linux-kernel - Re: [linux-sunxi] Re: [PATCH v3 17/25] dt-bindings: panel: Add Bananapi S070WV20-CT16 ICN6211 MIPI-DSI to RGB bridge

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Message-ID: <CAGRGNgX0eWUEzBUU0H_NRJitQmOgF1PxZP0BgQV+OMcDDuVVhA@mail.gmail.com>
Date:   Wed, 7 Nov 2018 21:20:03 +1100
From:   Julian Calaby <julian.calaby@...il.com>
To:     Jagan Teki <jagan@...rulasolutions.com>
Cc:     a.hajda@...sung.com, Chen-Yu Tsai <wens@...e.org>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Icenowy Zheng <icenowy@...c.io>, jernej.skrabec@...l.net,
        Vasily Khoruzhick <anarsoul@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        David Airlie <airlied@...ux.ie>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Michael Turquette <mturquette@...libre.com>, sboyd@...nel.org,
        "open list:COMMON CLK FRAMEWORK" <linux-clk@...r.kernel.org>,
        Michael Nazzareno Trimarchi <michael@...rulasolutions.com>,
        "Mailing List, Arm" <linux-arm-kernel@...ts.infradead.org>,
        devicetree <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-sunxi@...glegroups.com
Subject: Re: [linux-sunxi] Re: [PATCH v3 17/25] dt-bindings: panel: Add
 Bananapi S070WV20-CT16 ICN6211 MIPI-DSI to RGB bridge

Hi Jagan,

On Wed, Nov 7, 2018 at 5:13 AM Jagan Teki <jagan@...rulasolutions.com> wrote:
>
> On Wed, Oct 31, 2018 at 2:46 PM Julian Calaby <julian.calaby@...il.com> wrote:
> >
> > Hi Jagan,
> >
> > On Wed, Oct 31, 2018 at 7:58 PM Chen-Yu Tsai <wens@...e.org> wrote:
> > >
> > > On Wed, Oct 31, 2018 at 4:53 PM Andrzej Hajda <a.hajda@...sung.com> wrote:
> > > >
> > > > On 26.10.2018 16:43, Jagan Teki wrote:
> > > > > Bananapi S070WV20-CT16 ICN6211 is 800x480, 4-lane MIPI-DSI to RGB
> > > > > bridge panel, which is available on same PCB with 24-bit RGB interface.
> > > > >
> > > > > So, this patch adds DSI specific binding details on existing
> > > > > dt-bindings file.
> > > > >
> > > > > Signed-off-by: Jagan Teki <jagan@...rulasolutions.com>
> > > > > ---
> > > > > Changes for v3:
> > > > > - Use existing binding doc and update dsi details
> > > > > Changes for v2:
> > > > > - none
> > > > >
> > > > >  .../display/panel/bananapi,s070wv20-ct16.txt  | 31 +++++++++++++++++--
> > > > >  1 file changed, 29 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/Documentation/devicetree/bindings/display/panel/bananapi,s070wv20-ct16.txt b/Documentation/devicetree/bindings/display/panel/bananapi,s070wv20-ct16.txt
> > > > > index 35bc0c839f49..b7855dc7c66f 100644
> > > > > --- a/Documentation/devicetree/bindings/display/panel/bananapi,s070wv20-ct16.txt
> > > > > +++ b/Documentation/devicetree/bindings/display/panel/bananapi,s070wv20-ct16.txt
> > > > > @@ -1,12 +1,39 @@
> > > > >  Banana Pi 7" (S070WV20-CT16) TFT LCD Panel
> > > > >
> > > > > +S070WV20-CT16 is 7" 800x480 panel connected through a 24-bit RGB interface.
> > > > > +
> > > > > +Depending on the variant, the PCB attached to the panel module either
> > > > > +supports DSI, or DSI + 24-bit RGB. DSI is converted to 24-bit RGB via
> > > > > +an onboard ICN6211 MIPI DSI - RGB bridge chip, then fed to the panel
> > > > > +itself
> > > >
> > > > As I understand this is display board, which contains 'pure' RGB panel
> > > > S070WV20-CT16 and optionally ICN6211 DSI->RGB bridge.
> > > > These are separate devices, just connected by vendor to simplify its
> > > > assembly. Why don't you create then bridge driver for ICN6211 and RGB
> > > > panel driver for S070WV20-CT16 - it looks more generic.
> > > > Then you can describe both in dts and voila.
> > > > Creating drivers for every combo of devices (panel + bridge), just
> > > > because some vendor sells them together seems incorrect - we have
> > > > devicetree for it.
> > >
> > > Rob suggested this, and also the opposite: using the same
> > > "bananapi,s070wv20-ct16"
> > > compatible string for both types of connections, and have the driver deal with
> > > detecting the bus type.
> > >
> > > The thing about the bridge chip is that there's no available datasheet that
> > > describes all the parts of the init sequence, in fact none at all. I managed
> > > to work out some bits, but the others remain a mystery and must be hard-coded
> > > to match the panel. That would work against having a generic bridge driver.
> >
> > To me it seems logical that we'd model it as another step in the graph
> > between the DSI component and the panel. It's conceivable that some
> > other manufacturer will probably buy these for their panels and having
> > a somewhat generic driver seems vaguely future proof to me.
> >
> > As I see it, the weird init process belongs to the bridge chip and the
> > timings belong to the panel and we shouldn't "confuse" them by giving
> > them the same compatible.
>
> But the problem here is due to lack of information about the panel,
> the init process command opcode data values seem to based on the panel
> timings values. This look weird and ie reason for going into separate
> panel driver with different compatible.

I'd missed that particular wrinkle.

In that case, it makes sense to produce a panel driver with the bridge
chip's compatible.

Thanks,

-- 
Julian Calaby

Email: julian.calaby@...il.com
Profile: http://www.google.com/profiles/julian.calaby/