arm64: idreg-override: use early FDT mapping in ID map

Instead of calling into the kernel to map the FDT into the kernel page
tables before even calling start_kernel(), let's switch to the initial,
temporary mapping of the device tree that has been added to the ID map.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20220624150651.1358849-16-ardb@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/head.S

@@ -472,6 +472,7 @@ SYM_FUNC_START_LOCAL(__primary_switched)
 #endif
 	mov	x0, x21				// pass FDT address in x0
 	bl	early_fdt_map			// Try mapping the FDT early
+	mov	x0, x22				// pass FDT address in x0
 	bl	init_feature_override		// Parse cpu feature overrides
 #ifdef CONFIG_RANDOMIZE_BASE
 	tst	x23, ~(MIN_KIMG_ALIGN - 1)	// already running randomized?

arch/arm64/kernel/idreg-override.c

@@ -201,16 +201,11 @@ static __init void __parse_cmdline(const char *cmdline, bool parse_aliases)
 	} while (1);
 }
 
-static __init const u8 *get_bootargs_cmdline(void)
+static __init const u8 *get_bootargs_cmdline(const void *fdt)
 {
 	const u8 *prop;
-	void *fdt;
 	int node;
 
-	fdt = get_early_fdt_ptr();
-	if (!fdt)
-		return NULL;
-
 	node = fdt_path_offset(fdt, "/chosen");
 	if (node < 0)
 		return NULL;
@@ -222,9 +217,9 @@ static __init const u8 *get_bootargs_cmdline(void)
 	return strlen(prop) ? prop : NULL;
 }
 
-static __init void parse_cmdline(void)
+static __init void parse_cmdline(const void *fdt)
 {
-	const u8 *prop = get_bootargs_cmdline();
+	const u8 *prop = get_bootargs_cmdline(fdt);
 
 	if (IS_ENABLED(CONFIG_CMDLINE_FORCE) || !prop)
 		__parse_cmdline(CONFIG_CMDLINE, true);
@@ -234,9 +229,9 @@ static __init void parse_cmdline(void)
 }
 
 /* Keep checkers quiet */
-void init_feature_override(void);
+void init_feature_override(const void *fdt);
 
-asmlinkage void __init init_feature_override(void)
+asmlinkage void __init init_feature_override(const void *fdt)
 {
 	int i;
 
@@ -247,7 +242,7 @@ asmlinkage void __init init_feature_override(void)
 		}
 	}
 
-	parse_cmdline();
+	parse_cmdline(fdt);
 
 	for (i = 0; i < ARRAY_SIZE(regs); i++) {
 		if (regs[i]->override)