RISC-V: ACPI: Enable SPCR table for console output on RISC-V

The ACPI SPCR code has been used to enable console output for ARM64 and
X86. The same code can be reused for RISC-V. Furthermore, SPCR table is
mandated for headless system as outlined in the RISC-V BRS
Specification, chapter 6.
Signed-off-by: Sia Jee Heng <jeeheng.sia@starfivetech.com>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>
Link: https://lore.kernel.org/r/20240502073751.102093-2-jeeheng.sia@starfivetech.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Kconfig

@@ -14,6 +14,7 @@ config RISCV
 	def_bool y
 	select ACPI_GENERIC_GSI if ACPI
 	select ACPI_REDUCED_HARDWARE_ONLY if ACPI
+	select ACPI_SPCR_TABLE if ACPI
 	select ARCH_DMA_DEFAULT_COHERENT
 	select ARCH_ENABLE_HUGEPAGE_MIGRATION if HUGETLB_PAGE && MIGRATION
 	select ARCH_ENABLE_SPLIT_PMD_PTLOCK if PGTABLE_LEVELS > 2

arch/riscv/kernel/acpi.c

@@ -17,7 +17,9 @@
 #include <linux/efi.h>
 #include <linux/io.h>
 #include <linux/memblock.h>
+#include <linux/of_fdt.h>
 #include <linux/pci.h>
+#include <linux/serial_core.h>
 
 int acpi_noirq = 1;		/* skip ACPI IRQ initialization */
 int acpi_disabled = 1;
@@ -131,7 +133,7 @@ void __init acpi_boot_table_init(void)
 	if (param_acpi_off ||
 	    (!param_acpi_on && !param_acpi_force &&
 	     efi.acpi20 == EFI_INVALID_TABLE_ADDR))
-		return;
+		goto done;
 
 	/*
 	 * ACPI is disabled at this point. Enable it in order to parse
@@ -151,6 +153,14 @@ void __init acpi_boot_table_init(void)
 		if (!param_acpi_force)
 			disable_acpi();
 	}
+
+done:
+	if (acpi_disabled) {
+		if (earlycon_acpi_spcr_enable)
+			early_init_dt_scan_chosen_stdout();
+	} else {
+		acpi_parse_spcr(earlycon_acpi_spcr_enable, true);
+	}
 }
 
 static int acpi_parse_madt_rintc(union acpi_subtable_headers *header, const unsigned long end)