fbdev/deferred-io: Provide get_page hook in struct fb_deferred_io

Add a callback for drivers to provide framebuffer pages to fbdev's
deferred-I/O helpers. Implementations need to acquire a reference on
the page before returning it. Returning NULL generates a SIGBUS
signal.

This will be useful for DRM's fbdev emulation with GEM-shmem buffer
objects.

v2:
- fix typo in commit message (Javier)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240419083331.7761-8-tzimmermann@suse.de

drivers/video/fbdev/core/fb_defio.c

@@ -25,9 +25,13 @@
 
 static struct page *fb_deferred_io_get_page(struct fb_info *info, unsigned long offs)
 {
+	struct fb_deferred_io *fbdefio = info->fbdefio;
 	const void *screen_buffer = info->screen_buffer;
 	struct page *page = NULL;
 
+	if (fbdefio->get_page)
+		return fbdefio->get_page(info, offs);
+
 	if (is_vmalloc_addr(screen_buffer + offs))
 		page = vmalloc_to_page(screen_buffer + offs);
 	else if (info->fix.smem_start)

include/linux/fb.h

@@ -225,6 +225,7 @@ struct fb_deferred_io {
 	struct mutex lock; /* mutex that protects the pageref list */
 	struct list_head pagereflist; /* list of pagerefs for touched pages */
 	/* callback */
+	struct page *(*get_page)(struct fb_info *info, unsigned long offset);
 	void (*deferred_io)(struct fb_info *info, struct list_head *pagelist);
 };
 #endif