MIPS: cmdline: Clean up boot_command_line initialization

Our current code to initialize boot_command_line is a mess. Some of this
is due to the addition of too many options over the years, and some of
this is due to workarounds for early_init_dt_scan_chosen() performing
actions specific to options from other architectures that probably
shouldn't be in generic code.

Clean this up by introducing a new bootcmdline_init() function that
simplifies the initialization somewhat. The major changes are:

- Because bootcmdline_init() is a function it can return early in the
  CONFIG_CMDLINE_OVERRIDE case.

- We clear boot_command_line rather than inheriting whatever
  early_init_dt_scan_chosen() may have left us. This means we no longer
  need to set boot_command_line to a space character in an attempt to
  prevent early_init_dt_scan_chosen() from copying CONFIG_CMDLINE into
  boot_command_line without us knowing about it.

- Indirection via USE_PROM_CMDLINE, USE_DTB_CMDLINE, EXTEND_WITH_PROM &
  BUILTIN_EXTEND_WITH_PROM macros is removed; they seemingly served only
  to obfuscate the code.

- The logic is cleaner, clearer & commented.

Two minor drawbacks of this approach are:

1) We call of_scan_flat_dt(), which means we scan through the DT again.
   The overhead is fairly minimal & shouldn't be noticeable.

2) cmdline_scan_chosen() duplicates a small amount of the logic from
   early_init_dt_scan_chosen(). Alternatives might be to allow the
   generic FDT code to keep & expose a copy of the arguments taken from
   the /chosen node's bootargs property, or to introduce a function like
   early_init_dt_scan_chosen() that retrieves them without modification
   to handle CONFIG_CMDLINE. Neither of these sounds particularly
   cleaner though, and this way we at least keep the extra work in
   arch/mips.
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org

arch/mips/kernel/setup.c

@@ -538,11 +538,88 @@ static void __init check_kernel_sections_mem(void)
 	}
 }
 
-#define USE_PROM_CMDLINE	IS_ENABLED(CONFIG_MIPS_CMDLINE_FROM_BOOTLOADER)
-#define USE_DTB_CMDLINE		IS_ENABLED(CONFIG_MIPS_CMDLINE_FROM_DTB)
-#define EXTEND_WITH_PROM	IS_ENABLED(CONFIG_MIPS_CMDLINE_DTB_EXTEND)
-#define BUILTIN_EXTEND_WITH_PROM	\
-	IS_ENABLED(CONFIG_MIPS_CMDLINE_BUILTIN_EXTEND)
+static void __init bootcmdline_append(const char *s, size_t max)
+{
+	if (!s[0] || !max)
+		return;
+
+	if (boot_command_line[0])
+		strlcat(boot_command_line, " ", COMMAND_LINE_SIZE);
+
+	strlcat(boot_command_line, s, max);
+}
+
+static int __init bootcmdline_scan_chosen(unsigned long node, const char *uname,
+					  int depth, void *data)
+{
+	bool *dt_bootargs = data;
+	const char *p;
+	int l;
+
+	if (depth != 1 || !data ||
+	    (strcmp(uname, "chosen") != 0 && strcmp(uname, "chosen@0") != 0))
+		return 0;
+
+	p = of_get_flat_dt_prop(node, "bootargs", &l);
+	if (p != NULL && l > 0) {
+		bootcmdline_append(p, min(l, COMMAND_LINE_SIZE));
+		*dt_bootargs = true;
+	}
+
+	return 1;
+}
+
+static void __init bootcmdline_init(char **cmdline_p)
+{
+	bool dt_bootargs = false;
+
+	/*
+	 * If CMDLINE_OVERRIDE is enabled then initializing the command line is
+	 * trivial - we simply use the built-in command line unconditionally &
+	 * unmodified.
+	 */
+	if (IS_ENABLED(CONFIG_CMDLINE_OVERRIDE)) {
+		strlcpy(boot_command_line, builtin_cmdline, COMMAND_LINE_SIZE);
+		return;
+	}
+
+	/*
+	 * If the user specified a built-in command line &
+	 * MIPS_CMDLINE_BUILTIN_EXTEND, then the built-in command line is
+	 * prepended to arguments from the bootloader or DT so we'll copy them
+	 * to the start of boot_command_line here. Otherwise, empty
+	 * boot_command_line to undo anything early_init_dt_scan_chosen() did.
+	 */
+	if (IS_ENABLED(CONFIG_MIPS_CMDLINE_BUILTIN_EXTEND))
+		strlcpy(boot_command_line, builtin_cmdline, COMMAND_LINE_SIZE);
+	else
+		boot_command_line[0] = 0;
+
+	/*
+	 * If we're configured to take boot arguments from DT, look for those
+	 * now.
+	 */
+	if (IS_ENABLED(CONFIG_MIPS_CMDLINE_FROM_DTB))
+		of_scan_flat_dt(bootcmdline_scan_chosen, &dt_bootargs);
+
+	/*
+	 * If we didn't get any arguments from DT (regardless of whether that's
+	 * because we weren't configured to look for them, or because we looked
+	 * & found none) then we'll take arguments from the bootloader.
+	 * plat_mem_setup() should have filled arcs_cmdline with arguments from
+	 * the bootloader.
+	 */
+	if (IS_ENABLED(CONFIG_MIPS_CMDLINE_DTB_EXTEND) || !dt_bootargs)
+		bootcmdline_append(arcs_cmdline, COMMAND_LINE_SIZE);
+
+	/*
+	 * If the user specified a built-in command line & we didn't already
+	 * prepend it, we append it to boot_command_line here.
+	 */
+	if (IS_ENABLED(CONFIG_CMDLINE_BOOL) &&
+	    !IS_ENABLED(CONFIG_MIPS_CMDLINE_BUILTIN_EXTEND))
+		bootcmdline_append(builtin_cmdline, COMMAND_LINE_SIZE);
+}
 
