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

[Maxime Ripard <mripard@kernel.org>]

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On Sat, Sep 14, 2024 at 10:12:29PM GMT, Cristian Ciocaltea wrote:
> Hi Maxime,
> 
> On 9/9/24 6:13 PM, Maxime Ripard wrote:
> > 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
> 
> Thanks for all the details and references!
> 
> Unfortunately I had to drop the scrambling setup for now [1], as I
> encountered some issues while attempting to get this implemented as
> suggested.  Will get back to this and submit it separately when done.

Yeah, I think that's the best way forward for now :)

Maxime

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