linux-kernel - Re: [PATCH 8/8] arm64: dts: renesas: salvator: add a connection from adv748x codec (HDMI input) to the R-Car SoC

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Date:   Mon, 2 Mar 2020 16:07:06 +0100
From:   Alex Riesen <alexander.riesen@...itec.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
CC:     Kieran Bingham <kieran.bingham@...asonboard.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        "Laurent Pinchart" <laurent.pinchart@...asonboard.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        driverdevel <devel@...verdev.osuosl.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH 8/8] arm64: dts: renesas: salvator: add a connection from
 adv748x codec (HDMI input) to the R-Car SoC

Geert Uytterhoeven, Mon, Mar 02, 2020 14:47:46 +0100:
> On Mon, Mar 2, 2020 at 2:40 PM Alex Riesen <alexander.riesen@...itec.com> wrote:
> > > > --- 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?

In the endpoint definition, arch/arm64/boot/dts/renesas/r8a7795-es1-salvator-x.dts

So the frequency can be set at the run-time, perhaps even derived from
endpoint connected to the output. In this case, rsnd_endpoint3,
which has the "mclk-fs" setting. Not sure if the sampling rate
can be set to something else for the HDMI, though.

> Even if they are, technically this is a clock output of the ADV7482.

... which I hope to correct as soon as I steal the hardware from whoever stole
it from me...

> > > > 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.

The bindings documentation file, which we're talking about here and which does
not list the specifiable input clocks in its properties, is it the

    Documentation/devicetree/bindings/media/i2c/adv748x.txt

?

And this absence of documentation also means that whatever clocks (both input
in "clocks=" and output in "#clock-cells") listed in a specific .dts are just
an integration detail?

Does this below makes more sense, than?

    video-receiver@70 {
        compatible = "adi,adv7482";
        clocks = <&rcar_sound 3>;
        clock-names = "clk-hdmi-video";
        adv748x_mclk: mclk {
            compatible = "fixed-clock";
            #clock-cells =  <0>;
            /* frequency hard-coded for illustration */
            clock-frequency = <12288000>;
            clock-output-names = "clk-hdmi-i2s-mclk";
        };
    };

Now I'm a bit hazy on how to declare that the MCLK output of the
video-receiver@70 is connected to the Audio Clock C of the SoC...
Probably remove use of "audio_clk_c" completely?

> > > > @@ -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.

That ... might require some trial and testing: when I add them to the group,
the reset defaults will be overridden by the platform initialization, which is
not necessarily the reset default. Will see.

> >
> > 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.

Something like this?

    &rcar_sound {
        clocks = ...,
                 <&adv748x_mclk>,
                 <&cpg CPG_CORE CPG_AUDIO_CLK_I>;
        clock-names = ...,
                      "clk_c",
                      "clk_i";
    };

Regards,
Alex