Modified Device Tree for BeagleBone Black Running Yocto Poky to Enable UART4 causing Kernel Halt - linux

Question

I have recently tried to take some steps into learning how to use Yocto and leverage some of the functionality provided. I have a BeagleBone Black and wanted to use this as my device to build a custom kernel using Yocto. I have been following the following tutorial series to understand the basics of yocto: https://www.youtube.com/watch?v=9vsu67uMcko&list=PLEBQazB0HUyTpoJoZecRK6PpDG31Y7RPB.
My current objective is to patch the existing device tree used by the Poky reference distribution to enable UART4, the UART port that is accessible through the BeagleBone Black through GPIO header.
I have taken a look at the am335x-boneblack.dts file and the included dtsi files and understand which files need to be modified to enable the UART4 port. In specific, I see that the am335x-bone-common.dtsi and am33xx-l4.dtsi files have pertinent snippets of information related to UART.
The patch file that I add to a new layer and apply is shown below:
diff --git a/orig/am335x-bone-common.dtsi b/altered/am335x-bone-common.dtsi
index 0ccdc7c..8a2541c 100644
--- a/arch/arm/boot/dts/am335x-bone-common.dtsi
+++ b/arch/arm/boot/dts/am335x-bone-common.dtsi
## -95,6 +95,14 ##
AM33XX_PADCONF(AM335X_PIN_UART0_TXD, PIN_OUTPUT_PULLDOWN, MUX_MODE0)
>;
};
+
+ uart4_pins: pinmux_uart4_pins {
+ pinctrl-single,pins = <
+ AM33XX_PADCONF(AM335X_PIN_GPMC_WAIT0, PIN_INPUT_PULLUP, MUX_MODE6)
+ AM33XX_PADCONF(AM335X_PIN_GPMC_WPN, PIN_OUTPUT_PULLDOWN, MUX_MODE6)
+ >;
+ };
+
clkout2_pin: pinmux_clkout2_pin {
pinctrl-single,pins = <
## -191,6 +199,13 ##
status = "okay";
};
+&uart4 {
+ pinctrl-names = "default";
+ pinctrl-0 = <&uart4_pins>;
+
+ status = "okay";
+};
+
&usb0 {
dr_mode = "peripheral";
interrupts-extended = <&intc 18 &tps 0>;
diff --git a/orig/am33xx-l4.dtsi b/altered/am33xx-l4.dtsi
index c9629cb..637a1a7 100644
--- a/arch/arm/boot/dts/am33xx-l4.dtsi
+++ b/arch/arm/boot/dts/am33xx-l4.dtsi
## -1802,7 +1802,7 ##
clock-frequency = <48000000>;
reg = <0x0 0x1000>;
interrupts = <45>;
- status = "disabled";
+ status = "okay";
};
};
When I build my kernel, the resulting .dtb file reflects the changes, however when I try to boot off of the SD card, the beaglebone halts at
Starting kernel ...
If anyone can offer some help, I'd be greatful!
Regards

