Re: [PATCH v6 02/11] ARM: dts: renesas: r8a7740: Add clocks for FSI

[Bui Duc Phuc]

Dear all,

Sorry for sending two additional emails regarding FSIA. They were not
in HTML format and therefore were rejected by the mailing list.
To avoid further confusion, I would like to summarize my findings here
and also include some additional observations regarding FSIB.

1. Regarding FSIA

The 12.288 MHz oscillator (OSC X8) only provides a reference/system
clock and does not by itself determine whether the FSI operates in
Master or Slave mode.
The Master/Slave relationship is determined by which device drives the
BCLK and LRCLK signals. Therefore, assuming that the current MCLK
source remains valid,
switching the FSI to Master mode would primarily require rerouting
BCLK and LRCLK, together with the corresponding software configuration
changes.
Alternatively, the codec MCLK could also be sourced directly from the
FSI MCLK output by rerouting the MCLK connection.
In that configuration, the FSI would provide all audio clocks (MCLK,
BCLK, and LRCLK) and operate as the clock master.

2. Regarding FSIB

I could not find any FSIB-related connections in the available
Armadillo board schematic. The schematic only shows FSIA.
Therefore, there appear to be two possible explanations:

 1. The available hardware design documentation is incomplete, or

 2. The connection between FSIB and HDMI is implemented internally
inside the SoC,
    so the FSIB signals are not exposed in the board schematic.

Personally, I believe the second explanation is more likely.
At present, the only signal related to FSIB that I can identify in the
schematic is FSIACK, which is shared between FSIA and FSIB.
I cannot find any of the other FSIB signals. In addition, after
tracing the Linux 4.2 source code, I found that for the FSIB-to-HDMI
path,
only one FSIB-related pin is configured:  ' fsib_mclk_in '
Furthermore, when the audio stream is started and the clock rate is
configured, the driver calls: fsi_clk_set_rate_cpg() rather than
the external clock configuration function (fsi_clk_set_rate_external()).

At the moment, I have not investigated the HDMI subsystem in depth,
and I do not know how the internal connection between HDMI
and FSIB is implemented inside the SoC. In addition, the original HDMI
driver source code has already been removed, so HDMI is not supported.
Currently, FSIB is not ready for testing on the current kernel.
As I also mentioned in the cover letter:
>   - FSI master mode is currently compile-tested only. Full verification
>     requires a dedicated HDMI driver (FSIB) or hardware modifications
>     (resoldering board resistors) (FSIA).


3. Regarding Geert's question

> I have one more general question.
> arch/arm/boot/dts/renesas/r8a7740-armadillo800eva.dts configures
> audio for FSI (fsia_pins), but does not fill in a clock-frequency
> in fsiack_clk.  Instead, it fills in 12.288 MHz in fsibck_clk, while
> the schematics call it FSIACK.
> Apparently the FSIACK pin is shared with FSIBCK on R-Mobile A1, so
> which function is used depends on pin control.  However, the DTS does
> not perform any pin configuration for this pin?
>

Based on the current schematic and source code analysis, I think that
if we continue to support FSIB for this use case,
the DTS should explicitly configure the FSIB clock and pinmux, for example:

&fsibck_clk {
        clock-frequency = <12288000>;
};

fsia_pins: sounda {
        groups = "fsia_sclk_in",
                 "fsia_mclk_out",
                 "fsia_data_in_1",
                 "fsia_data_out_0";
        function = "fsia";
};

+ fsib_pins: soundb {
+       groups = "fsib_mclk_in";
+       function = "fsib";
+ };

This configuration appears to match the current resistor population
shown in the schematic:
FSIA operates in Slave mode & FSIB operates in Master mode.
However, this conclusion is based only on the currently available
schematic and source code analysis.
Additional hardware documentation would be helpful to better
understand the internal HDMI/FSIB clock architecture and to confirm
this assumption.

Best regards,
Phuc