[PATCH v3 11/11] RISC-V: provide Kconfig & commandline options to control parsing "riscv,isa"

[Conor Dooley]

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 at ventanamicro.com>
Suggested-by: Palmer Dabbelt <palmer at rivosinc.com>
Signed-off-by: Conor Dooley <conor.dooley at microchip.com>
---
Changes in v3:
- Invert the Kconfig entry. It's now default y & not hidden by
  NONPORTABLE, but its entablement will now activate the fallback
- Add a commandline option to enable the fallback on kernels that do not
  enable it in Kconfig, as Drew suggested
- Default the global var to the Kconfig option & override it with the
  commandline one, rather than have checks for IS_ENABLED() and for the
  commandline option in riscv_fill_hwcap() &
  riscv_early_of_processor_hartid()
---
 .../admin-guide/kernel-parameters.txt          |  7 +++++++
 arch/riscv/Kconfig                             | 18 ++++++++++++++++++
 arch/riscv/include/asm/hwcap.h                 |  1 +
 arch/riscv/kernel/cpu.c                        |  6 ++++++
 arch/riscv/kernel/cpufeature.c                 | 14 +++++++++++++-
 5 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index d910fba25f2c..1bd435f60055 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5437,6 +5437,13 @@
 			[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]
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 1d39efe2b940..a9a473b67182 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -841,6 +841,24 @@ 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 of "riscv,isa-base" and
+	  "riscv,isa-extensions".
+
 endmenu # "Boot options"
 
 config BUILTIN_DTB
diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
index e3cda14a486b..b7b58258f6c7 100644
--- a/arch/riscv/include/asm/hwcap.h
+++ b/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);
 
diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
index 28d5af21f544..1acf3679600d 100644
--- a/arch/riscv/kernel/cpu.c
+++ b/arch/riscv/kernel/cpu.c
@@ -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);
diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 2c4503fa984f..5945dfc5f806 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -471,6 +471,18 @@ 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];
@@ -490,7 +502,7 @@ 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);
 		}
-- 
2.40.1