linux-kernel - Re: [PATCH] drm: bridge/dw_hdmi: Filter modes

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Date:	Thu, 18 Jun 2015 00:30:40 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Doug Anderson <dianders@...omium.org>
Cc:	Philipp Zabel <p.zabel@...gutronix.de>,
	Thierry Reding <treding@...dia.com>,
	Heiko Stuebner <heiko@...ech.de>,
	David Airlie <airlied@...ux.ie>,
	Andy Yan <andy.yan@...k-chips.com>,
	Yakir Yang <ykk@...k-chips.com>,
	Fabio Estevam <fabio.estevam@...escale.com>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm: bridge/dw_hdmi: Filter modes > 165MHz for DVI

On Wed, Jun 17, 2015 at 04:14:07PM -0700, Doug Anderson wrote:
> If you plug in a DVI monitor to your HDMI port, you need to filter out
> clocks > 165MHz.  That's because 165MHz is the maximum clock rate that
> we can run single-link DVI at.
> 
> If you want to run high resolutions to DVI, you'd need some type of an
> active adapter that pretended that it was HDMI, interpreted the
> signal, and produced a new dual link DVI signal at a lower clock rate.
> 
> Signed-off-by: Doug Anderson <dianders@...omium.org>
> ---
> Note: this patch was tested against a 3.14 kernel with backports.  It
> was only compile tested against linuxnext, but the code is
> sufficiently similar that I'm convinced it will work there.

Really?  I have to wonder what your testing was...

        hdmi->vic = drm_match_cea_mode(mode);

        if (!hdmi->vic) {
                dev_dbg(hdmi->dev, "Non-CEA mode used in HDMI\n");
                hdmi->hdmi_data.video_mode.mdvi = true;
        } else {
                dev_dbg(hdmi->dev, "CEA mode used vic=%d\n", hdmi->vic);
                hdmi->hdmi_data.video_mode.mdvi = false;
        }

mdvi indicates whether the _currently set mode_ is a CEA mode or not (imho,
it's mis-named).  It doesn't indicate whether we have a HDMI display device
or a DVI display device connected, which seems to be what you want to use
it for below.

To sort that, what you need to do is detect a HDMI display device using
drm_detect_hdmi_monitor() on the EDID received from the device before
parsing the modes, and save that value in a dw_hdmi struct member, and
I'd suggest that it's a top-level struct member, not buried in 'hdmi_data'
or 'video_mode'.

-- 
FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up
according to speedtest.net.
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