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

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Message-ID: <f8b17995-ce53-45ab-8e68-c7087dbc9786@collabora.com>
Date: Sat, 14 Sep 2024 22:12:29 +0300
From: Cristian Ciocaltea <cristian.ciocaltea@...labora.com>
To: Maxime Ripard <mripard@...nel.org>
Cc: Andrzej Hajda <andrzej.hajda@...el.com>,
 Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>,
 Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
 Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
 Daniel Vetter <daniel@...ll.ch>, Sandy Huang <hjc@...k-chips.com>,
 Heiko Stübner <heiko@...ech.de>,
 Andy Yan <andy.yan@...k-chips.com>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Mark Yao <markyao0591@...il.com>,
 Sascha Hauer <s.hauer@...gutronix.de>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-rockchip@...ts.infradead.org, devicetree@...r.kernel.org,
 kernel@...labora.com, Alexandre ARNOUD <aarnoud@...com>,
 Luis de Arquer <ldearquer@...il.com>, Algea Cao <algea.cao@...k-chips.com>
Subject: Re: [PATCH v6 1/3] drm/bridge: synopsys: Add DW HDMI QP TX Controller
 support library

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.

Regards,
Cristian

[1] https://lore.kernel.org/lkml/20240914-b4-rk3588-bridge-upstream-v7-0-2b1348137123@collabora.com/