Re: [PATCH] drm: bridge/dw_hdmi: Filter modes > 165MHz for DVI

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Wed, Jun 17, 2015 at 07:52:14PM -0700, Doug Anderson wrote:
> Russell,
> 
> On Wed, Jun 17, 2015 at 4:30 PM, Russell King - ARM Linux
> <linux@arm.linux.org.uk> wrote:
> > 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@chromium.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'.
> 
> OK, so clearly my patch won't work against mainline.  I guess it's a
> good thing that I pointed out that it was only tested locally (would
> have been better to test against mainline, but I don't think that's so
> easy since there are several unlanded patches in mainline for
> Rockchip).

As far as I'm aware, Freescale's original BSP version was the same, as is
their later BSPs, and Jon's maintained 3.14-stable kernel.

> As pointed out by others at <http://crosreview.com/278255>, locally
> our kernel has a slightly older version of
> <https://lkml.org/lkml/2015/2/28/291>, which would change mdvi to be
> as needed.

Please don't post unreliable lkml.org URLs, please use some other archive
site.  I can't access this URL at the moment.

> ...so I guess my change is blocked on someone reviewing/landing that
> series.  If that series is rejected (or is changed sufficiently so
> that mdvi no longer is set via drm_detect_hdmi_monitor() then my patch
> will need to be re-spun.

That's not what I said.  I said mdvi is set according to whether the mode
being set is a CEA mode or not.  We need something set according to
the return value of drm_detect_hdmi_monitor(), which will tell us if the
connected sink is a HDMI device or a DVI device (based upon the EDID.)

A thought occurs to me this morning though: what happens if you connect
a DVI monitor to an AV receiver which is then connected to this device.
Does the resulting EDID contain the HDMI vendor ID?  If it does, it
means that drm_detect_hdmi_monitor() will return true, indicating that
the connected device is HDMI, and we will still allow modes greater than
165MHz.

That's probably a scenario that should be checked at some point... and
it would throw a question mark over whether this is the correct approach
to limit the video modes.

-- 
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