x86, early-printk: Update earlyprintk documentation (and kill x86 copy)

Documentation/kernel-parameters.txt and
Documentation/x86/x86_64/boot-options.txt contain virtually
identical text describing earlyprintk.

This consolidates the two copies and updates the documentation a
bit.  No one ever documented the:

	earlyprintk=serial,0x1008,115200

syntax, nor mentioned that ARM is now a supported earlyprintk
arch.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Rob Landley <rob@landley.net>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave@sr71.net>
Link: http://lkml.kernel.org/r/20130410210338.E2930E98@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 14 insertions, 2 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 4609e81dbc37..b5cfd047becb 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -757,19 +757,31 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                         (mmio) or 32-bit (mmio32).
                         The options are the same as for ttyS, above.
 
-        earlyprintk=    [X86,SH,BLACKFIN]
+        earlyprintk=    [X86,SH,BLACKFIN,ARM]
                         earlyprintk=vga
                         earlyprintk=xen
                         earlyprintk=serial[,ttySn[,baudrate]]
+                        earlyprintk=serial[,0x...[,baudrate]]
                         earlyprintk=ttySn[,baudrate]
                         earlyprintk=dbgp[debugController#]
 
+                        earlyprintk is useful when the kernel crashes before
+                        the normal console is initialized. It is not enabled by
+                        default because it has some cosmetic problems.
+
                         Append ",keep" to not disable it when the real console
                         takes over.
 
                         Only vga or serial or usb debug port at a time.
 
-                        Currently only ttyS0 and ttyS1 are supported.
+                        Currently only ttyS0 and ttyS1 may be specified by
+                        name.  Other I/O ports may be explicitly specified
+                        on some architectures (x86 and arm at least) by
+                        replacing ttySn with an I/O port address, like this:
+                                earlyprintk=serial,0x1008,115200
+                        You can find the port for a given device in
+                        /proc/tty/driver/serial:
+                                2: uart:ST16650V2 port:00001008 irq:18 ...
 
                         Interaction with the standard serial driver is not
                         very good.