Re: Valid DP connection without EDID?

From: Adam Jackson <ajax@redhat.com>
To: Takashi Iwai <tiwai@suse.de>
Cc: mmarek@suse.cz, intel-gfx@lists.freedesktop.org
Subject: Re: Valid DP connection without EDID?
Date: Mon, 17 Sep 2012 12:24:36 -0400	[thread overview]
Message-ID: <1347899076.25266.46.camel@atropine> (raw)
In-Reply-To: <s5hsjahxae5.wl%tiwai@suse.de>

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On Mon, 2012-09-17 at 11:32 +0200, Takashi Iwai wrote:
> At Mon, 17 Sep 2012 19:11:34 +1000,
> Dave Airlie wrote:
> > On Mon, Sep 17, 2012 at 6:28 PM, Takashi Iwai <tiwai@suse.de> wrote:
> > > At Mon, 17 Sep 2012 18:21:47 +1000,
> > > Dave Airlie wrote:
> > >> On Mon, Sep 17, 2012 at 6:16 PM, Takashi Iwai <tiwai@suse.de> wrote:
> > >> > At Fri, 14 Sep 2012 11:25:03 -0400,
> > >> > Adam Jackson wrote:
> > >> >>
> > >> >> Can you be more specific here?  What DPCD does it return?
> > >> >
> > >> > It shows "DPCD: 110a820100030181"

So dpcd[0x5] is 0x03, and drm_dp_helper.h says:

#define DP_DOWNSTREAMPORT_PRESENT           0x005
# define DP_DWN_STRM_PORT_PRESENT           (1 << 0)
# define DP_DWN_STRM_PORT_TYPE_MASK         0x06
/* 00b = DisplayPort */
/* 01b = Analog */
/* 10b = TMDS or HDMI */
/* 11b = Other */
# define DP_FORMAT_CONVERSION               (1 << 3)

So there we go, it's a branch device with VGA downstream of it.  We need
to figure out the connectivity of the downstream port.

> > > Checking DPCD bits might work, but I have no exact spec of DPCD, so no
> > > idea whether the above is really wrong or not.
> > 
> > Is there a VGA port on the docking station, it might have a DP->VGA in
> > it, or dual-link DVI port.
> 
> Ah, right, that's the missing piece!
> 
> It's a DP->VGA conversion indeed.  Plugging to a VGA connector appears
> as DP2.  Oh well, I didn't think of this possibility at all.
> 
> But then this makes me wonder how to solve the problem at best.
> It's obviously a bogus connection status returned from the driver.
> DP2 advertises itself as if connected.

I wouldn't quite say it's bogus, more that we're describing the
connection status of "the nearest DP device" as if it were the
connection status of the whole wire.

I started a patch series for this a bit ago, I'll send it on
momentarily.

- ajax

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