IMX8MM RMII mode 50MHz output disappears during kernel booting

[IlyaShorikov]

Hello!

We are using custom board based on imx8m mini and there was a problem setting the RMII mode.
In u-boot code was edited and 50Mhz ref clock output exist on pad ENET_TD2 (observing it on oscilloscope),

One of U-Boot change:

static int setup_fec(void)
{
struct iomuxc_gpr_base_regs *const iomuxc_gpr_regs
= (struct iomuxc_gpr_base_regs *) IOMUXC_GPR_BASE_ADDR;

setbits_le32(&iomuxc_gpr_regs->gpr[1], IOMUXC_GPR_GPR1_GPR_ENET1_TX_CLK_SEL_MASK);
setup_iomux_fec();
return set_clk_enet(ENET_50MHZ);
}

but during kernel is booting 50Mhz reference clock disappears (observing it on oscilloscope).
I tried to find a dependencies of that behavior:
1. change or remove pinmux settings
2. change fec1 settings and disable it
but there are no dependencies was found.

Device tree code:

fec1grp {
  fsl,pins = <
    MX8MM_IOMUXC_ENET_MDC_ENET1_MDC              0x3
    MX8MM_IOMUXC_ENET_MDIO_ENET1_MDIO            0x3
    MX8MM_IOMUXC_ENET_TX_CTL_ENET1_RGMII_TX_CTL  0x1f
    MX8MM_IOMUXC_ENET_TXC_ENET1_TX_ER            0x1f
    MX8MM_IOMUXC_ENET_TD0_ENET1_RGMII_TD0        0x1f
    MX8MM_IOMUXC_ENET_TD1_ENET1_RGMII_TD1        0x1f
    MX8MM_IOMUXC_ENET_TD2_ENET1_TX_CLK           0x1f
    MX8MM_IOMUXC_ENET_RD0_ENET1_RGMII_RD0        0x91
    MX8MM_IOMUXC_ENET_RD1_ENET1_RGMII_RD1        0x91
    MX8MM_IOMUXC_ENET_RX_CTL_ENET1_RGMII_RX_CTL  0x91
    MX8MM_IOMUXC_ENET_RXC_ENET1_RX_ER            0x91
    MX8MM_IOMUXC_GPIO1_IO09_GPIO1_IO9            0x23
  >;
};

&fec1 {
  pinctrl-names = "default";
  pinctrl-0 = <&pinctrl_fec1>;
  phy-mode = "rmii";
  phy-reset-gpios = <&gpio1 9 0>;
  phy-handle = <&ethphy0>;
  status = "okay";
  
  mdio {
    #address-cells = <1>;
    #size-cells = <0>;
    ethphy0: ethernet-phy@0 {
      compatible = "ethernet-phy-ieee802.3-c22";
      reg = <1>;
    };
  };
};

During kernel booting ref clock disappears and link connection between LAN switch and eth PHY losts.

I will be glad for any help.

Thanks in advance.

[igorpadykov]

Hi Ilya

for rmii clock one can look at solution on

https://community.nxp.com/t5/i-MX-Processors/i-MX8MM-RMII-reference-clock/m-p/952927

Best regards
igor