[PATCH v6 1/3] drm/bridge: synopsys: Add DW HDMI QP TX Controller support library

[Maxime Ripard]

Hi,

On Fri, Sep 06, 2024 at 04:17:40AM GMT, Cristian Ciocaltea wrote:
> +static enum drm_connector_status
> +dw_hdmi_qp_bridge_detect(struct drm_bridge *bridge)
> +{
> +	struct dw_hdmi_qp *hdmi = bridge->driver_private;
> +	enum drm_connector_status status;
> +
> +	status = hdmi->phy.ops->read_hpd(hdmi, hdmi->phy.data);
> +
> +	dev_dbg(hdmi->dev, "%s conn=%d scramb=%d\n", __func__,
> +		status == connector_status_connected, hdmi->scramb_enabled);
> +
> +	if (hdmi->scramb_enabled) {
> +		cancel_delayed_work_sync(&hdmi->scramb_work);
> +
> +		if (status == connector_status_connected)
> +			dw_hdmi_qp_check_and_set_scramb(hdmi);
> +	}
> +
> +	return status;
> +}

Unfortunately, that won't work. The HDMI Spec has (HDMI 2.0, Section
6.1.3.1 - Scrambling Control):

The minimum time period between the write to the Scrambling_Enable bit,
and the transmission of a scrambled video signal is not specified;
however the Source shall not begin transmission of a scrambled video
signal before writing a 1 to the Scrambling_Enable bit. The maximum time
period between the write to the Scrambling_Enable bit and the
transmission of a scrambled video signal shall be 100 ms.

So you need to disable the output and enable it again.

vc4 does just that, you can have a look here:
https://elixir.bootlin.com/linux/v6.10.9/source/drivers/gpu/drm/vc4/vc4_hdmi.c#L410

Maxime
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