Initialize the ACPI core for RISC-V during boot.

ACPI tables and interpreter are initialized based on
the information passed from the firmware and the value of
the kernel parameter 'acpi'.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
 arch/riscv/kernel/acpi.c  | 113 ++++++++++++++++++++++++++++++++++++++
 arch/riscv/kernel/setup.c |  25 ++++++---
 2 files changed, 130 insertions(+), 8 deletions(-)

diff --git a/arch/riscv/kernel/acpi.c b/arch/riscv/kernel/acpi.c
index 13b26c87c136..35e7b24a30c8 100644
--- a/arch/riscv/kernel/acpi.c
+++ b/arch/riscv/kernel/acpi.c
@@ -16,6 +16,7 @@
 #include <linux/acpi.h>
 #include <linux/io.h>
 #include <linux/pci.h>
+#include <linux/efi.h>
 
 int acpi_noirq = 1;		/* skip ACPI IRQ initialization */
 int acpi_disabled = 1;
@@ -26,6 +27,118 @@ EXPORT_SYMBOL(acpi_pci_disabled);
 
 static unsigned int intc_count;
 static struct acpi_madt_rintc cpu_madt_rintc[NR_CPUS];
+static bool param_acpi_off __initdata;
+static bool param_acpi_on __initdata;
+static bool param_acpi_force __initdata;
+
+static int __init parse_acpi(char *arg)
+{
+	if (!arg)
+		return -EINVAL;
+
+	/* "acpi=off" disables both ACPI table parsing and interpreter */
+	if (strcmp(arg, "off") == 0)
+		param_acpi_off = true;
+	else if (strcmp(arg, "on") == 0) /* prefer ACPI over DT */
+		param_acpi_on = true;
+	else if (strcmp(arg, "force") == 0) /* force ACPI to be enabled */
+		param_acpi_force = true;
+	else
+		return -EINVAL;	/* Core will print when we return error */
+
+	return 0;
+}
+early_param("acpi", parse_acpi);
+
+/*
+ * acpi_fadt_sanity_check() - Check FADT presence and carry out sanity
+ *			      checks on it
+ *
+ * Return 0 on success,  <0 on failure
+ */
+static int __init acpi_fadt_sanity_check(void)
+{
+	struct acpi_table_header *table;
+	struct acpi_table_fadt *fadt;
+	acpi_status status;
+	int ret = 0;
+
+	/*
+	 * FADT is required on riscv; retrieve it to check its presence
+	 * and carry out revision and ACPI HW reduced compliancy tests
+	 */
+	status = acpi_get_table(ACPI_SIG_FADT, 0, &table);
+	if (ACPI_FAILURE(status)) {
+		const char *msg = acpi_format_exception(status);
+
+		pr_err("Failed to get FADT table, %s\n", msg);
+		return -ENODEV;
+	}
+
+	fadt = (struct acpi_table_fadt *)table;
+
+	if (!(fadt->flags & ACPI_FADT_HW_REDUCED)) {
+		pr_err("FADT not ACPI hardware reduced compliant\n");
+		ret = -EINVAL;
+	}
+
+	/*
+	 * acpi_get_table() creates FADT table mapping that
+	 * should be released after parsing and before resuming boot
+	 */
+	acpi_put_table(table);
+	return ret;
+}
+
+/*
+ * acpi_boot_table_init() called from setup_arch(), always.
+ *	1. find RSDP and get its address, and then find XSDT
+ *	2. extract all tables and checksums them all
+ *	3. check ACPI FADT HW reduced flag
+ *
+ * We can parse ACPI boot-time tables such as MADT after
+ * this function is called.
+ *
+ * On return ACPI is enabled if either:
+ *
+ * - ACPI tables are initialized and sanity checks passed
+ * - acpi=force was passed in the command line and ACPI was not disabled
+ *   explicitly through acpi=off command line parameter
+ *
+ * ACPI is disabled on function return otherwise
+ */
+void __init acpi_boot_table_init(void)
+{
+	/*
+	 * Enable ACPI instead of device tree unless
+	 * - ACPI has been disabled explicitly (acpi=off), or
+	 * - firmware has not populated ACPI ptr in EFI system table
+	 *   and ACPI has not been [force] enabled (acpi=on|force)
+	 */
+	if (param_acpi_off ||
+	    (!param_acpi_on && !param_acpi_force &&
+	     efi.acpi20 == EFI_INVALID_TABLE_ADDR))
+		return;
+
+	/*
+	 * ACPI is disabled at this point. Enable it in order to parse
+	 * the ACPI tables and carry out sanity checks
+	 */
+	enable_acpi();
+
+	/*
+	 * If ACPI tables are initialized and FADT sanity checks passed,
+	 * leave ACPI enabled and carry on booting; otherwise disable ACPI
+	 * on initialization error.
+	 * If acpi=force was passed on the command line it forces ACPI
+	 * to be enabled even if its initialization failed.
+	 */
+	if (acpi_table_init() || acpi_fadt_sanity_check()) {
+		pr_err("Failed to init ACPI tables\n");
+		if (!param_acpi_force)
+			disable_acpi();
+	}
+}
 
