powerpc: Support CMDLINE_EXTEND

Bring powerpc in line with other architectures that support extending or
overriding the bootloader provided command line.

The current behaviour is most like CMDLINE_FROM_BOOTLOADER where the
bootloader command line is preferred but the kernel config can provide a
fallback so CMDLINE_FROM_BOOTLOADER is the default. CMDLINE_EXTEND can
be used to append the CMDLINE from the kernel config to the one provided
by the bootloader.
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190801225006.21952-1-chris.packham@alliedtelesis.co.nz

arch/powerpc/Kconfig

@@ -874,15 +874,33 @@ config CMDLINE
 	  some command-line options at build time by entering them here.  In
 	  most cases you will need to specify the root device here.
 
+choice
+	prompt "Kernel command line type" if CMDLINE != ""
+	default CMDLINE_FROM_BOOTLOADER
+
+config CMDLINE_FROM_BOOTLOADER
+	bool "Use bootloader kernel arguments if available"
+	help
+	  Uses the command-line options passed by the boot loader. If
+	  the boot loader doesn't provide any, the default kernel command
+	  string provided in CMDLINE will be used.
+
+config CMDLINE_EXTEND
+	bool "Extend bootloader kernel arguments"
+	help
+	  The command-line arguments provided by the boot loader will be
+	  appended to the default kernel command string.
+
 config CMDLINE_FORCE
 	bool "Always use the default kernel command string"
-	depends on CMDLINE_BOOL
 	help
 	  Always use the default kernel command string, even if the boot
 	  loader passes other arguments to the kernel.
 	  This is useful if you cannot or don't want to change the
 	  command-line options your boot loader passes to the kernel.
 
+endchoice
+
 config EXTRA_TARGETS
 	string "Additional default image types"
 	help

arch/powerpc/kernel/prom_init.c

@@ -303,16 +303,24 @@ static char __init *prom_strstr(const char *s1, const char *s2)
 	return NULL;
 }
 
-static size_t __init prom_strlcpy(char *dest, const char *src, size_t size)
-{
-	size_t ret = prom_strlen(src);
+static size_t __init prom_strlcat(char *dest, const char *src, size_t count)
+{
+	size_t dsize = prom_strlen(dest);
+	size_t len = prom_strlen(src);
+	size_t res = dsize + len;
+
+	/* This would be a bug */
+	if (dsize >= count)
+		return count;
+
+	dest += dsize;
+	count -= dsize;
+	if (len >= count)
+		len = count-1;
+	memcpy(dest, src, len);
+	dest[len] = 0;
+	return res;
 
-	if (size) {
-		size_t len = (ret >= size) ? size - 1 : ret;
-		memcpy(dest, src, len);
-		dest[len] = '\0';
-	}
-	return ret;
 }
 
 #ifdef CONFIG_PPC_PSERIES
@@ -764,10 +772,14 @@ static void __init early_cmdline_parse(void)
 
 	prom_cmd_line[0] = 0;
 	p = prom_cmd_line;
-	if ((long)prom.chosen > 0)
+
+	if (!IS_ENABLED(CONFIG_CMDLINE_FORCE) && (long)prom.chosen > 0)
 		l = prom_getprop(prom.chosen, "bootargs", p, COMMAND_LINE_SIZE-1);
-	if (IS_ENABLED(CONFIG_CMDLINE_BOOL) && (l <= 0 || p[0] == '\0')) /* dbl check */
-		prom_strlcpy(prom_cmd_line, CONFIG_CMDLINE, sizeof(prom_cmd_line));
+
+	if (IS_ENABLED(CONFIG_CMDLINE_EXTEND) || l <= 0 || p[0] == '\0')
+		prom_strlcat(prom_cmd_line, " " CONFIG_CMDLINE,
+			     sizeof(prom_cmd_line));
+
 	prom_printf("command line: %s\n", prom_cmd_line);
 
 #ifdef CONFIG_PPC64