RISC-V: provide Kconfig & commandline options to control parsing "riscv,isa"

As it says on the tin, provide Kconfig option to control parsing the
"riscv,isa" devicetree property. If either option is used, the kernel
will fall back to parsing "riscv,isa", where "riscv,isa-base" and
"riscv,isa-extensions" are not present.
The Kconfig options are set up so that the default kernel configuration
will enable the fallback path, without needing the commandline option.
Suggested-by: Andrew Jones <ajones@ventanamicro.com>
Suggested-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230713-aviator-plausibly-a35662485c2c@wendySigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -5468,6 +5468,13 @@
 			[KNL] Disable ring 3 MONITOR/MWAIT feature on supported
 			CPUs.
 
+	riscv_isa_fallback [RISCV]
+			When CONFIG_RISCV_ISA_FALLBACK is not enabled, permit
+			falling back to detecting extension support by parsing
+			"riscv,isa" property on devicetree systems when the
+			replacement properties are not found. See the Kconfig
+			entry for RISCV_ISA_FALLBACK.
+
 	ro		[KNL] Mount root device read-only on boot
 
 	rodata=		[KNL]

arch/riscv/Kconfig

@@ -848,6 +848,24 @@ config XIP_PHYS_ADDR
 	  be linked for and stored to.  This address is dependent on your
 	  own flash usage.
 
+config RISCV_ISA_FALLBACK
+	bool "Permit falling back to parsing riscv,isa for extension support by default"
+	default y
+	help
+	  Parsing the "riscv,isa" devicetree property has been deprecated and
+	  replaced by a list of explicitly defined strings. For compatibility
+	  with existing platforms, the kernel will fall back to parsing the
+	  "riscv,isa" property if the replacements are not found.
+
+	  Selecting N here will result in a kernel that does not use the
+	  fallback, unless the commandline "riscv_isa_fallback" parameter is
+	  present.
+
+	  Please see the dt-binding, located at
+	  Documentation/devicetree/bindings/riscv/extensions.yaml for details
+	  on the replacement properties, "riscv,isa-base" and
+	  "riscv,isa-extensions".
+
 endmenu # "Boot options"
 
 config BUILTIN_DTB

arch/riscv/include/asm/hwcap.h

@@ -81,6 +81,7 @@ struct riscv_isa_ext_data {
 
 extern const struct riscv_isa_ext_data riscv_isa_ext[];
 extern const size_t riscv_isa_ext_count;
+extern bool riscv_isa_fallback;
 
 unsigned long riscv_isa_extension_base(const unsigned long *isa_bitmap);
 

arch/riscv/kernel/cpu.c

@@ -41,7 +41,7 @@ int riscv_of_processor_hartid(struct device_node *node, unsigned long *hart)
 	return 0;
 }
 
-int riscv_early_of_processor_hartid(struct device_node *node, unsigned long *hart)
+int __init riscv_early_of_processor_hartid(struct device_node *node, unsigned long *hart)
 {
 	const char *isa;
 
@@ -87,6 +87,12 @@ int riscv_early_of_processor_hartid(struct device_node *node, unsigned long *har
 	return 0;
 
 old_interface:
+	if (!riscv_isa_fallback) {
+		pr_warn("CPU with hartid=%lu is invalid: this kernel does not parse \"riscv,isa\"",
+			*hart);
+		return -ENODEV;
+	}
+
 	if (of_property_read_string(node, "riscv,isa", &isa)) {
 		pr_warn("CPU with hartid=%lu has no \"riscv,isa-base\" or \"riscv,isa\" property\n",
 			*hart);

arch/riscv/kernel/cpufeature.c

@@ -473,6 +473,18 @@ static int __init riscv_fill_hwcap_from_ext_list(unsigned long *isa2hwcap)
 	return 0;
 }
 
+#ifdef CONFIG_RISCV_ISA_FALLBACK
+bool __initdata riscv_isa_fallback = true;
+#else
+bool __initdata riscv_isa_fallback;
+static int __init riscv_isa_fallback_setup(char *__unused)
+{
+	riscv_isa_fallback = true;
+	return 1;
+}
+early_param("riscv_isa_fallback", riscv_isa_fallback_setup);
+#endif
+
 void __init riscv_fill_hwcap(void)
 {
 	char print_str[NUM_ALPHA_EXTS + 1];
@@ -492,7 +504,7 @@ void __init riscv_fill_hwcap(void)
 	} else {
 		int ret = riscv_fill_hwcap_from_ext_list(isa2hwcap);
 
-		if (ret) {
+		if (ret && riscv_isa_fallback) {
 			pr_info("Falling back to deprecated \"riscv,isa\"\n");
 			riscv_fill_hwcap_from_isa_string(isa2hwcap);
 		}