 static int acpi_parse_madt_rintc(union acpi_subtable_headers *header, const unsigned long end)
 {
diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
index 4335f08ffaf2..c2ee7f4427a1 100644
--- a/arch/riscv/kernel/setup.c
+++ b/arch/riscv/kernel/setup.c
@@ -8,6 +8,7 @@
  *  Nick Kossifidis <mick@ics.forth.gr>
  */
 
+#include <linux/acpi.h>
 #include <linux/init.h>
 #include <linux/mm.h>
 #include <linux/memblock.h>
@@ -276,14 +277,22 @@ void __init setup_arch(char **cmdline_p)
 
 	efi_init();
 	paging_init();
-#if IS_ENABLED(CONFIG_BUILTIN_DTB)
-	unflatten_and_copy_device_tree();
-#else
-	if (early_init_dt_verify(__va(XIP_FIXUP(dtb_early_pa))))
-		unflatten_device_tree();
-	else
-		pr_err("No DTB found in kernel mappings\n");
-#endif
+
+	/* Parse the ACPI tables for possible boot-time configuration */
+	acpi_boot_table_init();
+	if (acpi_disabled) {
+		if (IS_ENABLED(CONFIG_BUILTIN_DTB)) {
+			unflatten_and_copy_device_tree();
+		} else {
+			if (early_init_dt_verify(__va(XIP_FIXUP(dtb_early_pa))))
+				unflatten_device_tree();
+			else
+				pr_err("No DTB found in kernel mappings\n");
+		}
+	} else {
+		early_init_dt_verify(__va(XIP_FIXUP(dtb_early_pa)));
+	}
+
 	early_init_fdt_scan_reserved_mem();
 	misc_mem_init();
 
