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, 9 insertions, 2 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 4d880b3d1f35..62aab585d9d7 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -462,13 +462,20 @@ and is between 256 and 4096 characters. It is defined in the file
                         Documentation/networking/netconsole.txt for an
                         alternative.
 
-                uart,io,<addr>[,options]
-                uart,mmio,<addr>[,options]
+                uart[8250],io,<addr>[,options]
+                uart[8250],mmio,<addr>[,options]
                         Start an early, polled-mode console on the 8250/16550
                         UART at the specified I/O port or MMIO address,
                         switching to the matching ttyS device later.  The
                         options are the same as for ttyS, above.
 
+        earlycon=       [KNL] Output early console device and options.
+                uart[8250],io,<addr>[,options]
+                uart[8250],mmio,<addr>[,options]
+                        Start an early, polled-mode console on the 8250/16550
+                        UART at the specified I/O port or MMIO address.
+                        The options are the same as for ttyS, above.
+
         cpcihp_generic= [HW,PCI] Generic port I/O CompactPCI driver
                         Format:
                         <first_slot>,<last_slot>,<port>,<enum_bit>[,<debug>]