video: fb_defio: preserve user fb_ops

Modifying fb_ops directly to override fb_mmap with fb_deferred_io_mmap
and then resetting it to NULL afterwards causes problems all over the
place. First, it prevents making the fbops member of struct fb_info a
const pointer, which means we can't make struct fb_ops const
anywhere. Second, a few places have to go out of their way to restore
the original fb_mmap pointer that gets reset to NULL.

Since the only user of the fbops->fb_mmap hook is fb_mmap() in fbmem.c,
call fb_deferred_io_mmap() directly when deferred IO is enabled, and
avoid modifying fb_ops altogether.

Simply use info->fbdefio to determine whether deferred IO should be used
or not. This should be accurate enough for all use cases, although
perhaps not pedantically correct.

v2: Simplify considerably by calling fb_deferred_io_mmap() directly
(Daniel, Ville)

Cc: Jaya Kumar <[e-mail]>
Cc: [e-mail]
Cc: Daniel Vetter <[e-mail]>
Cc: Ville Syrjälä <[e-mail]>
Acked-by: Noralf Trønnes <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Signed-off-by: Jani Nikula <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/022[e-mail]

drivers/video/fbdev/core/fb_defio.c

···
 	vma->vm_private_data = info;
 	return 0;
 }
-EXPORT_SYMBOL(fb_deferred_io_mmap);
 
 /* workqueue callback */
 static void fb_deferred_io_work(struct work_struct *work)
···
 
 	BUG_ON(!fbdefio);
 	mutex_init(&fbdefio->lock);
-	info->fbops->fb_mmap = fb_deferred_io_mmap;
 	INIT_DELAYED_WORK(&info->deferred_work, fb_deferred_io_work);
 	INIT_LIST_HEAD(&fbdefio->pagelist);
 	if (fbdefio->delay == 0) /* set a default of 1 s */
···
 		page->mapping = NULL;
 	}
 
-	info->fbops->fb_mmap = NULL;
 	mutex_destroy(&fbdefio->lock);
 }
 EXPORT_SYMBOL_GPL(fb_deferred_io_cleanup);

drivers/video/fbdev/core/fbmem.c

···
 fb_mmap(struct file *file, struct vm_area_struct * vma)
 {
 	struct fb_info *info = file_fb_info(file);
-	struct fb_ops *fb;
+	int (*fb_mmap_fn)(struct fb_info *info, struct vm_area_struct *vma);
 	unsigned long mmio_pgoff;
 	unsigned long start;
 	u32 len;
 
 	if (!info)
 		return -ENODEV;
-	fb = info->fbops;
 	mutex_lock(&info->mm_lock);
-	if (fb->fb_mmap) {
+
+	fb_mmap_fn = info->fbops->fb_mmap;
+
+#if IS_ENABLED(CONFIG_FB_DEFERRED_IO)
+	if (info->fbdefio)
+		fb_mmap_fn = fb_deferred_io_mmap;
+#endif
+
+	if (fb_mmap_fn) {
 		int res;
 
 		/*
···
 		 * SME protection is removed ahead of the call
 		 */
 		vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
-		res = fb->fb_mmap(info, vma);
+		res = fb_mmap_fn(info, vma);
 		mutex_unlock(&info->mm_lock);
 		return res;
 	}