ACPI/IORT: Fix PMCG node single ID mapping handling

An IORT PMCG node can have no ID mapping if its overflow interrupt is
wire based therefore the code that parses the PMCG node can not assume
the node will always have a single mapping present at index 0.

Fix iort_get_id_mapping_index() by checking for an overflow interrupt
and mapping count.

Fixes: 24e51604 ("ACPI/IORT: Add support for PMCG")
Signed-off-by: Tuan Phan <tuanphan@os.amperecomputing.com>
Reviewed-by: Hanjun Guo <guoahanjun@huawei.com>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Link: https://lore.kernel.org/r/1589994787-28637-1-git-send-email-tuanphan@os.amperecomputing.comSigned-off-by: Will Deacon <will@kernel.org>

drivers/acpi/arm64/iort.c

@@ -388,6 +388,7 @@ static struct acpi_iort_node *iort_node_get_id(struct acpi_iort_node *node,
 static int iort_get_id_mapping_index(struct acpi_iort_node *node)
 {
 	struct acpi_iort_smmu_v3 *smmu;
+	struct acpi_iort_pmcg *pmcg;
 
 	switch (node->type) {
 	case ACPI_IORT_NODE_SMMU_V3:
@@ -415,6 +416,10 @@ static int iort_get_id_mapping_index(struct acpi_iort_node *node)
 
 		return smmu->id_mapping_index;
 	case ACPI_IORT_NODE_PMCG:
+		pmcg = (struct acpi_iort_pmcg *)node->node_data;
+		if (pmcg->overflow_gsiv || node->mapping_count == 0)
+			return -EINVAL;
+
 		return 0;
 	default:
 		return -EINVAL;