earlycon: Let users set the clock frequency

Some platforms, namely AMD Picasso, use non standard uart clocks (48M),
witch makes it impossible to use with earlycon.

Let the user select its own frequency.
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20221123-serial-clk-v3-1-49c516980ae0@chromium.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -1195,10 +1195,10 @@
 			specified, the serial port must already be setup and
 			configured.
 
-		uart[8250],io,<addr>[,options]
-		uart[8250],mmio,<addr>[,options]
-		uart[8250],mmio32,<addr>[,options]
-		uart[8250],mmio32be,<addr>[,options]
+		uart[8250],io,<addr>[,options[,uartclk]]
+		uart[8250],mmio,<addr>[,options[,uartclk]]
+		uart[8250],mmio32,<addr>[,options[,uartclk]]
+		uart[8250],mmio32be,<addr>[,options[,uartclk]]
 		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.
@@ -1207,7 +1207,9 @@
 			If none of [io|mmio|mmio32|mmio32be], <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.
+			unspecified, the h/w is not initialized. 'uartclk' is
+			the uart clock frequency; if unspecified, it is set
+			to 'BASE_BAUD' * 16.
 
 		pl011,<addr>
 		pl011,mmio32,<addr>

drivers/tty/serial/earlycon.c

@@ -120,7 +120,13 @@ static int __init parse_options(struct earlycon_device *device, char *options)
 	}
 
 	if (options) {
+		char *uartclk;
+
 		device->baud = simple_strtoul(options, NULL, 0);
+		uartclk = strchr(options, ',');
+		if (uartclk && kstrtouint(uartclk + 1, 0, &port->uartclk) < 0)
+			pr_warn("[%s] unsupported earlycon uart clkrate option\n",
+				options);
 		length = min(strcspn(options, " ") + 1,
 			     (size_t)(sizeof(device->options)));
 		strscpy(device->options, options, length);
@@ -139,7 +145,8 @@ static int __init register_earlycon(char *buf, const struct earlycon_id *match)
 		buf = NULL;
 
 	spin_lock_init(&port->lock);
-	port->uartclk = BASE_BAUD * 16;
+	if (!port->uartclk)
+		port->uartclk = BASE_BAUD * 16;
 	if (port->mapbase)
 		port->membase = earlycon_map(port->mapbase, 64);