[PATCH 10/10] drm/fb-helper: Acquire modeset lock around shadow-buffer flushing

[Thomas Zimmermann]

Flushing the fbdev's shadow buffer requires vmap'ing the BO memory, which
in turn requires pinning the BO. While being pinned, the BO cannot be moved
into VRAM for scanout. Consequently, a concurrent modeset operation that
involves the fbdev framebuffer would likely fail.

Resolve this problem be acquiring the modeset lock of the planes that use
the fbdev framebuffer. On non-atomic drivers, also acquire the mode-config
lock. This serializes the flushing of the framebuffer with concurrent
modeset operations.

Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
---
 drivers/gpu/drm/drm_fb_helper.c | 43 +++++++++++++++++++++++++++++++--
 1 file changed, 41 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 5a22c744378c..af485c71a42a 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -394,20 +394,59 @@ static void drm_fb_helper_damage_blit_real(struct drm_fb_helper *fb_helper,
 static int drm_fb_helper_damage_blit(struct drm_fb_helper *fb_helper,
 				     struct drm_clip_rect *clip)
 {
+	struct drm_device *dev = fb_helper->dev;
+	struct drm_framebuffer *fb = fb_helper->fb;
 	struct drm_client_buffer *buffer = fb_helper->buffer;
+	struct drm_modeset_acquire_ctx ctx;
+	struct drm_framebuffer *plane_fb;
+	struct drm_plane *plane;
 	struct dma_buf_map map, dst;
 	int ret;
 
+	if (!drm_drv_uses_atomic_modeset(dev))
+		mutex_lock(&dev->mode_config.mutex);
+
+	drm_modeset_acquire_init(&ctx, 0);
+
+retry:
+	drm_for_each_plane(plane, dev) {
+		ret = drm_modeset_lock(&plane->mutex, &ctx);
+		if (ret == -EDEADLK) {
+			ret = drm_modeset_backoff(&ctx);
+			if (!ret)
+				goto retry;
+		} else if (ret) {
+			goto out;
+		}
+
+		if (drm_drv_uses_atomic_modeset(dev))
+			plane_fb = plane->state->fb;
+		else
+			plane_fb = plane->fb;
+
+		if (plane_fb != fb) {
+			drm_modeset_unlock(&plane->mutex);
+			continue;
+		}
+	}
+
 	ret = drm_client_buffer_vmap(buffer, &map);
 	if (ret)
-		return ret;
+		goto out;
 
 	dst = map;
 	drm_fb_helper_damage_blit_real(fb_helper, clip, &dst);
 
 	drm_client_buffer_vunmap(buffer);
 
-	return 0;
+out:
+	drm_modeset_drop_locks(&ctx);
+	drm_modeset_acquire_fini(&ctx);
+
+	if (!drm_drv_uses_atomic_modeset(dev))
+		mutex_unlock(&dev->mode_config.mutex);
+
+	return ret;
 }
 
 static void drm_fb_helper_damage_work(struct work_struct *work)
-- 
2.29.2