serial: convert early_uart to earlycon for 8250

Beacuse SERIAL_PORT_DFNS is removed from include/asm-i386/serial.h and
include/asm-x86_64/serial.h.  the serial8250_ports need to be probed late in
serial initializing stage.  the console_init=>serial8250_console_init=>
register_console=>serial8250_console_setup will return -ENDEV, and console
ttyS0 can not be enabled at that time.  need to wait till uart_add_one_port in
drivers/serial/serial_core.c to call register_console to get console ttyS0.
that is too late.

Make early_uart to use early_param, so uart console can be used earlier.  Make
it to be bootconsole with CON_BOOT flag, so can use console handover feature.
and it will switch to corresponding normal serial console automatically.

new command line will be:
	console=uart8250,io,0x3f8,9600n8
	console=uart8250,mmio,0xff5e0000,115200n8
or
	earlycon=uart8250,io,0x3f8,9600n8
	earlycon=uart8250,mmio,0xff5e0000,115200n8

it will print in very early stage:
	Early serial console at I/O port 0x3f8 (options '9600n8')
	console [uart0] enabled
later for console it will print:
	console handover: boot [uart0] -> real [ttyS0]

Signed-off-by: <yinghai.lu@sun.com>
Cc: Andi Kleen <ak@suse.de>
Cc: Bjorn Helgaas <bjorn.helgaas@hp.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Gerd Hoffmann <kraxel@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index 2adbed4e10f3..cab42cbd920d 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -62,8 +62,22 @@ config SERIAL_8250_CONSOLE
           kernel will automatically use the first serial line, /dev/ttyS0, as
           system console.
 
+          you can set that using a kernel command line option such as
+          "console=uart8250,io,0x3f8,9600n8"
+          "console=uart8250,mmio,0xff5e0000,115200n8".
+          and it will switch to normal serial console when correponding port is
+          ready.
+          "earlycon=uart8250,io,0x3f8,9600n8"
+          "earlycon=uart8250,mmio,0xff5e0000,115200n8".
+          it will not only setup early console.
+
           If unsure, say N.
 
+config FIX_EARLYCON_MEM
+        bool
+        depends on X86
+        default y
+
 config SERIAL_8250_GSC
         tristate
         depends on SERIAL_8250 && GSC