-- 
2.34.1

On Thu, Feb 16, 2023 at 11:50:40PM +0530, Sunil V L wrote:
> Initialize the ACPI core for RISC-V during boot.
> 
> ACPI tables and interpreter are initialized based on
> the information passed from the firmware and the value of
> the kernel parameter 'acpi'.
> 
> Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
> Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
>  arch/riscv/kernel/acpi.c  | 113 ++++++++++++++++++++++++++++++++++++++
>  arch/riscv/kernel/setup.c |  25 ++++++---
>  2 files changed, 130 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/riscv/kernel/acpi.c b/arch/riscv/kernel/acpi.c
> index 13b26c87c136..35e7b24a30c8 100644
> --- a/arch/riscv/kernel/acpi.c
> +++ b/arch/riscv/kernel/acpi.c
> @@ -16,6 +16,7 @@
>  #include <linux/acpi.h>
>  #include <linux/io.h>
>  #include <linux/pci.h>
> +#include <linux/efi.h>
>  
>  int acpi_noirq = 1;		/* skip ACPI IRQ initialization */
>  int acpi_disabled = 1;
> @@ -26,6 +27,118 @@ EXPORT_SYMBOL(acpi_pci_disabled);
>  
>  static unsigned int intc_count;
>  static struct acpi_madt_rintc cpu_madt_rintc[NR_CPUS];
> +static bool param_acpi_off __initdata;
> +static bool param_acpi_on __initdata;
> +static bool param_acpi_force __initdata;
> +
> +static int __init parse_acpi(char *arg)
> +{
> +	if (!arg)
> +		return -EINVAL;
> +
> +	/* "acpi=off" disables both ACPI table parsing and interpreter */
> +	if (strcmp(arg, "off") == 0)
> +		param_acpi_off = true;
> +	else if (strcmp(arg, "on") == 0) /* prefer ACPI over DT */
> +		param_acpi_on = true;
> +	else if (strcmp(arg, "force") == 0) /* force ACPI to be enabled */
> +		param_acpi_force = true;
> +	else
> +		return -EINVAL;	/* Core will print when we return error */
> +
> +	return 0;
> +}
> +early_param("acpi", parse_acpi);
> +
> +/*
> + * acpi_fadt_sanity_check() - Check FADT presence and carry out sanity
> + *			      checks on it
> + *
> + * Return 0 on success,  <0 on failure
> + */
> +static int __init acpi_fadt_sanity_check(void)
> +{
> +	struct acpi_table_header *table;
> +	struct acpi_table_fadt *fadt;
> +	acpi_status status;
> +	int ret = 0;
> +
> +	/*
> +	 * FADT is required on riscv; retrieve it to check its presence
> +	 * and carry out revision and ACPI HW reduced compliancy tests
> +	 */
> +	status = acpi_get_table(ACPI_SIG_FADT, 0, &table);
> +	if (ACPI_FAILURE(status)) {
> +		const char *msg = acpi_format_exception(status);
> +
> +		pr_err("Failed to get FADT table, %s\n", msg);
> +		return -ENODEV;
> +	}
> +
> +	fadt = (struct acpi_table_fadt *)table;
> +
> +	if (!(fadt->flags & ACPI_FADT_HW_REDUCED)) {

Do we also need to check for ACPI version 5.0+ when checking for HW
reduced?

> +		pr_err("FADT not ACPI hardware reduced compliant\n");
> +		ret = -EINVAL;
> +	}
> +
> +	/*
> +	 * acpi_get_table() creates FADT table mapping that
> +	 * should be released after parsing and before resuming boot
> +	 */
> +	acpi_put_table(table);
> +	return ret;
> +}
> +
> +/*
> + * acpi_boot_table_init() called from setup_arch(), always.
> + *	1. find RSDP and get its address, and then find XSDT
> + *	2. extract all tables and checksums them all
> + *	3. check ACPI FADT HW reduced flag
> + *
> + * We can parse ACPI boot-time tables such as MADT after
> + * this function is called.
> + *
> + * On return ACPI is enabled if either:
> + *
> + * - ACPI tables are initialized and sanity checks passed
> + * - acpi=force was passed in the command line and ACPI was not disabled
> + *   explicitly through acpi=off command line parameter
> + *
> + * ACPI is disabled on function return otherwise
> + */
> +void __init acpi_boot_table_init(void)
> +{
> +	/*
> +	 * Enable ACPI instead of device tree unless
> +	 * - ACPI has been disabled explicitly (acpi=off), or
> +	 * - firmware has not populated ACPI ptr in EFI system table
> +	 *   and ACPI has not been [force] enabled (acpi=on|force)
> +	 */
> +	if (param_acpi_off ||
> +	    (!param_acpi_on && !param_acpi_force &&
> +	     efi.acpi20 == EFI_INVALID_TABLE_ADDR))
> +		return;
> +
> +	/*
> +	 * ACPI is disabled at this point. Enable it in order to parse
> +	 * the ACPI tables and carry out sanity checks
> +	 */
> +	enable_acpi();
> +
> +	/*
> +	 * If ACPI tables are initialized and FADT sanity checks passed,
> +	 * leave ACPI enabled and carry on booting; otherwise disable ACPI
> +	 * on initialization error.
> +	 * If acpi=force was passed on the command line it forces ACPI
> +	 * to be enabled even if its initialization failed.
> +	 */
> +	if (acpi_table_init() || acpi_fadt_sanity_check()) {
> +		pr_err("Failed to init ACPI tables\n");
> +		if (!param_acpi_force)
> +			disable_acpi();
> +	}
> +}

A lot of the above code is common with arm64. It'd be nice to share that,
but maybe refactoring can be done on top of this.

