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

[Bui Duc Phuc]

Hi Geert,

> Reviewed-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
> i.e. will queue in renesas-devel for v7.3.
>

Thank you for the review and for the Reviewed-by tag.

> 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?
>

I checked the schematic in more detail.
On sheet 12 (FSIA section), the FSIACK signal is connected to the
WM8978 MCLK pin (pin 11).
It is also connected to the X8 oscillator output (pin 3) through R121.
By default, the board populates R120 (0 ohm), which routes the signal
to R8A7740 pin K5 (FSIAOMC).
If R120 is removed and R250 is populated instead, the signal is routed
to pin G3 (FSIACK).

>From my understanding, the default resistor configuration places FSIA
in slave mode.
In this configuration, FSIA receives BCLK and LRCLK from the WM8978,
while the codec receives the 12.288 MHz MCLK from oscillator X8.

This matches the current DTS configuration:

simple-audio-card,codec {
          sound-dai = <&wm8978>;
          bitclock-master;
          frame-master;
          system-clock-frequency = <12288000>;
};

Therefore, I do not think it is necessary to provide a 12.288 MHz
frequency for fsiack_clk (xcka) in the current configuration.
If the hardware were reconfigured to operate FSIA in master mode, that
might need to be revisited.

I have considered testing the alternative resistor configuration
(removing R120 and populating R250)
to switch FSIA into master mode, but I have not done so due to the
risk of damaging the board.

As for the relationship between FSIACK and FSIBCK, I am not sure.
The schematic I have does not appear to describe FSIB at all.
The HDMI section only documents video signals and does not mention audio,
so I do not have enough information to determine whether FSIACK and
FSIBCK share the same pin on this board.

> Note that I have never tried audio on Armadillo myself.
>

You should give it a try sometime.
The audio quality of this board is actually not bad for hardware that
is more than a decade old :-)

Best regards,
Phuc