Answer 1

So, it would seem that the kernel doesn't really hang when enabling UART4.
For some reason, UART4 gets mapped to ttyS0 - which breaks the console output.
Setup:
Beaglebone Black - Yocto Kirkstone - Machine: beaglebone-yocto.
This is the relevant part from am335x-bone-common.dtsi:
uart0_pins: pinmux_uart0_pins {
pinctrl-single,pins = <
AM33XX_PADCONF(AM335X_PIN_UART0_RXD, PIN_INPUT_PULLUP, MUX_MODE0) /* Debug Header - UART0_RX */
AM33XX_PADCONF(AM335X_PIN_UART0_TXD, PIN_OUTPUT_PULLDOWN, MUX_MODE0) /* Debug Header - UART0_TX */
>;
};
uart1_pins: pinmux_uart1_pins {
pinctrl-single,pins = <
AM33XX_PADCONF(AM335X_PIN_UART1_RXD, PIN_INPUT_PULLUP, MUX_MODE0) /* P9-26 UART1_RX */
AM33XX_PADCONF(AM335X_PIN_UART1_TXD, PIN_OUTPUT_PULLDOWN, MUX_MODE0) /* P9-24 UART1_TX */
>;
};
uart2_pins: pinmux_uart2_pins {
pinctrl-single,pins = <
AM33XX_PADCONF(AM335X_PIN_SPI0_SCLK, PIN_INPUT_PULLUP, MUX_MODE1) /* P9-22 UART2_RX */
AM33XX_PADCONF(AM335X_PIN_SPI0_D0, PIN_OUTPUT_PULLDOWN, MUX_MODE1) /* P9-21 UART2_TX */
>;
};
uart4_pins: pinmux_uart4_pins {
pinctrl-single,pins = <
AM33XX_PADCONF(AM335X_PIN_GPMC_WAIT0, PIN_INPUT, MUX_MODE6) /* P9-11 UART4_RX */
AM33XX_PADCONF(AM335X_PIN_GPMC_WPN, PIN_OUTPUT_PULLDOWN, MUX_MODE6) /* P9-13 UART4_TX */
>;
};
Once the board boots - I can SSH into it from Ethernet. Here's the output from pinctrl and serial:
root#beaglebone-yocto:~# cat /sys/kernel/debug/pinctrl/pinctrl-maps
Pinctrl maps:
device 44e10800.pinmux
state default
type MUX_GROUP (2)
controlling device 44e10800.pinmux
group pinmux_clkout2_pin
function pinmux_clkout2_pin
device 48022000.serial
state default
type MUX_GROUP (2)
controlling device 44e10800.pinmux
group pinmux_uart1_pins
function pinmux_uart1_pins
device 48024000.serial
state default
type MUX_GROUP (2)
controlling device 44e10800.pinmux
group pinmux_uart2_pins
function pinmux_uart2_pins
device 4819c000.i2c
state default
type MUX_GROUP (2)
controlling device 44e10800.pinmux
group pinmux_i2c2_pins
function pinmux_i2c2_pins
device 481a8000.serial
state default
type MUX_GROUP (2)
controlling device 44e10800.pinmux
group pinmux_uart4_pins
function pinmux_uart4_pins
device 4a100000.switch
state default
type MUX_GROUP (2)
controlling device 44e10800.pinmux
group cpsw_default
function cpsw_default
device 4a100000.switch
state sleep
type MUX_GROUP (2)
controlling device 44e10800.pinmux
group cpsw_sleep
function cpsw_sleep
device 4a101000.mdio
state default
type MUX_GROUP (2)
controlling device 44e10800.pinmux
group davinci_mdio_default
function davinci_mdio_default
device 4a101000.mdio
state sleep
type MUX_GROUP (2)
controlling device 44e10800.pinmux
group davinci_mdio_sleep
function davinci_mdio_sleep
device 44e09000.serial
state default
type MUX_GROUP (2)
controlling device 44e10800.pinmux
group pinmux_uart0_pins
function pinmux_uart0_pins
device 44e0b000.i2c
state default
type MUX_GROUP (2)
controlling device 44e10800.pinmux
group pinmux_i2c0_pins
function pinmux_i2c0_pins
device 48060000.mmc
state default
type MUX_GROUP (2)
controlling device 44e10800.pinmux
group pinmux_mmc1_pins
function pinmux_mmc1_pins
device 481d8000.mmc
state default
type MUX_GROUP (2)
controlling device 44e10800.pinmux
group pinmux_emmc_pins
function pinmux_emmc_pins
root#beaglebone-yocto:~# cat /proc/tty/driver/serial
serinfo:1.0 driver revision:
0: uart:8250 mmio:0x481A8000 irq:35 tx:871 rx:0 RTS|CTS|DTR|DSR
1: uart:8250 mmio:0x48022000 irq:24 tx:0 rx:0 CTS|DSR|CD|RI
2: uart:8250 mmio:0x48024000 irq:25 tx:0 rx:0 CTS|DSR
3: uart:8250 mmio:0x44E09000 irq:18 tx:0 rx:0 CTS|D
As you can see, UART4 gets mapped to /dev/ttyS0 and UART0 gets mapped to /dev/ttyS3 after bootup.
I'm not sure why UART4 gets mapped to ttyS0 and not ttyS4.
I can't even see /dev/ttyS4 despite it being an alias in the am33xx.dtsi
root#beaglebone-yocto:~# ls -la /dev/ttyS*
crw--w---- 1 root tty 4, 64 Mar 9 12:45 /dev/ttyS0
crw-rw---- 1 root dialout 4, 65 Jan 1 2000 /dev/ttyS1
crw-rw---- 1 root dialout 4, 66 Jan 1 2000 /dev/ttyS2
crw-rw---- 1 root dialout 4, 67 Jan 1 2000 /dev/ttyS3