drm/fb-helper: Fix vertical damage clipping

Don't clip the damage rectangle against the viewport. This only
works if the viewport is located at the beginning of the video
memory and the video memory doesn't extend the screen (i.e., if
there's no overallocation).

Fbdev emulation transfers data from write operations into a
possible shadow buffer, then into a GEM buffer object, and finally
via graphics driver onto the screen.

If callers write outside the currently visible area, clipping the
damage rectangle against the viewport will loose these updates in
the shadow buffer and the fbdev's buffer object will contain stale
data. Panning the viewport to the stale area of the buffer will
display obsolete data.

Instead, mark all written areas as damaged, so that the damage
handler updates the buffer object from the shadow buffer for all
such areas. The graphics driver's later has the option of clipping
the damaged area against the viewport when updating the screen
from the buffer object.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220209161617.3553-3-tzimmermann@suse.de

drivers/gpu/drm/drm_fb_helper.c

@@ -706,8 +706,7 @@ void drm_fb_helper_deferred_io(struct fb_info *info,
 
 	if (min < max) {
 		y1 = min / info->fix.line_length;
-		y2 = min_t(u32, DIV_ROUND_UP(max, info->fix.line_length),
-			   info->var.yres);
+		y2 = DIV_ROUND_UP(max, info->fix.line_length);
 		drm_fb_helper_damage(info, 0, y1, info->var.xres, y2 - y1);
 	}
 }