fbdev: fbmem: add a helper to determine if an aperture is used by a fw fb

Add a function for drivers to check if the a firmware initialized
fb is corresponds to their aperture. This allows drivers to check if the
device corresponds to what the firmware set up as the display device.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=215203
Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1840
Signed-off-by: Alex Deucher <[e-mail]>

drivers/video/fbdev/core/fbmem.c

···
 EXPORT_SYMBOL(remove_conflicting_framebuffers);
 
 /**
+ * is_firmware_framebuffer - detect if firmware-configured framebuffer matches
+ * @a: memory range, users of which are to be checked
+ *
+ * This function checks framebuffer devices (initialized by firmware/bootloader)
+ * which use memory range described by @a. If @a matchesm the function returns
+ * true, otherwise false.
+ */
+bool is_firmware_framebuffer(struct apertures_struct *a)
+{
+	bool do_free = false;
+	bool found = false;
+	int i;
+
+	if (!a) {
+		a = alloc_apertures(1);
+		if (!a)
+			return false;
+
+		a->ranges[0].base = 0;
+		a->ranges[0].size = ~0;
+		do_free = true;
+	}
+
+	mutex_lock(&registration_lock);
+	/* check all firmware fbs and kick off if the base addr overlaps */
+	for_each_registered_fb(i) {
+		struct apertures_struct *gen_aper;
+
+		if (!(registered_fb[i]->flags & FBINFO_MISC_FIRMWARE))
+			continue;
+
+		gen_aper = registered_fb[i]->apertures;
+		if (fb_do_apertures_overlap(gen_aper, a)) {
+			found = true;
+			break;
+		}
+	}
+	mutex_unlock(&registration_lock);
+
+	if (do_free)
+		kfree(a);
+
+	return found;
+}
+EXPORT_SYMBOL(is_firmware_framebuffer);
+
+/**
  * remove_conflicting_pci_framebuffers - remove firmware-configured framebuffers for PCI devices
  * @pdev: PCI device
  * @name: requesting driver name

include/linux/fb.h

···
 					       const char *name);
 extern int remove_conflicting_framebuffers(struct apertures_struct *a,
 					   const char *name, bool primary);
+extern bool is_firmware_framebuffer(struct apertures_struct *a);
 extern int fb_prepare_logo(struct fb_info *fb_info, int rotate);
 extern int fb_show_logo(struct fb_info *fb_info, int rotate);
 extern char* fb_get_buffer_offset(struct fb_info *info, struct fb_pixmap *buf, u32 size);