xen/arm: Don't use pl011 UART by default for early printk

Add CONFIG_EARLY_PRINTK options in configs/arm{32,64}.mk to let the user
to choose if he wants to have early output, ie before the console is initialized.

This code is specific for each UART. When CONFIG_EARLY_PRINTK is enabled,
Xen will only be able to run on a board with this UART.

If a developper wants to add support for a new UART, he must implement the
following assembly macro/define:
   - EALY_UART_BASE_ADDRESS: variable which contains the physical base address
   for the UART
   - early_uart_init: initialize the UART
   - early_uart_ready: check and wait until the UART can transmit a new
   character
   - early_uart_transmit: transmit a character

For more details about the parameters of each function,
see arm{32,64}/debug-pl011.inc comments.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/docs/misc/arm/early-printk.txt b/docs/misc/arm/early-printk.txt
new file mode 100644
index 0000000000..4065811e82
--- /dev/null
+++ b/docs/misc/arm/early-printk.txt
@@ -0,0 +1,14 @@
+How to enable early printk
+
+Early printk can only be enabled if debug=y. You may want to enable it if
+you are debbuging code that executes before the console is initialized.
+
+Note that selecting this option will limit Xen to a single UART definition.
+Attempting to boot Xen image on a different platform *will not work*, so this
+option should not be enable for Xens that are intended to be portable.
+
+CONFIG_EARLY_PRINTK=mach
+where mach is the name of the machine:
+  - vexpress: printk with pl011 for versatile express
+
+By default early printk is disabled.