linux-kernel - Re: [PATCH 2/3 v3] drm: bridge/dw-hdmi: Enable ISCR1, ISCR2 and ACP packets

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Date:	Thu, 4 Aug 2016 16:32:07 +0100
From:	Russell King - ARM Linux <linux@...linux.org.uk>
To:	Jose Abreu <Jose.Abreu@...opsys.com>
Cc:	dri-devel@...ts.freedesktop.org,
	Carlos Palminha <CARLOS.PALMINHA@...opsys.com>,
	Archit Taneja <architt@...eaurora.org>,
	David Airlie <airlied@...ux.ie>,
	Fabio Estevam <fabio.estevam@...escale.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Takashi Iwai <tiwai@...e.de>,
	Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>,
	Thierry Reding <treding@...dia.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3 v3] drm: bridge/dw-hdmi: Enable ISCR1, ISCR2 and ACP
 packets

On Thu, Aug 04, 2016 at 11:44:50AM +0100, Jose Abreu wrote:
> Currently ISCR and ACP packets are not being sent causing
> HDMI compliance tests like CTS 7-19 HDMI 1.4b to fail.

Hmm.  Reading the HDMI specifications (v1.3, being the publically
available one), the specification does _not_ say that a source _must_
transmit ISRC and ACP packets if Supports_AI is set.

What it does say is that:
* a sink shall set Supports_AI if it supports at least one function
  which uses ISRC or ACP packets, and it shall accept such packets.
* a source shall not transmit ISRC or ACP packets to a sink that
  has Supports_AI=0.

I don't see anything that mandates the transmission of ISRC and ACP
packets when Supports_AI is set.

Moreover, transmission of wrong ISRC information probably goes against
the HDMI spec section 8.8, which requires specific ISRC status values
in HB1 - I don't see how that _could_ be handled, as the kernel has no
knowledge about the EAN of the audio content being played, nor when a
track starts or finishes.

Please clarify.

> +	/* Set ISCR1, ISCR2, and ACP packets to automatic scheduling */
> +	if (hdmi->sink_supports_ai) {
> +		dev_dbg(hdmi->dev, "sink supports AI packets\n");
> +		hdmi_writeb(hdmi, 0x06, HDMI_FC_ISCR1_0);
> +		hdmi_writeb(hdmi, 0x03, HDMI_FC_DATAUTO0);
> +		hdmi_writeb(hdmi, 0x01, HDMI_FC_DATAUTO1);
> +		hdmi_writeb(hdmi, 0x11, HDMI_FC_DATAUTO2);

So what data gets sent in these packets - I guess whatever random data
happens to be in the HDMI_FC_ACP* and HDMI_FC_ISCR1_* registers at the
time the sending is enabled.  I don't think that's particularly a good
idea.

> @@ -1474,8 +1490,13 @@ static int dw_hdmi_connector_get_modes(struct drm_connector *connector)
>  		/* Store the ELD */
>  		drm_edid_to_eld(connector, edid);
>  		kfree(edid);
> +
> +		hdmi->sink_supports_ai = connector->eld[5] & (0x1 << 1);

We have a definition for this bit - DRM_ELD_SUPPORTS_AI.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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