 /*
  * arch_mem_init - initialize memory management subsystem
@@ -570,48 +647,12 @@ static void __init arch_mem_init(char **cmdline_p)
 {
 	extern void plat_mem_setup(void);
 
-	/*
-	 * Initialize boot_command_line to an innocuous but non-empty string in
-	 * order to prevent early_init_dt_scan_chosen() from copying
-	 * CONFIG_CMDLINE into it without our knowledge. We handle
-	 * CONFIG_CMDLINE ourselves below & don't want to duplicate its
-	 * content because repeating arguments can be problematic.
-	 */
-	strlcpy(boot_command_line, " ", COMMAND_LINE_SIZE);
-
 	/* call board setup routine */
 	plat_mem_setup();
 	memblock_set_bottom_up(true);
 
-#if defined(CONFIG_CMDLINE_BOOL) && defined(CONFIG_CMDLINE_OVERRIDE)
-	strlcpy(boot_command_line, builtin_cmdline, COMMAND_LINE_SIZE);
-#else
-	if ((USE_PROM_CMDLINE && arcs_cmdline[0]) ||
-	    (USE_DTB_CMDLINE && !boot_command_line[0]))
-		strlcpy(boot_command_line, arcs_cmdline, COMMAND_LINE_SIZE);
-
-	if (EXTEND_WITH_PROM && arcs_cmdline[0]) {
-		if (boot_command_line[0])
-			strlcat(boot_command_line, " ", COMMAND_LINE_SIZE);
-		strlcat(boot_command_line, arcs_cmdline, COMMAND_LINE_SIZE);
-	}
-
-#if defined(CONFIG_CMDLINE_BOOL)
-	if (builtin_cmdline[0]) {
-		if (boot_command_line[0])
-			strlcat(boot_command_line, " ", COMMAND_LINE_SIZE);
-		strlcat(boot_command_line, builtin_cmdline, COMMAND_LINE_SIZE);
-	}
-
-	if (BUILTIN_EXTEND_WITH_PROM && arcs_cmdline[0]) {
-		if (boot_command_line[0])
-			strlcat(boot_command_line, " ", COMMAND_LINE_SIZE);
-		strlcat(boot_command_line, arcs_cmdline, COMMAND_LINE_SIZE);
-	}
-#endif
-#endif
+	bootcmdline_init(cmdline_p);
 	strlcpy(command_line, boot_command_line, COMMAND_LINE_SIZE);
-
 	*cmdline_p = command_line;
 
 	parse_early_param();