[RFC][PATCH v2] 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
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.

Suggestions or feedback 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 | 4 ++++
 drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.h | 1 +
 2 files changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.c b/drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.c
index 7ec93ae..b83556a 100644
--- a/drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.c
+++ b/drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.c
@@ -55,6 +55,7 @@ static void kirin_fbdev_output_poll_changed(struct drm_device *dev)
 {
 	struct kirin_drm_private *priv = dev->dev_private;
 
+	mutex_lock(&priv->fb_lock);
 	if (priv->fbdev) {
 		drm_fbdev_cma_hotplug_event(priv->fbdev);
 	} else {
@@ -63,6 +64,7 @@ static void kirin_fbdev_output_poll_changed(struct drm_device *dev)
 		if (IS_ERR(priv->fbdev))
 			priv->fbdev = NULL;
 	}
+	mutex_unlock(&priv->fb_lock);
 }
 #endif
 
@@ -95,6 +97,8 @@ static int kirin_drm_kms_init(struct drm_device *dev)
 	if (!priv)
 		return -ENOMEM;
 
+	mutex_init(&priv->fb_lock);
+
 	dev->dev_private = priv;
 	dev_set_drvdata(dev->dev, dev);
 
diff --git a/drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.h b/drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.h
index 7f60c649..9b6d2b1 100644
--- a/drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.h
+++ b/drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.h
@@ -23,6 +23,7 @@ struct kirin_drm_private {
 #ifdef CONFIG_DRM_FBDEV_EMULATION
 	struct drm_fbdev_cma *fbdev;
 #endif
+	struct mutex fb_lock;
 };
 
 extern const struct kirin_dc_ops ade_dc_ops;
-- 
2.7.4