Date:   Thu, 5 Jul 2018 09:03:58 +0200
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     Jernej Škrabec <jernej.skrabec@...l.net>,
        Rob Herring <robh+dt@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        Mark Rutland <mark.rutland@....com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        devicetree <devicetree@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>
Subject: Re: [linux-sunxi] Re: [PATCH v3 09/24] drm/sun4i: Don't skip TCONs
 if they don't have channel 0

On Sun, Jul 01, 2018 at 11:11:06PM +0800, Chen-Yu Tsai wrote:
> On Sun, Jul 1, 2018 at 4:27 PM, Jernej Škrabec <jernej.skrabec@...l.net> wrote:
> > Dne četrtek, 28. junij 2018 ob 08:24:34 CEST je Chen-Yu Tsai napisal(a):
> >> On Thu, Jun 28, 2018 at 12:45 PM, Jernej Škrabec
> >>
> >> <jernej.skrabec@...l.net> wrote:
> >> > Dne četrtek, 28. junij 2018 ob 03:51:31 CEST je Chen-Yu Tsai napisal(a):
> >> >> On Mon, Jun 25, 2018 at 8:02 PM, Jernej Skrabec <jernej.skrabec@...l.net>
> >> >
> >> > wrote:
> >> >> > TV TCONs (channel 1 only) are always connected to TV or HDMI encoder.
> >> >> > Because of that, all output endpoints on such TCON node will point to a
> >> >> > encoder which is part of component framework.
> >> >> >
> >> >> > Correct current graph traversing algorithm in such way that it doesn't
> >> >> > skip output enpoints with id 0 on TV TCONs.
> >> >>
> >> >> No. Our bindings say that endpoint 0 _is_ channel 0. For TCONs that don't
> >> >> have channel 0, it must be skipped.
> >> >
> >> > I'm not sure where this is stated. I read TCON binding again. Can you
> >> > please point me to it?
> >>
> >> https://elixir.bootlin.com/linux/latest/source/Documentation/devicetree/bind
> >> ings/display/sunxi/sun4i-drm.txt#L169
> >>
> >> Our TCON driver still expects RGB or LVDS panel / bridges on endpoint 0.
> >
> > Yes, but that can only happen on TCON which has channel 0. TV TCONs (those
> > with channel 1 only) can't have panels or bridges connected to them, because
> > they are internally always connected to either HDMI or TV encoder or both.
> > Actually, R40 is the only SoC, where same TV TCON can be connected to TV
> > encoder or HDMI. Others have specialized TV TCONs, which are connect to only
> > one encoder.
> >
> > IMO TV TCONs are really just stripped down LCD TCONs to support one (or max
> > two) specific encoder.
> 
> I agree. We've seen these first in the H3, and the reverse, TCONs only with
> LCD, on the A23/A33.
> 
> >> So I guess this was sort of implied historically. It's no longer true.
> >> This is something we should probably fix.
> >
> > Fixed in what way? You mean update bindings to mention that TCON output
> > endpoint 0 is reserved for panels or bridges?
> 
> Either that, or have the drm driver look at other endpoints. I guess we
> should ask Maxime if this is already done or not, since the DSI driver
> isn't endpoint 0 in the A33 dtsi.

The DSI driver isn't really a good example for this, since the panel
isn't described as part of the OF graph, but DSI binding require that
it's a child of the DSI controller node.

> >>
> >> In practice our drivers don't look at it (yet), but rely on the downstream
> >> encoder type to determine which channel to use.
> >>
> >> But please add the "allwinner,tcon-channel" property as specified in
> >> the binding.
> >
> > It's my understanding of TCON binding documentation that property
> > "allwinner,tcon-channel" is needed only if TCON supports both channels. TV
> > TCON clearly supports only channel 1. In that case, there is no doubt to which
> > channel output endpoint belongs.
> >
> > If that's not true, dt bindings documentation should be reworded to contain
> > word "needed" or something similar. Currently, no DT for newer SoC contains
> > that property (for example, A83T, H3, H5 or even A33).

Yeah, but that's mainly because we have a single output enabled for
each channel on those newer SoCs. When / if we enable the RGB and LVDS
output for example, we will have to set this.

> > On A33 this is even more interesting, since tcon0 has only channel
> > 0 and has DSI output endpoint with number 1. According to TCON
> > binding docs, if "allwinner,tcon-channel" is not preset, endpoint
> > number represent channel. So, that would mean DSI needs channel 1
> > on tcon which supports clearly only channel 0. So either there
> > TCON bindings documentation needs updates or DT for A33 has to be
> > updated.
> 
> Maxime? You did the A33 DSI stuff.

I guess it's missing on the A33 DSI endpoint.

Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com

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