>  
>  static int acpi_parse_madt_rintc(union acpi_subtable_headers *header, const unsigned long end)
>  {
> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> index 4335f08ffaf2..c2ee7f4427a1 100644
> --- a/arch/riscv/kernel/setup.c
> +++ b/arch/riscv/kernel/setup.c
> @@ -8,6 +8,7 @@
>   *  Nick Kossifidis <mick@ics.forth.gr>
>   */
>  
> +#include <linux/acpi.h>
>  #include <linux/init.h>
>  #include <linux/mm.h>
>  #include <linux/memblock.h>
> @@ -276,14 +277,22 @@ void __init setup_arch(char **cmdline_p)
>  
>  	efi_init();
>  	paging_init();
> -#if IS_ENABLED(CONFIG_BUILTIN_DTB)
> -	unflatten_and_copy_device_tree();
> -#else
> -	if (early_init_dt_verify(__va(XIP_FIXUP(dtb_early_pa))))
> -		unflatten_device_tree();
> -	else
> -		pr_err("No DTB found in kernel mappings\n");
> -#endif
> +
> +	/* Parse the ACPI tables for possible boot-time configuration */
> +	acpi_boot_table_init();
> +	if (acpi_disabled) {
> +		if (IS_ENABLED(CONFIG_BUILTIN_DTB)) {
> +			unflatten_and_copy_device_tree();
> +		} else {
> +			if (early_init_dt_verify(__va(XIP_FIXUP(dtb_early_pa))))
> +				unflatten_device_tree();
> +			else
> +				pr_err("No DTB found in kernel mappings\n");
> +		}
> +	} else {
> +		early_init_dt_verify(__va(XIP_FIXUP(dtb_early_pa)));
> +	}
> +
>  	early_init_fdt_scan_reserved_mem();
>  	misc_mem_init();
>  
> -- 
> 2.34.1
>

Otherwise,

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>

On Mon, Feb 20, 2023 at 09:07:43PM +0100, Andrew Jones wrote:
> On Thu, Feb 16, 2023 at 11:50:40PM +0530, Sunil V L wrote:
> > Initialize the ACPI core for RISC-V during boot.
> > 
> > ACPI tables and interpreter are initialized based on
> > the information passed from the firmware and the value of
> > the kernel parameter 'acpi'.
> > 
> > Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
> > Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > ---
> >  arch/riscv/kernel/acpi.c  | 113 ++++++++++++++++++++++++++++++++++++++
> >  arch/riscv/kernel/setup.c |  25 ++++++---
> >  2 files changed, 130 insertions(+), 8 deletions(-)
> > 
> > diff --git a/arch/riscv/kernel/acpi.c b/arch/riscv/kernel/acpi.c
> > index 13b26c87c136..35e7b24a30c8 100644
> > --- a/arch/riscv/kernel/acpi.c
> > +++ b/arch/riscv/kernel/acpi.c
> > @@ -16,6 +16,7 @@
> >  #include <linux/acpi.h>
> >  #include <linux/io.h>
> >  #include <linux/pci.h>
> > +#include <linux/efi.h>
> >  
> >  int acpi_noirq = 1;		/* skip ACPI IRQ initialization */
> >  int acpi_disabled = 1;
> > @@ -26,6 +27,118 @@ EXPORT_SYMBOL(acpi_pci_disabled);
> >  
> >  static unsigned int intc_count;
> >  static struct acpi_madt_rintc cpu_madt_rintc[NR_CPUS];
> > +static bool param_acpi_off __initdata;
> > +static bool param_acpi_on __initdata;
> > +static bool param_acpi_force __initdata;
> > +
> > +static int __init parse_acpi(char *arg)
> > +{
> > +	if (!arg)
> > +		return -EINVAL;
> > +
> > +	/* "acpi=off" disables both ACPI table parsing and interpreter */
> > +	if (strcmp(arg, "off") == 0)
> > +		param_acpi_off = true;
> > +	else if (strcmp(arg, "on") == 0) /* prefer ACPI over DT */
> > +		param_acpi_on = true;
> > +	else if (strcmp(arg, "force") == 0) /* force ACPI to be enabled */
> > +		param_acpi_force = true;
> > +	else
> > +		return -EINVAL;	/* Core will print when we return error */
> > +
> > +	return 0;
> > +}
> > +early_param("acpi", parse_acpi);
> > +
> > +/*
> > + * acpi_fadt_sanity_check() - Check FADT presence and carry out sanity
> > + *			      checks on it
> > + *
> > + * Return 0 on success,  <0 on failure
> > + */
> > +static int __init acpi_fadt_sanity_check(void)
> > +{
> > +	struct acpi_table_header *table;
> > +	struct acpi_table_fadt *fadt;
> > +	acpi_status status;
> > +	int ret = 0;
> > +
> > +	/*
> > +	 * FADT is required on riscv; retrieve it to check its presence
> > +	 * and carry out revision and ACPI HW reduced compliancy tests
> > +	 */
> > +	status = acpi_get_table(ACPI_SIG_FADT, 0, &table);
> > +	if (ACPI_FAILURE(status)) {
> > +		const char *msg = acpi_format_exception(status);
> > +
> > +		pr_err("Failed to get FADT table, %s\n", msg);
> > +		return -ENODEV;
> > +	}
> > +
> > +	fadt = (struct acpi_table_fadt *)table;
> > +
> > +	if (!(fadt->flags & ACPI_FADT_HW_REDUCED)) {
> 
> Do we also need to check for ACPI version 5.0+ when checking for HW
> reduced?
> 
We need to add version check of FADT once spec is released. Will
update it at that time.

