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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Mon, 2 Mar 2020 14:47:46 +0100
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Subject: Re: [PATCH 8/8] arm64: dts: renesas: salvator: add a connection from
 adv748x codec (HDMI input) to the R-Car SoC
To:     Alex Riesen <alexander.riesen@cetitec.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Kieran Bingham <kieran.bingham@ideasonboard.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Hans Verkuil <hverkuil-cisco@xs4all.nl>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        driverdevel <devel@driverdev.osuosl.org>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        Linux-Renesas <linux-renesas-soc@vger.kernel.org>
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Hi Alex,

On Mon, Mar 2, 2020 at 2:40 PM Alex Riesen <alexander.riesen@cetitec.com> wrote:
> Geert Uytterhoeven, Mon, Mar 02, 2020 13:28:13 +0100:
> > On Mon, Jan 13, 2020 at 3:24 PM Alex Riesen
> > <alexander.riesen@cetitec.com> wrote:
> > > Not sure if all variants of the Salvator board have the HDMI decoder
> > > chip (the ADV7482) connected to the SSI4 on R-Car SoC, as it is on
> > > Salvator-X ES1, so the the ADV7482 endpoint and connection definitions
> > > are placed in the board file.
> >
> > Both Salvator-X and Salvator-XS have SSI4 wired to the ADV7482.
> >
> > > I do assume though that all Salvator variants have the CLK_C clock line
> > > hard-wired to the ADV7482 HDMI decoder, and remove it from the list of
> > > clocks provided by the R-Car sound system.
> >
> > Yes, both Salvator-X and Salvator-XS have it wired that way.
>
> Ok, seems like I can move that part into the common file as well.
> Integrations of ADV7482 and R-Car which use salvator-common.dts can still
> redefine the endpoint settings in their board files, right?

Indeed.

> > > --- a/arch/arm64/boot/dts/renesas/salvator-common.dtsi
> > > +++ b/arch/arm64/boot/dts/renesas/salvator-common.dtsi
> > > @@ -322,6 +322,10 @@
> > >         clock-frequency = <22579200>;
> > >  };
> > >
> > > +&audio_clk_c {
> > > +       clock-frequency = <12288000>;
> > > +};
> >
> > Does the ADV7482 always generate a 12.288 MHz clock signal?
> > Or is this programmable?
>
> Oops. It looks like it is and the value is derived from the sampling rate
> (48kHz) and the master clock multiplier. Both hard-coded in the board file.

Where are these hardcoded in the board file?
Even if they are, technically this is a clock output of the ADC7482.

> > > video-receiver@70 {
> > >     compatible = "adi,adv7482";
> > > ...
> > > +   clocks = <&rcar_sound 3>, <&audio_clk_c>;
> > > +   clock-names = "clk-hdmi-video", "clk-hdmi-i2s-mclk";
> >
> > The above declares the Audio CLK C to be a clock input of the ADV7482, while
> > it is an output.
>
> I would gladly give it right direction if I *really* understood what I was
> doing...

:-)

> > Furthermore, the DT bindings do not document that clocks can be specified.
>
> Should the DT bindings document that the clock cannot be specified than?

It currently does say so, as it doesn't list "clocks" in its properties section.

> > > @@ -686,7 +700,8 @@
> > >         };
> > >
> > >         sound_pins: sound {
> > > -               groups = "ssi01239_ctrl", "ssi0_data", "ssi1_data_a";
> > > +               groups = "ssi01239_ctrl", "ssi0_data", "ssi1_data_a",
> > > +                        "ssi4_data";
> >
> > Missing "ss4_ctrl", for the SCK4 and WS4 pins.
>
> I'll add them.
> As the device seems to function even without thoes, does this mean the pins in
> the group are used "on demand" by whatever needs them?

Probably the SCK4/WS4 functions are the reset-state defaults.

> > > @@ -760,8 +775,18 @@
> > >                  <&cpg CPG_MOD 1020>, <&cpg CPG_MOD 1021>,
> > >                  <&cpg CPG_MOD 1019>, <&cpg CPG_MOD 1018>,
> > >                  <&audio_clk_a>, <&cs2000>,
> > > -                <&audio_clk_c>,
> >
> > Why remove it? This is the list of clock inputs, not outputs.
>
> ...probably because I was thinking the specification was exactly the other way
> around.
>
> Does a "clocks = ..." statement always mean input clocks?

Yes it does.
If a device has clock outputs and is thus a clock provider, it should
have a #clock-cells property, and this should be documented in the bindings.

A clock consumer will refer to clocks of a provider using the "clocks"
property, specifying a clock specifier (phandle and zero or more indices)
for each clock referenced.

> I shall correct that and re-test (might take a while, I don't have the
> hardware anymore).

Oops.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds