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

[Daniel Vetter]

On Fri, Feb 24, 2017 at 05:25:16PM -0800, John Stultz wrote:
> 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!

I feel a bit like a broken record, but:

Instead of reinventing broken delayed fbdev setup everywhere, can we
polish Thierry's patches to move that into the fbdev helpers in the core?
hisilicon isn't the only driver the (re)invents this weel, it'd be really
awesome if we could fix this bug just once ...

For reference, the patches:

http://markmail.org/message/d3jc4vebkndtvlkf#query:+page:1+mid:d3jc4vebkndtvlkf+state:results

I guess Thierry got sidetracked on these, but except for a bit of locking
scheme polish I think they've been mostly ready. Shouldn't be much work to
refresh them, hunt for other new drivers reinventing this wheel, do the
locking polish maybe and then get them landed.

Thanks, Daniel

> 
> Cc: Xinliang Liu <z.liuxinliang at hisilicon.com>
> Cc: Rongrong Zou <zourongrong at gmail.com>
> Cc: Xinwei Kong <kong.kongxinwei at hisilicon.com>
> Cc: Chen Feng <puck.chen at hisilicon.com>
> Cc: David Airlie <airlied at linux.ie>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Sean Paul <seanpaul at chromium.org>
> Cc: dri-devel at lists.freedesktop.org
> Signed-off-by: John Stultz <john.stultz at 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
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch