How to use early printk functionality for ARM64? - linux

Question

I am trying to use early printk for arm64 architecture but there is no such functionality.There is something called as earlycon in arm64 that replaces earlyprintk, but its usage is not known to me. I need to print the messages to serial console before real console is initialized. Is there any way to do that in arm64 using earlycon. I applied a patch called arm64: Add simple earlyprintk support to kernel 3.18 but it did not apply successfully. And what is used in arm64 instead of printch?

Answer 1

One can link the functional uart driver either using device tree chosen node or using boot argument.
Before getting into how to use it, you should check whether the platform UART driver supports EARLYCON. For example, directly copied from owl-uart.c
static int __init
owl_uart_early_console_setup(struct earlycon_device *device, const char *opt)
{
if (!device->port.membase)
return -ENODEV;
device->con->write = owl_uart_early_console_write;
return 0;
}
OF_EARLYCON_DECLARE(owl, "actions,owl-uart",
owl_uart_early_console_setup);
Here you can see the UART driver registers it's early console functionality, which will be called by ARCH init code (which runs before start_kernel).
Two options,
Using DTS entry: Use "stdout-path" option in chosen node to specify the serial/uart driver to use
uart3: serial#e0126000 {
compatible = "actions,s900-uart", "actions,owl-uart";
reg = <0x0 0xe0126000 0x0 0x2000>;
interrupts = <GIC_SPI 32 IRQ_TYPE_LEVEL_HIGH>;
status = "okay";
};
aliases {
serial3 = &uart3;
};
chosen {
stdout-path = "serial3:115200n8";
};
Again the example is for Actions SoC from here. With this entry in place, one need to give bootargs as "earlycon" either from u-boot or using the same chosen node "bootargs" option.
Another way is to explicitly specify the driver name using "earlycon" as bootargs like earlyprink. earlycon=owl-uart,e0126000 will ask the kernel to use the earylconsole definition in UART driver and uses the base address for Virtual and physical address mapping.
You can find more details here.