Re: [RFC PATCH 10/11] drm/bridge: ti-sn65dsi86: Support DisplayPort (non-eDP) mode

From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Doug Anderson <dianders@chromium.org>
Cc: Jernej Skrabec <jernej.skrabec@siol.net>,
	Neil Armstrong <narmstrong@baylibre.com>,
	Jonas Karlman <jonas@kwiboo.se>,
	Kieran Bingham <kieran.bingham@ideasonboard.com>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	Stephen Boyd <swboyd@chromium.org>,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	Andrzej Hajda <a.hajda@samsung.com>
Subject: Re: [RFC PATCH 10/11] drm/bridge: ti-sn65dsi86: Support DisplayPort (non-eDP) mode
Date: Fri, 4 Mar 2022 17:49:23 +0200	[thread overview]
Message-ID: <YiI1A+dtLXMQzkVf@pendragon.ideasonboard.com> (raw)
In-Reply-To: <CAD=FV=WSHpu3Ub=0Gbq01o_s-SsUa_Q6uC7Z3PaxFL00eeJbdw@mail.gmail.com>

On Wed, Feb 23, 2022 at 10:20:18AM -0800, Doug Anderson wrote:
> On Wed, Feb 23, 2022 at 10:05 AM Kieran Bingham wrote:
> >
> > > > > +       /* For DisplayPort, disable scrambling mode. */
> > > > > +       if (pdata->bridge.type == DRM_MODE_CONNECTOR_DisplayPort)
> > > > > +               regmap_update_bits(pdata->regmap, SN_TRAINING_SETTING_REG,
> > > > > +                                  SCRAMBLE_DISABLE, SCRAMBLE_DISABLE);
> > > >
> > > > I'm assuming that this is the important part of your patch? Would be
> > > > sorta nice to include the "why" in your comment. Why do you want to
> > > > disable scrambling mode for DP but not for eDP? Maybe you care about
> > > > compatibility but not EMI if you're hooking up to random DP things?
> > >
> > > I'll investigate and include proper documentation in v2 (or drop the
> > > change altogether if it's not required).
> >
> > And indeed, this part is important. If I drop this hunk - then I get no
> > display output.
> >
> > I'm afraid I don't (yet) know the reasons 'why' to extend the comment,
> > beyond "Scrambling is not supported for DP".
> >
> > If anyone already does, please feel free to provide the text, and I'll
> > include it in the next revision, or I'll try to do some more digging
> > into this part.
> 
> I don't know _tons_ about it, but I later learned that the "alternate"
> scrambler is used for eDP and the normal scrambler is used for DP. I
> don't have any background about why they are different other than what
> looks to be intentionally making the two things incompatible.

I think it was done for DRM purpose, to prevent signals meant for a
panel to be connected to a device that could capture video from a DP
source.

> ...so I guess that would make it pretty clear why you can't use the
> alternate scrambler for DP. I haven't personally done the research to
> know if you can be officially DP compliant with the scrambler
> disabled. I also don't know why the ti-sn65dsi86 makes it so difficult
> to switch to the standard scrambler or if it works at all... ;-)

-- 
Regards,

Laurent Pinchart