ACPI: arm64: cpuidle: Tolerate platforms with no deep PSCI idle states

Commit cac173b ("ACPI: processor: idle: Rework the handling of
acpi_processor_ffh_lpi_probe()") moved the acpi_processor_ffh_lpi_probe()
call from acpi_processor_setup_cpuidle_dev(), where its return value was
ignored, to acpi_processor_get_power_info(), where it is now treated as
a hard failure. As a result, platforms where psci_acpi_cpu_init_idle()
returned -ENODEV stopped registering any cpuidle states, forcing CPUs to
busy-poll when idle.

On NVIDIA Grace (aarch64) systems with PSCIv1.1, pr->power.count is 1
(only WFI, no deep PSCI states beyond it), so the previous
"count = pr->power.count - 1; if (count <= 0) return -ENODEV;" check
returned -ENODEV for all 72 CPUs and disabled cpuidle entirely.

The lpi_states count is already validated in acpi_processor_get_lpi_info(),
so the check here is redundant. Simplify the loop to iterate over
lpi_states[1..power.count). When only WFI is present, the loop body
simply does not execute and the function returns 0, which is the correct
outcome: there is nothing to validate for FFH and no error to report.

Suggested-by: Huisong Li <lihuisong@huawei.com>
Cc: stable@vger.kernel.org
Fixes: cac173b ("ACPI: processor: idle: Rework the handling of acpi_processor_ffh_lpi_probe()")
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Sudeep Holla <sudeep.holla@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: leitao

drivers/acpi/arm64/cpuidle.c

@@ -16,7 +16,7 @@
 
 static int psci_acpi_cpu_init_idle(unsigned int cpu)
 {
-	int i, count;
+	int i;
 	struct acpi_lpi_state *lpi;
 	struct acpi_processor *pr = per_cpu(processors, cpu);
 
@@ -30,14 +30,10 @@ static int psci_acpi_cpu_init_idle(unsigned int cpu)
 	if (!psci_ops.cpu_suspend)
 		return -EOPNOTSUPP;
 
-	count = pr->power.count - 1;
-	if (count <= 0)
-		return -ENODEV;
-
-	for (i = 0; i < count; i++) {
+	for (i = 1; i < pr->power.count; i++) {
 		u32 state;
 
-		lpi = &pr->power.lpi_states[i + 1];
+		lpi = &pr->power.lpi_states[i];
 		/*
 		 * Only bits[31:0] represent a PSCI power_state while
 		 * bits[63:32] must be 0x0 as per ARM ACPI FFH Specification