coreboot_tables.c: Expose the ACPI RSDP

The ACPI RSDP can only be found in:
- legacy BIOS region
- via UEFI service

On some systems like ARM that legacy BIOS region is not an option, so
to avoid needing UEFI it makes sense to expose the RSDP via a coreboot
table entry.

This also adds the respective unit test.

Change-Id: I591312a2c48f0cbbb03b2787e4b365e9c932afff
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/62573
Reviewed-by: Lance Zhao
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

tests/lib/coreboot_table-test.c

@@ -210,6 +210,12 @@ void arch_write_tables(uintptr_t coreboot_table)
 {
 }
 
+static const uintptr_t ebda_base = 0xf0000;
+uintptr_t get_coreboot_rsdp(void)
+{
+	return ebda_base;
+}
+
 struct resource mock_bootmem_ranges[] = {
 	{.base = 0x1000, .size = 0x2000, .flags = LB_MEM_RAM},
 	{.base = 0x0000, .size = 0x4000, .flags = LB_MEM_RAM},
@@ -476,6 +482,12 @@ static void test_write_tables(void **state)
 			assert_int_equal(ram_code(), board_config->ram_code);
 			assert_int_equal(sku_id(), board_config->sku_id);
 			break;
+		case LB_TAG_ACPI_RSDP:
+			assert_int_equal(sizeof(struct lb_acpi_rsdp), record->size);
+
+			const struct lb_acpi_rsdp *acpi_rsdp = (struct lb_acpi_rsdp *)record;
+			assert_int_equal(ebda_base, unpack_lb64(acpi_rsdp->rsdp_pointer));
+			break;
 		default:
 			fail_msg("Unexpected tag found in record. Tag ID: 0x%x", record->tag);
 		}