1How to enable early printk
2
3Early printk can only be enabled if CONFIG_DEBUG=y or in EXPERT mode. You
4may want to enable it if you are debugging code that executes before the
5console is initialized.
6
7Note that selecting this option will limit Xen to a single UART definition.
8Attempting to boot Xen image on a different platform *will not work*, so this
9option should not be enable for Xens that are intended to be portable.
10
11Select one of the "Early printk via * UART" in the choice possible for
12"Early printk" in the "Debugging options" of Kconfig. You will then need to
13set other options, which depends on the driver selected.
14
15CONFIG_EARLY_UART_BASE_ADDRESS is a mandatory argument, it is the base
16physical address of the UART to use.
17
18Other options depends on the driver selected:
19  - 8250
20    - CONFIG_EARLY_UART_8250_REG_SHIFT is, optionally, the left-shift to
21      apply to the register offsets within the uart.
22  - pl011
23    - CONFIG_EARLY_UART_PL011_BAUD_RATE is, optionally, a baud rate which
24      should be used to configure the UART at start of day.
25
26      If CONFIG_EARLY_UART_PL011_BAUD_RATE  is set to 0 then the code will
27      not try to initialize the UART, so that bootloader or firmware
28      settings can be used for maximum compatibility.
29
30    - CONFIG_EARLY_UART_PL011_MMIO32 is, optionally, used to enable 32-bit
31      only accesses to registers.
32  - scif
33    - CONFIG_EARLY_UART_SCIF_VERSION_* is, optionally, the interface version
34      of the UART. Default to version NONE.
35
36  - For all other uarts there are no additional options.
37
38By default early printk is disabled.
39