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Date:   Thu, 5 Mar 2020 15:36:28 +0100
From:   Alex Riesen <alexander.riesen@cetitec.com>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
CC:     Kieran Bingham <kieran.bingham@ideasonboard.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
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        Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Driver Development <devel@driverdev.osuosl.org>,
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        Linux Kernel <linux-kernel@vger.kernel.org>,
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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
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Hi Geert,

Geert Uytterhoeven, Mon, Mar 02, 2020 17:13:30 +0100:
> On Mon, Mar 2, 2020 at 5:09 PM Alex Riesen <alexander.riesen@cetitec.com> wrote:
> > Geert Uytterhoeven, Mon, Mar 02, 2020 16:32:32 +0100:
> > >
> > > The #clock-cells should be in the main video-receiver node.
> > > Probably there is more than one clock output, so #clock-cells may be 1?
> >
> > AFAICS, the device can provide only this one clock line (audio master clock
> > for I2S output)... I shall re-check, just in case.

And you're right, of course: the audio output formatting module of the ADV748x
devices provides a set of clock lines related to the I2S pins: the already
discussed master clock, left-right channel clock and the serial clock (bit
clock?).

> > > There is no need for a fixed-clock compatible, nor for clock-frequency
> > > and clock-output-names.
> > >
> > > But most important: this should be documented in the adv748x DT bindings,
> > > and implemented in the adv748x driver.
> >
> > So if the driver is to export that clock for the kernel (like in this case),
> > it must implement its support?
> 
> Exactly.  Unless that pin is hardcoded to output a fixed clock, in which case
> you can just override the existing audio_clk_c rate.

Just to try it out (I'll set #clock-cells to 1), I registered a fixed rate
clock in the driver, added a clock provider:

adv748x_probe:

    clk = clk_register_fixed_rate(state->dev,
				  "clk-hdmi-i2s-mclk",
				  NULL     /* parent_name */,
				  0        /* flags */,
				  12288000 /* rate */);
    of_clk_add_provider(state->dev->of_node, of_clk_src_simple_get, clk);

And removed the audio_clk_c frequency setting. I also replaced the audio_clk_c
in the list of input clocks of the R-Car-side sound card with the phandle of
the adv7482 main node:

salvator-common.dtsi:

    &i2c4 {
	status = "okay";

	adv7482_hdmi_decoder: video-receiver@70 {
	    #clock-cells = <0>; // to be replaced with <1>
	};
    };

    &rcar_sound {
	clocks = ..., <&adv7482_hdmi_decoder>, ...;
    };

As everything continues to work as before, I assume that at least the clock
dependencies were resolved.

Is there a way to verify that the added input clock is actually used?
IOW, if its frequency is actually has been programmed into the ssi4 (R-Car
receiving hardware) registers, and not just a left-over from previuos attempts
or plain default setting?

As the ADV748x devices seem to provide also the clocks for video outputs, will
it make any sense to place the clock definition into the port node?
Or should all provided clocks be indexed in the main device node?

Regards,
Alex