Re: [PATCH] drm/bridge: anx7625: Drop device lock before drm_helper_hpd_irq_event()

[Nícolas F. R. A. Prado]

Hi,

On Mon, Jan 16, 2023 at 03:22:34PM +0800, Chen-Yu Tsai wrote:
> The device lock is used to serialize the low level power sequencing
> operations. Since drm_helper_hpd_irq_event() could end up calling
> .atomic_enable, which also calls power sequencing functions through
> runtime PM, this results in a real deadlock. This was observed on an
> MT8192-based Chromebook's external display (with appropriate patches [1]
> and DT changes applied).
> 
> Move the drm_helper_hpd_irq_event() call outside of the lock range. The
> lock only needs to be held so that the device status can be read back.
> This is the bare minimum change to avoid the deadlock. The lock could
> be dropped completely and have pm_runtime_get_if_in_use() increase the
> reference count, but this is not the same as pm_runtime_suspended().
> This also causes the internal display of the same device to not
> function correctly. Both the internal and external display of said
> device each use one anx7625 bridge.
> 
> [1] https://lore.kernel.org/dri-devel/20230112042104.4107253-1-treapking@xxxxxxxxxxxx/
> 
> Fixes: 60487584a79a ("drm/bridge: anx7625: refactor power control to use runtime PM framework")
> Signed-off-by: Chen-Yu Tsai <wenst@xxxxxxxxxxxx>
> ---
> FWIW I'm aware that this driver could be refactored a lot better.
> The work function might be simplified and merged into the threaded
> interrupt handler. The .detect op should be reading the HPD state
> from the hardware, not some cached state.
> 
>  drivers/gpu/drm/bridge/analogix/anx7625.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/analogix/anx7625.c b/drivers/gpu/drm/bridge/analogix/anx7625.c
> index 7e1fb93a6ce4..bf1770b79bba 100644
> --- a/drivers/gpu/drm/bridge/analogix/anx7625.c
> +++ b/drivers/gpu/drm/bridge/analogix/anx7625.c
> @@ -1597,18 +1597,17 @@ static void anx7625_work_func(struct work_struct *work)
>  
>  	mutex_lock(&ctx->lock);
>  
> -	if (pm_runtime_suspended(&ctx->client->dev))
> -		goto unlock;
> +	if (pm_runtime_suspended(&ctx->client->dev)) {
> +		mutex_unlock(&ctx->lock);
> +		return;
> +	}
>  
>  	event = anx7625_hpd_change_detect(ctx);
> -	if (event < 0)

Are you intentionally dropping this early-return on error?

> -		goto unlock;
> +
> +	mutex_unlock(&ctx->lock);
>  
>  	if (ctx->bridge_attached)
>  		drm_helper_hpd_irq_event(ctx->bridge.dev);
> -
> -unlock:
> -	mutex_unlock(&ctx->lock);
>  }
>  
>  static irqreturn_t anx7625_intr_hpd_isr(int irq, void *data)
> -- 
> 2.39.0.314.g84b9a713c41-goog
>