Re: [PATCH v3 09/13] drm: bridge: dw_hdmi: Use delayed_work to debounce hotplug event

[Nicolas Frattaroli <nicolas.frattaroli@collabora.com>]

On Friday, 3 April 2026 20:52:55 Central European Summer Time Jonas Karlman wrote:
> HDMI Specification Version 1.4b chapter 8.5 mentions:
> 
>   An HDMI Sink shall not assert high voltage level on its Hot Plug
>   Detect pin when the E-EDID is not available for reading.
> 
>   A Source may use a high voltage level Hot Plug Detect signal to
>   initiate the reading of E-EDID data.
> 
>   An HDMI Sink shall indicate any change to the contents of the E-EDID
>   by driving a low voltage level pulse on the Hot Plug Detect pin. This
>   pulse shall be at least 100 msec.
> 
> Use a work queue to debounce reacting on HPD events to better handle a
> HPD low voltage level pulse when a sink changes the EDID.
> 
> The 1100 msec hotplug debounce timeout was arbitrarily picked to match
> other drivers using same const, and testing using a Raspberry Pi Monitor
> seem to use a 200-300 msec pulse when going from standby to power on
> state.
> 
> Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
> ---
> v3: New patch
> ---
>  drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 24 +++++++++++++++++++----
>  1 file changed, 20 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> index f3f8144ae98e..4d079b689b3b 100644
> --- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> +++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> @@ -51,6 +51,8 @@
>  
>  #define HDMI14_MAX_TMDSCLK	340000000
>  
> +#define HOTPLUG_DEBOUNCE_MS	1100
> +
>  static const u16 csc_coeff_default[3][4] = {
>  	{ 0x2000, 0x0000, 0x0000, 0x0000 },
>  	{ 0x0000, 0x2000, 0x0000, 0x0000 },
> @@ -192,6 +194,7 @@ struct dw_hdmi {
>  	hdmi_codec_plugged_cb plugged_cb;
>  	struct device *codec_dev;
>  	enum drm_connector_status last_connector_result;
> +	struct delayed_work hpd_work;
>  };
>  
>  const struct dw_hdmi_plat_data *dw_hdmi_to_plat_data(struct dw_hdmi *hdmi)
> @@ -2528,6 +2531,7 @@ static void dw_hdmi_connector_force(struct drm_connector *connector)
>  
>  	mutex_lock(&hdmi->mutex);
>  	hdmi->force = connector->force;
> +	hdmi->last_connector_result = connector->status;
>  	dw_hdmi_update_phy_mask(hdmi);
>  	mutex_unlock(&hdmi->mutex);
>  }
> @@ -3046,6 +3050,16 @@ void dw_hdmi_setup_rx_sense(struct dw_hdmi *hdmi, bool hpd, bool rx_sense)
>  }
>  EXPORT_SYMBOL_GPL(dw_hdmi_setup_rx_sense);
>  
> +static void dw_hdmi_hpd_work(struct work_struct *work)
> +{
> +	struct dw_hdmi *hdmi = container_of(work, struct dw_hdmi, hpd_work.work);
> +
> +	if (hdmi->bridge.dev) {
> +		drm_helper_hpd_irq_event(hdmi->bridge.dev);
> +		drm_bridge_hpd_notify(&hdmi->bridge, hdmi->last_connector_result);

I think you'll want to `mutex_lock(&hdmi->mutex);` (or better yet,
`guard(mutex)(&hdmi->mutex)` or `scoped_guard(mutex, &hdmi->mutex) {`)
here before reading last_connector_result, otherwise you'll have a
data race with any writers.

> +	}
> +}
> +
>  static irqreturn_t dw_hdmi_irq(int irq, void *dev_id)
>  {
>  	struct dw_hdmi *hdmi = dev_id;
> @@ -3097,10 +3111,8 @@ static irqreturn_t dw_hdmi_irq(int irq, void *dev_id)
>  			status == connector_status_connected ?
>  			"plugin" : "plugout");
>  
> -		if (hdmi->bridge.dev) {
> -			drm_helper_hpd_irq_event(hdmi->bridge.dev);
> -			drm_bridge_hpd_notify(&hdmi->bridge, status);
> -		}
> +		mod_delayed_work(system_percpu_wq, &hdmi->hpd_work,
> +				 msecs_to_jiffies(HOTPLUG_DEBOUNCE_MS));
>  	}
>  
>  	hdmi_writeb(hdmi, intr_stat, HDMI_IH_PHY_STAT0);
> @@ -3420,6 +3432,8 @@ struct dw_hdmi *dw_hdmi_probe(struct platform_device *pdev,
>  		goto err_res;
>  	}
>  
> +	INIT_DELAYED_WORK(&hdmi->hpd_work, dw_hdmi_hpd_work);
> +
>  	ret = devm_request_threaded_irq(dev, irq, dw_hdmi_hardirq,
>  					dw_hdmi_irq, IRQF_SHARED,
>  					dev_name(dev), hdmi);
> @@ -3552,6 +3566,8 @@ EXPORT_SYMBOL_GPL(dw_hdmi_probe);
>  
>  void dw_hdmi_remove(struct dw_hdmi *hdmi)
>  {
> +	disable_delayed_work_sync(&hdmi->hpd_work);
> +
>  	drm_bridge_remove(&hdmi->bridge);
>  
>  	if (hdmi->audio && !IS_ERR(hdmi->audio))
> 

Kind regards,
Nicolas Frattaroli



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