[PATCH v4] LoongArch: Fix CONFIG_CMDLINE_EXTEND and CONFIG_CMDLINE_BOOTLOADER handling

[Dong Zhihong]

Make CONFIG_CMDLINE work with CONFIG_CMDLINE_EXTEND and
CONFIG_CMDLINE_BOOTLOADER. The touched function is bootcmdline_init()`.
There's already code handling CONFIG_CMDLINE_FORCE, which replaces
`boot_command_line` with CONFIG_CMDLINE and immediately`goto out`. It'd be
similar way to handle CONFIG_CMDLINE_EXTEND and CONFIG_CMDLINE_BOOTLOADER,
so some code is added after OF_FLATTREE part to handle them.

Signed-off-by: Dong Zhihong <donmor3000@xxxxxxxxxxx>
---

v4 -> v3: Make CONFIG_CMDLINE appended to the end of command line (Huacai);
	Removed unnecessary #ifdef since CONFIG_CMDLINE is always a string on
	LoongArch
	Reworded comments
	Reworded the subject of commit message (Huacai)
v3 -> v2: Reworded the commit message again to make it imperative (Ruoyao)
v2 -> v1: Reworded the commit message so it's more imperative (Markus);
	Added `goto out` to FDT part (Huacai)

 arch/loongarch/kernel/setup.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/loongarch/kernel/setup.c b/arch/loongarch/kernel/setup.c
index 78a00359bde3..a98f33553acf 100644
--- a/arch/loongarch/kernel/setup.c
+++ b/arch/loongarch/kernel/setup.c
@@ -332,9 +332,26 @@ static void __init bootcmdline_init(char **cmdline_p)
 			strlcat(boot_command_line, " ", COMMAND_LINE_SIZE);
 
 		strlcat(boot_command_line, init_command_line, COMMAND_LINE_SIZE);
+		goto out;
 	}
 #endif
 
+	/*
+	 * Append built-in command to the retrieved one if
+	 * CONFIG_CMDLINE_EXTEND is enabled.
+	 */
+	if (IS_ENABLED(CONFIG_CMDLINE_EXTEND) && CONFIG_CMDLINE[0]) {
+		strlcat(boot_command_line, " ", COMMAND_LINE_SIZE);
+		strlcat(boot_command_line, CONFIG_CMDLINE, COMMAND_LINE_SIZE);
+		goto out;
+	}
+
+	/*
+	 * Use built-in command line if nothing is retrieved from boot loader.
+	 */
+	if (!boot_command_line[0])
+		strscpy(boot_command_line, CONFIG_CMDLINE, COMMAND_LINE_SIZE);
+
 out:
 	*cmdline_p = boot_command_line;
 }
-- 
2.25.1