> > +		pr_err("FADT not ACPI hardware reduced compliant\n");
> > +		ret = -EINVAL;
> > +	}
> > +
> > +	/*
> > +	 * acpi_get_table() creates FADT table mapping that
> > +	 * should be released after parsing and before resuming boot
> > +	 */
> > +	acpi_put_table(table);
> > +	return ret;
> > +}
> > +
> > +/*
> > + * acpi_boot_table_init() called from setup_arch(), always.
> > + *	1. find RSDP and get its address, and then find XSDT
> > + *	2. extract all tables and checksums them all
> > + *	3. check ACPI FADT HW reduced flag
> > + *
> > + * We can parse ACPI boot-time tables such as MADT after
> > + * this function is called.
> > + *
> > + * On return ACPI is enabled if either:
> > + *
> > + * - ACPI tables are initialized and sanity checks passed
> > + * - acpi=force was passed in the command line and ACPI was not disabled
> > + *   explicitly through acpi=off command line parameter
> > + *
> > + * ACPI is disabled on function return otherwise
> > + */
> > +void __init acpi_boot_table_init(void)
> > +{
> > +	/*
> > +	 * Enable ACPI instead of device tree unless
> > +	 * - ACPI has been disabled explicitly (acpi=off), or
> > +	 * - firmware has not populated ACPI ptr in EFI system table
> > +	 *   and ACPI has not been [force] enabled (acpi=on|force)
> > +	 */
> > +	if (param_acpi_off ||
> > +	    (!param_acpi_on && !param_acpi_force &&
> > +	     efi.acpi20 == EFI_INVALID_TABLE_ADDR))
> > +		return;
> > +
> > +	/*
> > +	 * ACPI is disabled at this point. Enable it in order to parse
> > +	 * the ACPI tables and carry out sanity checks
> > +	 */
> > +	enable_acpi();
> > +
> > +	/*
> > +	 * If ACPI tables are initialized and FADT sanity checks passed,
> > +	 * leave ACPI enabled and carry on booting; otherwise disable ACPI
> > +	 * on initialization error.
> > +	 * If acpi=force was passed on the command line it forces ACPI
> > +	 * to be enabled even if its initialization failed.
> > +	 */
> > +	if (acpi_table_init() || acpi_fadt_sanity_check()) {
> > +		pr_err("Failed to init ACPI tables\n");
> > +		if (!param_acpi_force)
> > +			disable_acpi();
> > +	}
> > +}
> 
> A lot of the above code is common with arm64. It'd be nice to share that,
> but maybe refactoring can be done on top of this.
>
Okay.
 
