[PATCH] [MIPS] Fix and enhance built-in kernel command line

[Dmitri Vorobiev <dmitri.vorobiev@movial.com>]

Currently, MIPS kernels silently overwrite kernel command-line
parameters hardcoded in CONFIG_CMDLINE by the ones received
from firmware.  Therefore, using firmware remains the only
reliable method to transfer the command-line parameters, which
is not always desirable or convenient, and the CONFIG_CMDLINE
option is thereby effectively rendered useless.

This patch fixes the problem described above and introduces
a more flexible scheme of handling the kernel command line,
in a manner identical to what is currently used for x86.
The default behavior, i.e. when CONFIG_CMDLINE_BOOL is not
defined, retains the existing semantics, and firmware
command-line arguments override the hardcoded ones.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.com>
---
 arch/mips/Kconfig.debug  |   45 +++++++++++++++++++++++++++++++++++++++++----
 arch/mips/kernel/setup.c |   24 ++++++++++++++++++++----
 2 files changed, 61 insertions(+), 8 deletions(-)

diff --git a/arch/mips/Kconfig.debug b/arch/mips/Kconfig.debug
index 364ca89..9835030 100644
--- a/arch/mips/Kconfig.debug
+++ b/arch/mips/Kconfig.debug
@@ -6,15 +6,52 @@ config TRACE_IRQFLAGS_SUPPORT
 
 source "lib/Kconfig.debug"
 
+config CMDLINE_BOOL
+       bool "Built-in kernel command line"
+       default n
+       help
+         For most systems, it is firmware or second stage bootloader that
+         by default specifies the kernel command line options.  However,
+         it might be necessary or advantageous to either override the
+         default kernel command line or add a few extra options to it.
+         For such cases, this option allows you to hardcode your own
+         command line options directly into the kernel.  For that, you
+         should choose 'Y' here, and fill in the extra boot arguments
+         in CONFIG_CMDLINE.
+
+         The built-in options will be concatenated to the default command
+         line if CMDLINE_OVERRIDE is set to 'N'. Otherwise, the default
+         command line will be ignored and replaced by the built-in string.
+
+         Most MIPS systems will normally expect 'N' here and rely upon
+         the command line from the firmware or the second-stage bootloader.
+
+config CMDLINE_OVERRIDE
+       bool "Built-in command line overrides firware arguments"
+       default n
+       depends on CMDLINE_BOOL
+       help
+         By setting this option to 'Y' you will have your kernel ignore
+         command line arguments from firmware or second stage bootloader.
+         Instead, the built-in command line will be used exclusively.
+
+         Normally, you will choose 'N' here.
+
 config CMDLINE
        string "Default kernel command string"
+       depends on CMDLINE_BOOL
        default ""
        help
          On some platforms, there is currently no way for the boot loader to
-         pass arguments to the kernel. For these platforms, you can supply
-         some command-line options at build time by entering them here.  In
-         other cases you can specify kernel args so that you don't have
-         to set them up in board prom initialization routines.
+         pass arguments to the kernel.  For these platforms, and for the cases
+         when you want to add some extra options to the command line or ignore
+         the default command line, you can supply some command-line options at
+         build time by entering them here.  In other cases you can specify
+         kernel args so that you don't have to set them up in board prom
+         initialization routines.
+
+         For more information, see the CMDLINE_BOOL and CMDLINE_OVERRIDE
+         options.
 
 config DEBUG_STACK_USAGE
        bool "Enable stack utilization instrumentation"
diff --git a/arch/mips/kernel/setup.c b/arch/mips/kernel/setup.c
index bd55f71..f9513f9 100644
--- a/arch/mips/kernel/setup.c
+++ b/arch/mips/kernel/setup.c
@@ -58,8 +58,12 @@ EXPORT_SYMBOL(mips_machtype);
 
 struct boot_mem_map boot_mem_map;
 
-static char command_line[COMMAND_LINE_SIZE];
-       char arcs_cmdline[COMMAND_LINE_SIZE] = CONFIG_CMDLINE;
+static char __initdata command_line[COMMAND_LINE_SIZE];
+char __initdata arcs_cmdline[COMMAND_LINE_SIZE];
+
+#ifdef CONFIG_CMDLINE_BOOL
+static char __initdata builtin_cmdline[COMMAND_LINE_SIZE] = CONFIG_CMDLINE;
+#endif
 
 /*
  * mips_io_port_base is the begin of the address space to which x86 style
@@ -458,8 +462,20 @@ static void __init arch_mem_init(char **cmdline_p)
        pr_info("Determined physical RAM map:\n");
        print_memory_map();
 
-       strlcpy(command_line, arcs_cmdline, sizeof(command_line));
-       strlcpy(boot_command_line, command_line, COMMAND_LINE_SIZE);
+#ifdef CONFIG_CMDLINE_BOOL
+#ifdef CONFIG_CMDLINE_OVERRIDE
+       strlcpy(boot_command_line, builtin_cmdline, COMMAND_LINE_SIZE);
+#else
+       if (builtin_cmdline[0]) {
+               strlcat(arcs_cmdline, " ", COMMAND_LINE_SIZE);
+               strlcat(arcs_cmdline, builtin_cmdline, COMMAND_LINE_SIZE);
+       }
+       strlcpy(boot_command_line, arcs_cmdline, COMMAND_LINE_SIZE);
+#endif
+#else
+       strlcpy(boot_command_line, arcs_cmdline, COMMAND_LINE_SIZE);
+#endif
+       strlcpy(command_line, boot_command_line, COMMAND_LINE_SIZE);
 
        *cmdline_p = command_line;
 
-- 
1.6.3.3