[RFC][PATCH] drm: kirin: Add a mutex to avoid fb initialization race

[John Stultz <john.stultz@linaro.org>]

In some cases I've been seeing a race where two framebuffers
would be initialized, as kirin_fbdev_output_poll_changed()
might get called quickly in succession, resulting in the fb
initialization happening twice. This could cause the system
to boot up with a blank screen.

This patch adds a simple mutex to serialize it and seems to
avoid the race.

Obviously I suspect this patch isn't the best solution, but
I wanted to send it out as something concrete to discuss the
bug.

Suggestions or feedback for a better solution would be greatly
appreciated.

Cc: Xinliang Liu <z.liuxinliang@hisilicon.com>
Cc: Rongrong Zou <zourongrong@gmail.com>
Cc: Xinwei Kong <kong.kongxinwei@hisilicon.com>
Cc: Chen Feng <puck.chen@hisilicon.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
 drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.c b/drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.c
index ebd5f4f..80c607f 100644
--- a/drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.c
+++ b/drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.c
@@ -50,11 +50,13 @@ static int kirin_drm_kms_cleanup(struct drm_device *dev)
 	return 0;
 }
 
+static DEFINE_MUTEX(fb_lock);
 #ifdef CONFIG_DRM_FBDEV_EMULATION
 static void kirin_fbdev_output_poll_changed(struct drm_device *dev)
 {
 	struct kirin_drm_private *priv = dev->dev_private;
 
+	mutex_lock(&fb_lock);
 	if (priv->fbdev) {
 		drm_fbdev_cma_hotplug_event(priv->fbdev);
 	} else {
@@ -64,6 +66,7 @@ static void kirin_fbdev_output_poll_changed(struct drm_device *dev)
 		if (IS_ERR(priv->fbdev))
 			priv->fbdev = NULL;
 	}
+	mutex_unlock(&fb_lock);
 }
 #endif
 
-- 
2.7.4