MIPS: Add support for extending builtin cmdline

Allow the builtin command line to be extended by what the bootloader
passes in.  For example, the bootloader can pass specific arguments
depending on the boot mode, and these should override the defaults in
the builtin cmdline.

The default MIPS_CMDLINE_FROM_BOOTLOADER option prepends the
bootloader's cmdline to the builtin cmdline so is not suitable for this
purpose.

Signed-off-by: Rabin Vincent <rabinv@axis.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/13181/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -2930,6 +2930,10 @@ choice
 
 	config MIPS_CMDLINE_FROM_BOOTLOADER
 		bool "Bootloader kernel arguments if available"
+
+	config MIPS_CMDLINE_BUILTIN_EXTEND
+		depends on CMDLINE_BOOL
+		bool "Extend builtin kernel arguments with bootloader arguments"
 endchoice
 
 endmenu

arch/mips/kernel/setup.c

@@ -640,6 +640,8 @@ static void __init request_crashkernel(struct resource *res)
 #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 arch_mem_init(char **cmdline_p)
 {
@@ -684,6 +686,12 @@ static void __init arch_mem_init(char **cmdline_p)
 			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
 	strlcpy(command_line, boot_command_line, COMMAND_LINE_SIZE);