> >  
> >  static int acpi_parse_madt_rintc(union acpi_subtable_headers *header, const unsigned long end)
> >  {
> > diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> > index 4335f08ffaf2..c2ee7f4427a1 100644
> > --- a/arch/riscv/kernel/setup.c
> > +++ b/arch/riscv/kernel/setup.c
> > @@ -8,6 +8,7 @@
> >   *  Nick Kossifidis <mick@ics.forth.gr>
> >   */
> >  
> > +#include <linux/acpi.h>
> >  #include <linux/init.h>
> >  #include <linux/mm.h>
> >  #include <linux/memblock.h>
> > @@ -276,14 +277,22 @@ void __init setup_arch(char **cmdline_p)
> >  
> >  	efi_init();
> >  	paging_init();
> > -#if IS_ENABLED(CONFIG_BUILTIN_DTB)
> > -	unflatten_and_copy_device_tree();
> > -#else
> > -	if (early_init_dt_verify(__va(XIP_FIXUP(dtb_early_pa))))
> > -		unflatten_device_tree();
> > -	else
> > -		pr_err("No DTB found in kernel mappings\n");
> > -#endif
> > +
> > +	/* Parse the ACPI tables for possible boot-time configuration */
> > +	acpi_boot_table_init();
> > +	if (acpi_disabled) {
> > +		if (IS_ENABLED(CONFIG_BUILTIN_DTB)) {
> > +			unflatten_and_copy_device_tree();
> > +		} else {
> > +			if (early_init_dt_verify(__va(XIP_FIXUP(dtb_early_pa))))
> > +				unflatten_device_tree();
> > +			else
> > +				pr_err("No DTB found in kernel mappings\n");
> > +		}
> > +	} else {
> > +		early_init_dt_verify(__va(XIP_FIXUP(dtb_early_pa)));
> > +	}
> > +
> >  	early_init_fdt_scan_reserved_mem();
> >  	misc_mem_init();
> >  
> > -- 
> > 2.34.1
> >
> 
> Otherwise,
> 
> Reviewed-by: Andrew Jones <ajones@ventanamicro.com>

Thanks,
Sunil

On Fri, Feb 24, 2023 at 06:06:22PM +0530, Sunil V L wrote:
> On Mon, Feb 20, 2023 at 09:07:43PM +0100, Andrew Jones wrote:
> > On Thu, Feb 16, 2023 at 11:50:40PM +0530, Sunil V L wrote:
...
> > > +	fadt = (struct acpi_table_fadt *)table;
> > > +
> > > +	if (!(fadt->flags & ACPI_FADT_HW_REDUCED)) {
> > 
> > Do we also need to check for ACPI version 5.0+ when checking for HW
> > reduced?
> > 
> We need to add version check of FADT once spec is released. Will
> update it at that time.
> 

I was thinking we need the version check already just for the flag.
The spec has a footnote that says

"The description of HW_REDUCED_ACPI provided here applies to ACPI
 specifications 5.0 and later"

It doesn't really matter in practice since no RISC-V machines can
boot with ACPI less than a version that support the new RISC-V
tables... But I'd rather we either document that fact, or just do
the check.

Thanks,
drew

On Fri, Feb 24, 2023 at 02:07:12PM +0100, Andrew Jones wrote:
> On Fri, Feb 24, 2023 at 06:06:22PM +0530, Sunil V L wrote:
> > On Mon, Feb 20, 2023 at 09:07:43PM +0100, Andrew Jones wrote:
> > > On Thu, Feb 16, 2023 at 11:50:40PM +0530, Sunil V L wrote:
> ...
> > > > +	fadt = (struct acpi_table_fadt *)table;
> > > > +
> > > > +	if (!(fadt->flags & ACPI_FADT_HW_REDUCED)) {
> > > 
> > > Do we also need to check for ACPI version 5.0+ when checking for HW
> > > reduced?
> > > 
> > We need to add version check of FADT once spec is released. Will
> > update it at that time.
> > 
> 
> I was thinking we need the version check already just for the flag.
> The spec has a footnote that says
> 
> "The description of HW_REDUCED_ACPI provided here applies to ACPI
>  specifications 5.0 and later"
> 
> It doesn't really matter in practice since no RISC-V machines can
> boot with ACPI less than a version that support the new RISC-V
> tables... But I'd rather we either document that fact, or just do
> the check.
> 
Okay Let me add the current FADT version of 6.5 which for sure
supports HW_REDUCED flag. When the spec gets released, we can update with
actual revision we need for RISC-V.

Thanks,
Sunil