earlycon: 8250: Document kernel command line options

Document the expected behavior of kernel command lines of the forms:
    console=uart[8250],io|mmio|mmio32,<addr>[,options]
    console=uart[8250],<addr>[,options]
and
    earlycon=uart[8250],io|mmio|mmio32,<addr>[,options]
    earlycon=uart[8250],<addr>[,options]
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/kernel-parameters.txt

@@ -713,10 +713,18 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
 		uart[8250],io,<addr>[,options]
 		uart[8250],mmio,<addr>[,options]
+		uart[8250],mmio32,<addr>[,options]
+		uart[8250],0x<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.
+			switching to the matching ttyS device later.
+			MMIO inter-register address stride is either 8-bit
+			(mmio) or 32-bit (mmio32).
+			If none of [io|mmio|mmio32], <addr> is assumed to be
+			equivalent to 'mmio'. 'options' are specified in the
+			same format described for ttyS above; if unspecified,
+			the h/w is not re-initialized.
+
 		hvc<n>	Use the hypervisor console device <n>. This is for
 			both Xen and PowerPC hypervisors.
 
@@ -944,11 +952,15 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 		uart[8250],io,<addr>[,options]
 		uart[8250],mmio,<addr>[,options]
 		uart[8250],mmio32,<addr>[,options]
+		uart[8250],0x<addr>[,options]
 			Start an early, polled-mode console on the 8250/16550
 			UART at the specified I/O port or MMIO address.
 			MMIO inter-register address stride is either 8-bit
 			(mmio) or 32-bit (mmio32).
-			The options are the same as for ttyS, above.
+			If none of [io|mmio|mmio32], <addr> is assumed to be
+			equivalent to 'mmio'. 'options' are specified in the
+			same format described for "console=ttyS<n>"; if
+			unspecified, the h/w is not initialized.
 
 		pl011,<addr>
 			Start an early, polled-mode console on a pl011 serial

drivers/tty/serial/8250/8250_core.c

@@ -3471,12 +3471,13 @@ static int univ8250_console_setup(struct console *co, char *options)
  *	@options: ptr to option string from console command line
  *
  *	Only attempts to match console command lines of the form:
- *	    console=uart<>,io|mmio|mmio32,<addr>,<options>
- *	    console=uart<>,<addr>,options
+ *	    console=uart[8250],io|mmio|mmio32,<addr>[,<options>]
+ *	    console=uart[8250],0x<addr>[,<options>]
  *	This form is used to register an initial earlycon boot console and
  *	replace it with the serial8250_console at 8250 driver init.
  *
  *	Performs console setup for a match (as required by interface)
+ *	If no <options> are specified, then assume the h/w is already setup.
  *
  *	Returns 0 if console matches; otherwise non-zero to use default matching
  */