efi/libstub: Make efi_printk() input argument const char*

To help the compiler figure out that efi_printk() will not modify
the string it is given, make the input argument type const char*.

While at it, simplify the implementation as well.
Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/libstub/efi-stub-helper.c

@@ -26,20 +26,15 @@ bool __pure __efi_soft_reserve_enabled(void)
 	return !efi_nosoftreserve;
 }
 
-void efi_printk(char *str)
+void efi_printk(const char *str)
 {
-	char *s8;
-
-	for (s8 = str; *s8; s8++) {
-		efi_char16_t ch[2] = { 0 };
-
-		ch[0] = *s8;
-		if (*s8 == '\n') {
-			efi_char16_t nl[2] = { '\r', 0 };
-			efi_char16_printk(nl);
-		}
+	while (*str) {
+		efi_char16_t ch[] = { *str++, L'\0' };
 
-		efi_char16_printk(ch);
+		if (ch[0] == L'\n')
+			efi_char16_printk(L"\r\n");
+		else
+			efi_char16_printk(ch);
 	}
 }
 

drivers/firmware/efi/libstub/efistub.h

@@ -625,7 +625,7 @@ efi_status_t check_platform_features(void);
 
 void *get_efi_config_table(efi_guid_t guid);
 
-void efi_printk(char *str);
+void efi_printk(const char *str);
 
 void efi_free(unsigned long size, unsigned long addr);