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

[liuxinliang]

Hi John,

On 2017/2/23 8:56, 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 fb
> initialization happening twice. This could cause the system

I might understand this race. This because two places call 
drm_helper_hpd_irq_event might cause the race:
One place is here
static int kirin_drm_kms_init(struct drm_device *dev)
{
...
     /* force detection after connectors init */
     (void)drm_helper_hpd_irq_event(dev);
...
}

another is the adv7533 interrupt thread handler
static int adv7511_irq_process(struct adv7511 *adv7511, bool process_hpd)
{
...
     if (process_hpd && irq0 & ADV7511_INT0_HPD && adv7511->bridge.encoder)
         drm_helper_hpd_irq_event(adv7511->connector.dev);
...
}

right?

I don't get a better way to fix this yet , I like to put fb_lock into 
kirin_drm_private.
And it seems hard to fix this in the core drm_helper_hpd_irq_event.

-xinliang

> 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 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 | 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
>