Changing Console UART

The Developer Kits are configured to use UART-A on the carrier boards for the SPL/U-Boot/Linux console. Which actual UART is used depends on the COM/uCOM board as can bee seen in this table.

Changing to use one of the other UARTs as console instead, likely due to the design choices on your custom carrier board, requires changes in several places and this page will help with that.

The code changes below are for iMX8M Mini uCOM which uses UART2 as the default console which will be changed to UART4.

The hexadecimal values below for the UARTs come from the imx8mm.dtsi file:

Serial - UART link	UART Address
serial0 = &uart1	uart1: serial@30860000
serial1 = &uart2	uart2: serial@30890000
serial2 = &uart3	uart3: serial@30880000
serial3 = &uart4	uart4: serial@30a60000

The important values for now are the old: uart2 / 30890000 and the new: uart4 / 30a60000.

note

The instructions below are for the iMX8M Mini uCOM but it is very similar to the iMX8M Nano uCOM so to do the UART change for an iMX8M Nano uCOM, simply replace the imx8mm with imx8mn in all paths and filenames below.

note

Swapping to a different UART than the UART4 is very similar but requires different changes to the dts files. Use these instructions as a starting point and to know where to change but exactly what to change, what pins to use etc. will be board dependent and is out of scope for this page.

U-Boot

Enable the UART and make it the console.

Open arch/arm/dts/imx8mm-ea-ucom-kit_v2-u-boot.dtsi:

-&pinctrl_uart2 {
+&pinctrl_uart4 {
        u-boot,dm-spl;
 };

-&uart2 {
+&uart4 {
        u-boot,dm-spl;
 };

Open arch/arm/dts/imx8mm-ea-ucom-kit_v2.dts:

        chosen {
-               bootargs = "console=ttymxc1,115200 earlycon=ec_imx6q,0x30890000,115200";
-               stdout-patch = &uart2;
+               bootargs = "console=ttymxc3,115200 earlycon=ec_imx6q,0x30a60000,115200";
+               stdout-patch = &uart4;
        };

-&uart2 { /* console */
+&uart2 { /* old console */
        pinctrl-names = "default";
        pinctrl-0 = <&pinctrl_uart2>;
+       status = "disabled";
+};
+
+&uart4 { /* new console on UART-B */
+       pinctrl-names = "default";
+       pinctrl-0 = <&pinctrl_uart4>;
        status = "okay";
 };

+               pinctrl_uart4: uart4grp {
+                       fsl,pins = <
+                               MX8MM_IOMUXC_UART4_RXD_UART4_DCE_RX     0x49
+                               MX8MM_IOMUXC_UART4_TXD_UART4_DCE_TX     0x49
+                       >;
+               };
+

Open board/embeddedartists/mx8mmea-ucom/mx8mmea-ucom.c:

 static iomux_v3_cfg_t const uart_pads[] = {
-       IMX8MM_PAD_UART2_RXD_UART2_RX | MUX_PAD_CTRL(UART_PAD_CTRL),
-       IMX8MM_PAD_UART2_TXD_UART2_TX | MUX_PAD_CTRL(UART_PAD_CTRL),
+       IMX8MM_PAD_UART4_RXD_UART4_RX | MUX_PAD_CTRL(UART_PAD_CTRL),
+       IMX8MM_PAD_UART4_TXD_UART4_TX | MUX_PAD_CTRL(UART_PAD_CTRL),
 };

        imx_iomux_v3_setup_multiple_pads(uart_pads, ARRAY_SIZE(uart_pads));

-       init_uart_clk(1);
+       init_uart_clk(3); //3=UART4

        return 0;
 }

Open include/configs/mx8mmea-ucom.h:

        "fdt_addr=0x43000000\0"                 \
        "fdt_high=0xffffffffffffffff\0" \
        "mtdparts=" MFG_NAND_PARTITION "\0" \
-       "console=ttymxc1,115200 earlycon=ec_imx6q,0x30890000,115200\0" \
-       "bootargs=console=ttymxc1,115200 earlycon=ec_imx6q,0x30890000,115200 ubi.mtd=5 "  \
+       "console=ttymxc3,115200 earlycon=ec_imx6q,0x30a60000,115200\0" \
+       "bootargs=console=ttymxc3,115200 earlycon=ec_imx6q,0x30a60000,115200 ubi.mtd=5 "  \
                "root=ubi0:nandrootfs rootfstype=ubifs "                     \
                MFG_NAND_PARTITION \
                "\0" \

        "bsp_script=boot.scr\0" \
        "image=Image\0" \
        "splashimage=0x50000000\0" \
-       "console=ttymxc1,115200\0" \
+       "console=ttymxc3,115200\0" \
        "fdt_addr_r=0x43000000\0"               \
        "fdt_addr=0x43000000\0"                 \
        "fdt_high=0xffffffffffffffff\0"         \

