: Hot-unplug firmware fb devices on forced removal

Hot-unplug all firmware-framebuffer devices as part of removing
them via remove_conflicting_framebuffers() et al. Releases all
memory regions to be acquired by native drivers.

Firmware, such as EFI, install a framebuffer while posting the
computer. After removing the firmware-framebuffer device from fbdev,
a native driver takes over the hardware and the firmware framebuffer
becomes invalid.

Firmware-framebuffer drivers, specifically simplefb, don't release
their device from Linux' device hierarchy. It still owns the firmware
framebuffer and blocks the native drivers from loading. This has been
observed in the vmwgfx driver. [1]

Initiating a device removal (i.e., hot unplug) as part of
remove_conflicting_framebuffers() removes the underlying device and
returns the memory range to the system.

[1] https://lore.kernel.org/dri-devel/20220117180359.18114-1-zack@kde.org/

v2:
	* rename variable 'dev' to 'device' (Javier)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reported-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
CC: stable@vger.kernel.org # v5.11+
Link: https://patchwork.freedesktop.org/patch/msgid/20220125091222.21457-2-tzimmermann@suse.de

drivers/video/fbdev/core/fbmem.c

@@ -25,6 +25,7 @@
 #include <linux/init.h>
 #include <linux/linux_logo.h>
 #include <linux/proc_fs.h>
+#include <linux/platform_device.h>
 #include <linux/seq_file.h>
 #include <linux/console.h>
 #include <linux/kmod.h>
@@ -1557,18 +1558,36 @@ static void do_remove_conflicting_framebuffers(struct apertures_struct *a,
 	/* check all firmware fbs and kick off if the base addr overlaps */
 	for_each_registered_fb(i) {
 		struct apertures_struct *gen_aper;
+		struct device *device;
 
 		if (!(registered_fb[i]->flags & FBINFO_MISC_FIRMWARE))
 			continue;
 
 		gen_aper = registered_fb[i]->apertures;
+		device = registered_fb[i]->device;
 		if (fb_do_apertures_overlap(gen_aper, a) ||
 			(primary && gen_aper && gen_aper->count &&
 			 gen_aper->ranges[0].base == VGA_FB_PHYS)) {
 
 			printk(KERN_INFO "fb%d: switching to %s from %s\n",
 			       i, name, registered_fb[i]->fix.id);
-			do_unregister_framebuffer(registered_fb[i]);
+
+			/*
+			 * If we kick-out a firmware driver, we also want to remove
+			 * the underlying platform device, such as simple-framebuffer,
+			 * VESA, EFI, etc. A native driver will then be able to
+			 * allocate the memory range.
+			 *
+			 * If it's not a platform device, at least print a warning. A
+			 * fix would add code to remove the device from the system.
+			 */
+			if (dev_is_platform(device)) {
+				registered_fb[i]->forced_out = true;
+				platform_device_unregister(to_platform_device(device));
+			} else {
+				pr_warn("fb%d: cannot remove device\n", i);
+				do_unregister_framebuffer(registered_fb[i]);
+			}
 		}
 	}
 }
@@ -1851,9 +1870,13 @@ EXPORT_SYMBOL(register_framebuffer);
 void
 unregister_framebuffer(struct fb_info *fb_info)
 {
-	mutex_lock(&registration_lock);
+	bool forced_out = fb_info->forced_out;
+
+	if (!forced_out)
+		mutex_lock(&registration_lock);
 	do_unregister_framebuffer(fb_info);
-	mutex_unlock(&registration_lock);
+	if (!forced_out)
+		mutex_unlock(&registration_lock);
 }
 EXPORT_SYMBOL(unregister_framebuffer);
 

include/linux/fb.h

@@ -502,6 +502,7 @@ struct fb_info {
 	} *apertures;
 
 	bool skip_vt_switch; /* no VT switch on suspend/resume required */
+	bool forced_out; /* set when being removed by another driver */
 };
 
 static inline struct apertures_struct *alloc_apertures(unsigned int max_num) {