linux-kernel - Re: [PATCH 06/11] dt-bindings: display: sun4i-drm: Add A83T HDMI pipeline

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Date:   Thu, 4 Jan 2018 19:52:10 +0100
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Jernej Škrabec <jernej.skrabec@...l.net>
Cc:     Rob Herring <robh@...nel.org>, airlied@...ux.ie,
        mark.rutland@....com, wens@...e.org, architt@...eaurora.org,
        a.hajda@...sung.com, Laurent.pinchart@...asonboard.com,
        mturquette@...libre.com, sboyd@...eaurora.org,
        Jose.Abreu@...opsys.com, narmstrong@...libre.com,
        dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-clk@...r.kernel.org, linux-sunxi@...glegroups.com
Subject: Re: [PATCH 06/11] dt-bindings: display: sun4i-drm: Add A83T HDMI
 pipeline

On Wed, Jan 03, 2018 at 10:32:26PM +0100, Jernej Škrabec wrote:
> Hi Rob,
> 
> Dne sreda, 03. januar 2018 ob 21:21:54 CET je Rob Herring napisal(a):
> > On Sat, Dec 30, 2017 at 10:01:58PM +0100, Jernej Skrabec wrote:
> > > This commit adds all necessary compatibles and descriptions needed to
> > > implement A83T HDMI pipeline.
> > > 
> > > Mixer is already properly described, so only compatible is added.
> > > 
> > > However, A83T TCON1, which is connected to HDMI, doesn't have channel 0,
> > > contrary to all TCONs currently described. Because of that, TCON
> > > documentation is extended.
> > > 
> > > A83T features Synopsys DW HDMI controller with a custom PHY which looks
> > > like Synopsys Gen2 PHY with few additions. Since there is no
> > > documentation, needed properties were found out through experimentation
> > > and reading BSP code.
> > > 
> > > At the end, example is added for newer SoCs, which features DE2 and DW
> > > HDMI.
> > > 
> > > Signed-off-by: Jernej Skrabec <jernej.skrabec@...l.net>
> > > ---
> > > 
> > >  .../bindings/display/sunxi/sun4i-drm.txt           | 188
> > >  ++++++++++++++++++++- 1 file changed, 181 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/Documentation/devicetree/bindings/display/sunxi/sun4i-drm.txt
> > > b/Documentation/devicetree/bindings/display/sunxi/sun4i-drm.txt index
> > > 9f073af4c711..3eca258096a5 100644
> > > --- a/Documentation/devicetree/bindings/display/sunxi/sun4i-drm.txt
> > > +++ b/Documentation/devicetree/bindings/display/sunxi/sun4i-drm.txt
> > > 
> > > @@ -64,6 +64,40 @@ Required properties:
> > >      first port should be the input endpoint. The second should be the
> > >      output, usually to an HDMI connector.
> > > 
> > > +DWC HDMI TX Encoder
> > > +-----------------------------
> > > +
> > > +The HDMI transmitter is a Synopsys DesignWare HDMI 1.4 TX controller IP
> > > +with Allwinner's own PHY IP. It supports audio and video outputs and CEC.
> > > +
> > > +These DT bindings follow the Synopsys DWC HDMI TX bindings defined in
> > > +Documentation/devicetree/bindings/display/bridge/dw_hdmi.txt with the
> > > +following device-specific properties.
> > > +
> > > +Required properties:
> > > +
> > > +  - compatible: value must be one of:
> > > +    * "allwinner,sun8i-a83t-dw-hdmi"
> > > +  - reg: two pairs of base address and size of memory-mapped region,
> > > first
> > > +    for controller and second for PHY
> > > +    registers.
> > 
> > Seems like the phy should be a separate node and use the phy binding.
> > You can use the phy binding even if you don't use the kernel phy
> > framework...
> 
> Unfortunately, it's not so straighforward. Phy is actually accessed through 
> I2C implemented in HDMI controller. Second memory region in this case has 
> small influence on phy. However, it has big influence on controller. For 
> example, magic number has to be written in one register in second memory 
> region in order to unlock read access to any register from first memory region 
> (controller). However, they shouldn't be merged to one region, because first 
> memory region requires byte access while second memory region can be accessed 
> per byte or word.
> 
> To complicate things more, later I want to add support for another SoC which 
> has same glue layer (unlocking read access, etc.) and uses memory mapped phy 
> registers in second memory region.
> 
> I think current binding is the least complicated way to represent this.

I agree with Rob here. I did a similar thing for the DSI patches I've
sent a few monthes ago and it turned out to not be that difficult, so
I'm sure you can come up with something :)

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com