-#define CONFIG_MXC_UART_BASE           UART2_BASE_ADDR
+#define CONFIG_MXC_UART_BASE           UART4_BASE_ADDR

Linux

Enable the UART and make it the console. Open arch/arm64/boot/dts/freescale/imx8mm-ea-ucom-kit_v3.dts:

        chosen {
-               bootargs = "console=ttymxc1,115200 earlycon=ec_imx6q,0x30890000,115200";
-               stdout-path = &uart2;
+               bootargs = "console=ttymxc3,115200 earlycon=ec_imx6q,0x30a60000,115200";
+               stdout-path = &uart4;
        };

+               pinctrl_uart4: uart4grp {
+                       fsl,pins = <
+                               MX8MM_IOMUXC_UART4_RXD_UART4_DCE_RX     0x140
+                               MX8MM_IOMUXC_UART4_TXD_UART4_DCE_TX     0x140
+                       >;
+               };

+/* By default UART4 is owned by the M core not the A53 and enabling it here
+   without also removing the ownership from the M core will result in a
+   kernel panic. */
+&uart4 {
+       pinctrl-names = "default";
+       pinctrl-0 = <&pinctrl_uart4>;
+       status = "okay";
+};

Yocto - Machine Configuration

Change the SERIAL_CONSOLES variable in Yocto. It affects which UART can be used for login. Open ea-bsp/sources/meta-ea/conf/machine/imx8mmea-ucom.conf and change this line:

-SERIAL_CONSOLES = "115200;ttymxc1"
+SERIAL_CONSOLES = "115200;ttymxc3"

Yocto - imx-atf

This step is only needed for iMX8M Mini or iMX8M Nano boards as they both use the imx-boot Yocto recipe to package U-Boot, SPL, ARM Trusted Firmware, DDR Firmware and HDMI Firmware into one binary. The imx-boot recipe uses imx-atf for one of the steps, and it contains the code that sets up the Resource Domain Controller (RDC). By default, the RDC assigns UART4 to the Cortex-M.

In your build folder, e.g. ea-bsp/b_8mm, run this command to check out the correct version of the imx-atf repo:

devtool modify imx-atf

The repo will be checked out locally under ea-bsp/b_8mm/workspaces/sources/imx-atf.

Open plat/imx/imx8m/imx8mm/imx8mm_bl31_setup.c and change in two locations (with and without Android):

diff --git a/plat/imx/imx8m/imx8mm/imx8mm_bl31_setup.c b/plat/imx/imx8m/imx8mm/imx8mm_bl31_setup.c
index 8702d5160..c5bb66040 100644
--- a/plat/imx/imx8m/imx8mm/imx8mm_bl31_setup.c
+++ b/plat/imx/imx8m/imx8mm/imx8mm_bl31_setup.c
@@ -61,7 +61,7 @@ static const struct imx_rdc_cfg rdc[] = {
        RDC_MDAn(RDC_MDA_M4, DID1),

        /* peripherals domain permission */
-       RDC_PDAPn(RDC_PDAP_UART4, D1R | D1W),
+       RDC_PDAPn(RDC_PDAP_UART4, D0R | D0W),
        RDC_PDAPn(RDC_PDAP_UART2, D0R | D0W),
        RDC_PDAPn(RDC_PDAP_UART1, D0R | D0W),

@@ -108,7 +108,7 @@ static const struct imx_rdc_cfg rdc[] = {
        RDC_MDAn(RDC_MDA_M4, DID1),

        /* peripherals domain permission */
-       RDC_PDAPn(RDC_PDAP_UART4, D1R | D1W),
+       RDC_PDAPn(RDC_PDAP_UART4, D0R | D0W),
        RDC_PDAPn(RDC_PDAP_UART2, D0R | D0W),
        RDC_PDAPn(RDC_PDAP_UART1, D0R | D0W),

Change UART_BASE - 5.15 distribution

Modify plat/imx/imx8m/imx8mm/platform.mk:

diff --git a/plat/imx/imx8m/imx8mm/platform.mk b/plat/imx/imx8m/imx8mm/platform.mk
index bfbcaed38..901bfe024 100644
--- a/plat/imx/imx8m/imx8mm/platform.mk
+++ b/plat/imx/imx8m/imx8mm/platform.mk
@@ -153,7 +153,7 @@ $(eval $(call add_define,BL32_BASE))
 BL32_SIZE              ?=      0x2000000
 $(eval $(call add_define,BL32_SIZE))

-IMX_BOOT_UART_BASE     ?=      0x30890000
+IMX_BOOT_UART_BASE     ?=      0x30a60000
 $(eval $(call add_define,IMX_BOOT_UART_BASE))

 $(eval $(call add_define,IMX8M_DDR4_DVFS))

Change UART_BASE - 6.1 distribution

In the 6.1 distribution the UART_BASE can be changed in <build dir>/conf/local.conf.

Open local.conf and add:

ATF_BOOT_UART_BASE = "0x30a60000"