[RFC][PATCH v2] drm: kirin: Add a mutex to avoid fb initialization race

[John Stultz]

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.liuxinli...@hisilicon.com>
Cc: Rongrong Zou <zourongr...@gmail.com>
Cc: Xinwei Kong <kong.kongxin...@hisilicon.com>
Cc: Chen Feng <puck.c...@hisilicon.com>
Cc: David Airlie <airl...@linux.ie>
Cc: Daniel Vetter <daniel.vet...@ffwll.ch>
Cc: Sean Paul <seanp...@chromium.org>
Cc: dri-de...@lists.freedesktop.org
Signed-off-by: John Stultz <john.stu...@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