[PATCH] drm/fb-helper: flush damage before unblanking

[Angelo Compagnucci <angelo.compagnucci@gmail.com>]

With a shadow buffer and deferred I/O, what userspace draws in fbdev
reaches the DRM framebuffer only once the deferred I/O work and then the
damage work have run. Unblanking turns the display on with the
framebuffer as it is: a program drawing its first picture and then
unblanking, such as a boot splash taking over the picture a bootloader
left on a panel, shows the previous framebuffer contents for a moment.

Seen on an Arietta G25 with an ILI9341 panel (tinydrm ili9341): enabling
the pipe sends the whole framebuffer to the panel, still black, and the
splash only replaces it once the damage work runs, a visible flicker.

On unblank, flush the deferred I/O and the damage work first.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
---
 drivers/gpu/drm/clients/drm_fbdev_helper.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/drivers/gpu/drm/clients/drm_fbdev_helper.c b/drivers/gpu/drm/clients/drm_fbdev_helper.c
index 415764ee086a..5c584dee3e55 100644
--- a/drivers/gpu/drm/clients/drm_fbdev_helper.c
+++ b/drivers/gpu/drm/clients/drm_fbdev_helper.c
@@ -150,6 +150,22 @@ static void drm_fb_helper_dpms(struct fb_info *info, int dpms_mode)
 	mutex_unlock(&fb_helper->lock);
 }
 
+/*
+ * Turning a display on sends it the framebuffer: first get there what was
+ * drawn in fbdev, still pending in deferred I/O or damage handling, else the
+ * display shows the framebuffer as it was before for a moment.
+ */
+static void drm_fb_helper_flush_damage(struct fb_info *info)
+{
+	struct drm_fb_helper *helper = info->par;
+
+#ifdef CONFIG_FB_DEFERRED_IO
+	if (info->fbdefio)
+		flush_delayed_work(&info->deferred_work);
+#endif
+	flush_work(&helper->damage_work);
+}
+
 /**
  * drm_fb_helper_blank - implementation for &fb_ops.fb_blank
  * @blank: desired blanking state
@@ -163,6 +179,7 @@ int drm_fb_helper_blank(int blank, struct fb_info *info)
 	switch (blank) {
 	/* Display: On; HSync: On, VSync: On */
 	case FB_BLANK_UNBLANK:
+		drm_fb_helper_flush_damage(info);
 		drm_fb_helper_dpms(info, DRM_MODE_DPMS_ON);
 		break;
 	/* Display: Off; HSync: On, VSync: On */
-- 
2.53.0